Systematic revision of the genus Peronia Fleming, 1822 (Gastropoda, Euthyneura, Pulmonata, Onchidiidae)

Abstract The genus Peronia Fleming, 1822 includes all the onchidiid slugs with dorsal gills. Its taxonomy is revised for the first time based on a large collection of fresh material from the entire Indo-West Pacific, from South Africa to Hawaii. Nine species are supported by mitochondrial (COI and 16S) and nuclear (ITS2 and 28S) sequences as well as comparative anatomy. All types available were examined and the nomenclatural status of each existing name in the genus is addressed. Of 31 Peronia species-group names available, 27 are regarded as invalid (twenty-one synonyms, sixteen of which are new, five nomina dubia, and one homonym), and four as valid: Peronia peronii (Cuvier, 1804), Peronia verruculata (Cuvier, 1830), Peronia platei (Hoffmann, 1928), and Peronia madagascariensis (Labbé, 1934a). Five new species names are created: P. griffithsi Dayrat & Goulding, sp. nov., P. okinawensis Dayrat & Goulding, sp. nov., P. setoensis Dayrat & Goulding, sp. nov., P. sydneyensis Dayrat & Goulding, sp. nov., and P. willani Dayrat & Goulding, sp. nov.Peronia species are cryptic externally but can be distinguished using internal characters, with the exception of P. platei and P. setoensis. The anatomy of most species is described in detail here for the first time. All the secondary literature is commented on and historical specimens from museum collections were also examined to better establish species distributions. The genus Peronia includes two species that are widespread across the Indo-West Pacific (P. verruculata and P. peronii) as well as endemic species: P. okinawensis and P. setoensis are endemic to Japan, and P. willani is endemic to Northern Territory, Australia. Many new geographical records are provided, as well as a key to the species using morphological traits.


Introduction
Onchidiid slugs live in the intertidal, worldwide, except at the poles. Their larvae are released in sea water and, in that sense, onchidiids are truly marine. As adult slugs, however, they breathe air through a lung and die if they are immersed in water for too long. The slugs of the genus Peronia Fleming, 1822a are found across the entire tropical and subtropical Indo-West Pacific, from South Africa to Hawaii. They primarily inhabit rocky shores and coral rubble, can occasionally be found on muddy sand, but are typically not found inside mangrove forests.
The genus Peronia includes all onchidiid slugs with a dorsal notum bearing ramified appendages, or dorsal gills, which are most easily seen when animals are relaxed. Dorsal gills tend to be retracted when live animals are crawling at low tide, and they can be hard to see on specimens preserved without relaxation. In fact, Cuvier did not mention dorsal gills in the original description of Onchidium peronii Cuvier, 1804, the first Peronia species ever recognized. Dorsal gills were first illustrated by Savigny (1817: pl. 2, fig. 3.5) on a plate of gastropods from the Red Sea in the famous Description de l'Egypte, and first described by Audouin (1826: 19) in the explanation of Savigny's plate. Dorsal gills are either present or absent on the dorsal notum of onchidiid slugs, and all slugs with dorsal gills belong to the genus Peronia (Dayrat et al. : 1861. For the past sixty years or so, authors have accepted only two valid Peronia species names for two species broadly distributed across the Indo-West Pacific (e.g., Solem 1959: 38-39;Marcus and Marcus 1970: 213-214;Britton 1984: 183): P. peronii (Cuvier, 1804) and P. verruculata (Cuvier, 1830). However, the differences between P. peronii and P. verruculata have remained unclear, to say the least, and both names have been used arbitrarily. More importantly, 31 species-group names are available for onchidiids with dorsal gills and their exact application has never been addressed. Indeed, the taxonomy of the genus Peronia is so challenging that people have avoided it for decades, and  is the last author who created species names for onchidiids with dorsal gills, except for the recent Peronia persiae Maniei et al., 2020a, regarded in the present work as a synonym of P. verruculata. The taxonomy of the genus Peronia is comprehensively revised here for the first time. The goals of the present revision are to determine how many Peronia species there are, where they are distributed, how they are related, how they can be identified, how many of the available species names are valid, and to create new names if needed.
All the available types of all onchidiid species were re-examined in the context of our revision of the whole family (Dayrat 2009;Dayrat et al. 2016Dayrat et al. , 2018Dayrat et al. , 2019aGoulding et al. 2018a, b, c), which served as a basis to establish a complete list of all the species names available in the genus Peronia. For the sake of clarity, important features (especially intestinal loops) of the types of Peronia nominal species are illustrated here. In many cases, lectotypes are designated in order to clarify the application of species names.
Fresh material was collected across the entire Indo-West Pacific, from South Africa to Japan, Hawaii, and eastern Australia. Special attention was paid to collecting fresh material from type localities. Specimens from which DNA could be extracted were also obtained from museum collections (the first author visited many collections around the world). Old museum specimens from which DNA could not be extracted were also examined, especially in cases of interesting geographical records or when specimens were included in important onchidiid studies (Semper 1880(Semper -1885Plate 1893;. Because they are notoriously cryptic, Peronia species were first delineated using DNA sequences. Then, the anatomy of the specimens was examined in order to determine diagnostic characters for each species as well as individual variation. As in our previous revisions (Dayrat et al. , 2018(Dayrat et al. , 2019aGoulding et al. 2018a, b, c), both mitochondrial and nuclear DNA sequences were used for species delineation and relationships.
Nine Peronia species are recognized here, five of which are new to science: P. griffithsi Dayrat & Goulding, sp. nov., P. madagascariensis  , P. okinawensis Dayrat & Goulding, sp. nov., P. peronii (Cuvier, 1804), P. platei , P. setoensis Dayrat & Goulding, sp. nov., P. sydneyensis Dayrat & Goulding, sp. nov., P. verruculata (Cuvier, 1830), and P. willani Dayrat & Goulding, sp. nov. Both P. madagascariensis and P. platei were only known from the original descriptions and are described anatomically in detail for the first time. Amazingly, the best anatomical description of P. peronii so far is Cuvier's (1804) original description, but many traits are described and illustrated here for the first time. Finally, the anatomy of all mitochondrial units of P. verruculata is described in detail for the first time from numerous localities, although some anatomical information was scattered in the literature for three of them (units #1, #3, and #4).
These nine species cannot be distinguished externally, except for the very large individuals of P. peronii (longer than 100 mm). However, details of the internal anatomy can help separate species, except for P. platei and P. setoensis which are both cryptic externally and internally. Geographic distribution varies greatly among Peronia species. Three species are broadly distributed across the Indo-West Pacific, from the western Indian Ocean to the West Pacific: P. griffithsi, P. peronii, and P. verruculata. The six other species are characterized by much narrower geographic ranges. Three species are even endemic: Peronia okinawensis and P. setoensis are endemic to Japan, and P. willani is endemic to the Northern Territory, Australia. Of the 31 Peronia species names available, four are valid and 27 are invalid: 21 synonyms (16 of which are new), five nomina dubia, and one junior secondary homonym. The large number of available names in Peronia is explained by a combination of three main factors. First, Peronia slugs have often been collected, because they are common across the Indo-West Pacific and because they mostly live in the rocky intertidal, which is more easily accessible than mangrove forests where most other onchidiids are found. Second, earlier zoologists created new species names without examining the types of existing nominal species and without proper knowledge of individual variation, which resulted in many names being added unnecessarily. Third, Peronia is a genus for which molecular data were critically needed, because species are externally cryptic; also, species could hardly be delineated just based on their internal anatomy because they differ only with respect to minute anatomical details. The fact that five new species names are needed in Peronia even though there already are 31 available names shows that a comprehensive revision was desperately needed.

Nomenclature
Establishing a complete list of available names for a taxon often requires an enormous amount of time but it is the keystone of any taxonomic revision, because otherwise it would be impossible to address the nomenclatural status of available names and to determine how many new species names are needed.
All available type specimens were re-examined beyond the taxon of interest (Peronia) because species names often are incorrectly classified when they are first created. For instance, Onchidium durum  was originally created for slugs with a smooth notum, but the types of O. durum clearly bear dorsal gills. Ignoring O. durum because it was created for slugs with a smooth notum would have led to an incomplete list of available Peronia species names. Several species names had to be transferred to Peronia, because they refer to slugs with dorsal gills, regardless of whether species were originally described with dorsal gills or not. When type specimens are not located, one needs to go through original species descriptions very carefully, and still beyond the taxon of interest. Reciprocally, not all species names ever classified in Peronia belong to Peronia: for instance, several specific names originally combined with Peronia refer to Onchidella species. Finally, many species names of doubtful application need to be commented upon.
In total, 51 species-group names had to be considered for the revision of Peronia. Of these, only 31 are available Peronia species names (Table 1). Indeed, ten of those Table 1. Alphabetic list of the 51 existing species-group names of which the nomenclatural status is addressed in the present work. Details can be found in the text: comments on the four valid Peronia species names, their synonyms, and the junior homonym are in the species remarks; comments on the fifteen nomina dubia and the ten names that must be classified in other genera are in the general discussion. Table 2. DNA extraction numbers and GenBank accession numbers for all the specimens included in the present study. The letter H next to an extraction number indicates the holotype. Sequences marked with an asterisk (*) are from our former publications (Dayrat et al. , 2018(Dayrat et al. , 2019aGoulding et al. 2018a, b, c). In addition, 11 COI sequences also marked with an asterisk (*) were obtained from GenBank (GB) and BOLD: four sequences from China (Sun et al. 2014), two from Singapore , two from Japan (Takagi et al. 2019), one from the Persian Gulf (unpublished), one from Gujarat, western India (unpublished), and one from Iran    LGEN099-14* GB Iran, Persian Gulf LC027608* 5500 MNHN-IM-2009- Vouchers used in Dayrat et al. (2011). Ten of our Peronia specimens were tentatively identified by Dayrat et al. (2011) at a time when nothing was known about the onchidiid species diversity in general and most especially in the genus Peronia. Most of those ten specimens were merely referred to with numbers (e.g., Peronia sp. 1). In order to avoid any confusion, those specimens are all included here so that correct species names are provided (  (NHMUK 20050628) identified as Peronia sp. 6 from Sulawesi, Indonesia. Types of existing species-group names. All type specimens available for all onchidiid species-group names have been examined in context of the revision of the entire family. Comments on many onchidiid types can be found in our previous revisions (Dayrat et al. , 2018(Dayrat et al. , 2019aGoulding et al. 2018a, b, c). In total, 118 type specimens (holotypes, lectotypes, paralectotypes, syntypes, etc.) are commented on here for the first time. Fifteen of those 118 type specimens are commented on in the general discussion because they are types of nomina dubia which may or may not refer to Peronia slugs. All the other (103) types are commented on in species descriptions because they are the types of 25 species-group names which must be classified in Peronia and which are not nomina dubia (Table 1).There are only two Peronia species names for which types could not be located: Scaphis lata Paraperonia jousseaumei Labbé, 1934a. Finally, 14 lectotypes are designated here in order to clarify the application of 14 species names, usually because syntypes belong to different species or come from very distant localities.
Many type specimens were not labeled as types and were found within the general collections. In most cases, it was easy to determine that specimens were types because the information on the labels would match perfectly to that of the original descriptions. However, finding Labbé's types was challenging, with the exception of the holotype, by monotypy, of Onchidium astridae Labbé, 1934b, preserved in Bruxelles (RBINS I.G.9223/MT.3822): it was not marked as a holotype, but the name Onchidium astridae is on the label, and the locality and collector information is matching.
The types of all the other Peronia species (and one subspecies) described by Labbé are preserved at the MNHN (the monograph in which those new taxa were described was almost exclusively based on material from the MNHN). The major issue with this material is that Labbé did not write any of his new species names on any of the labels. To be fair and fully accurate, there are actually three jars for which a specific name was written in pencil and in tiny letters on labels: one jar contains the type material of Onchidium durum (MNHN-IM-2000-33698), and two other jars contain part of the type material of Paraperonia gondwanae (MNHN-IM-2000-33683, MNHN-IM-2000. Eleven years ago, Dayrat (2009) considered that identifying the types of Labbé's onchidiid species names in the MNHN collection would be too risky (because specimens could be erroneously interpreted as types). However, after Virginie Héros (who is in charge of the Mollusk type collection at the MNHN) correctly remarked that it should still be possible to find some of Labbé's types, an excel file was generated including all the old onchidiid material preserved at the MNHN and all the material cited in  monograph. By comparing various information (localities, names of the collec-tors, collecting dates, specimen sizes), it then became clear that many specimens could be identified as types with great confidence, even though they were not labeled as types and Labbé's species names were not indicated on the labels.
For instance, originally, no jar clearly labeled as the type material of Scaphis carbonaria was found at the MNHN. However, of the old jars found at the MNHN with specimens from New Caledonia, only one matches perfectly the information provided in Labbé's original description of S. carbonaria: an individual collected in 1880 by Réveillère (with an identification as Peronia). Other jars with one or more specimens from New Caledonia were collected by Fisher in 1878 and by François in 1894. Therefore, it is extremely likely that the specimen collected by Réveillère in 1880 is the holotype, by monotypy, of Scaphis carbonaria (MNHN-IM-2000-33708). In many cases, however, identifying the types happened to be much more challenging because there were several jars with the same locality, the same collector, and the same collecting date. In order to avoid any future confusion, Labbé's types are commented on in great detail in species descriptions. There are only two of Labbé's species for which no type material could be confidently traced back at the MNHN: Scaphis lata Paraperonia jousseaumei Labbé, 1934a. Finally, the type material of Peronia persiae, recently described by Maniei et al. (2020a), was not borrowed for examination. Regardless, there is no doubt that P. persiae is a junior synonym of both P. verruculata (Cuvier, 1830) and P. gondwanae   (Table 1), because all the COI and 16S sequences published for P. persiae cluster within the mitochondrial unit #4 of P. verruculata.
Additional material examined (historical museum collections). In addition to the 189 specimens included in the molecular analyses (not including the eleven outgroups) and the 118 type specimens of existing nominal species, 297 old specimens were obtained from museum collections from which no DNA could be extracted. Those specimens correspond to a total of 60 jars. One jar contains 161 specimens. All other jars contain fewer than 15 specimens. These old museum specimens are not included in the anatomical species descriptions, except for the description of Peronia verruculata from the Red Sea. Instead, these additional specimens are commented on in the species remarks. The additional specimens were especially useful to provide geographic records from places which could not be visited, such as the Chagos Archipelago, Nicobar Islands, Persian Gulf, and Socotra. Identifying Peronia species using only anatomical traits is challenging but possible (see below). Finally, some of the historical specimens from museum collections were studied by previous authors, and their reexamination allowed us to confirm or reject many identifications from the literature.

Anatomical preparations and descriptions
Size (length/width) is indicated in millimeters (mm) for each specimen. Both the external morphology and the internal anatomy were studied. All anatomical observations were made under a dissecting microscope and drawn with a camera lucida. Radulae and male reproductive organs were prepared for scanning electron microscopy (Zeiss SIGMA Field Emission Scanning Electron Microscopy). Radulae were cleaned in 10% NaOH for a week, rinsed in distilled water, briefly cleaned in an ultrasonic water bath (less than a minute), sputter-coated with gold-palladium and examined by SEM. Soft parts (penis, accessory penial gland, etc.) were dehydrated in ethanol and critical point dried before coating.
Anatomical species descriptions are based on those 179 Peronia individuals for which sequences were generated for the present study as well as on the available type material for species with existing names (see below). To avoid unnecessary repetition, the description of anatomical features that are virtually identical between Peronia species (e.g., nervous system, heart, and stomach) is not repeated for each species. However, all the characters that are useful for species comparison (e.g., intestinal loops and male apparatus) are described for every species. Special attention has been given to illustrating the holotype and the type locality of each new species.
Species are being described following a phylogenetic order. The detailed description of Peronia verruculata is based on the mitochondrial unit #1, by far the most widespread (from Peninsular Malaysia to the West Pacific) and most abundant (55 specimens in our study), but variations in the other units are precisely reported and figure captions indicate the unit to which each illustrated individual belongs.

Types of intestinal loops
In onchidiids, types of intestinal loops are defined based on the pattern of the intestine on the dorsal aspect of the digestive gland (with the digestive gland still in place). Plate (1893) first distinguished four types of intestinal loops (types I to IV) and  later added a type V. Only the types I and V are found in Peronia. Hoffmann (1928: 51, pl. 3, fig. 11) noted before Labbé that intestinal loops of type V differ from other types and he referred to them as type Ia. Labbé's terminology (type V) is preferred because past authors have adopted it and because a type V is very different from a type I. The different types of intestinal loops and their individual variation are best revealed by coloring sections of the intestine differently (Dayrat et al. 2019b, c, d): a clockwise intestinal loop is colored in blue, a counterclockwise intestinal loop is colored in yellow, and a transitional loop between them is colored in green (Fig. 1).
The intestine first appears dorsally on the right side. In intestinal loops of type I, the intestine starts by forming a clockwise (blue) loop which does not make a complete circle. As a result, the transitional (green) loop is oriented to the right ( Fig. 1A-F). In two species with intestinal loops of type I (P. okinawensis and P. peronii), the transitional loop is oriented between 12 and 3 o'clock ( Fig. 1D-F). In the three other species with intestinal loops of type I (P. sydneyensis, P. verruculata, and P. willani), the transitional loop is oriented between 3 and 6 o'clock ( Fig. 1A-C). In intestinal loops of type V, the intestine starts by forming immediately a counterclockwise (yellow) loop. In intestinal loops of type V, the counterclockwise loop is oriented between 10 and 11 o'clock ( Fig. 1G-I). Four Peronia species are characterized by intestinal loops of type V: P. griffithsi, P. madagascariensis, P. platei, and P. setoensis.
Three independent sets of phylogenetic analyses were performed: 1) Maximum Likelihood and Bayesian analyses with concatenated mitochondrial COI and 16S sequences; 2) Maximum Parsimony analyses with concatenated nuclear ITS2 and 28S sequences; 3) Maximum Parsimony analyses with ITS2 haplotype sequences. Maximum Parsimony analyses were conducted in PAUP v 4.0 (Swofford 2002) with gaps coded as a fifth character state, and 100 bootstrap replicates conducted using a full heuristic search. Prior to Maximum Likelihood and Bayesian phylogenetic analyses, the best-fitting evolutionary model was selected for each locus separately using the Model Selection option from Topali v2.5 (Milne et al. 2004): a GTR + G model was independently selected for COI and 16S. Maximum Likelihood analyses were performed using PhyML (Guindon and Gascuel 2003) as implemented in Topali. Node support was evaluated using bootstrapping with 100 replicates. Bayesian analyses were performed using MrBayes v3.1.2 (Ronquist and Huelsenbeck 2003) as implemented in Topali, with five simultaneous runs of 1.5×10 6 generations each, sample frequency of 100, and burn in of 25% (and posterior probabilities were also calculated). Topali did not detect any issue with respect to convergence. All analyses were run several times and yielded the same result.
In addition, genetic distances between COI sequences were calculated in MEGA 7 as uncorrected p-distances. COI sequences were also translated into amino acid sequences in MEGA using the invertebrate mitochondrial genetic code to check for the presence of stop codons (no stop codon was found).

Molecular phylogenetic analyses
The monophyly of Peronia is strongly supported in all analyses except in the mitochondrial ML analyses (bootstrap of 58), which confirms that all onchidiid slugs with dorsal gills belong to the same clade (Figs 2-4).
Seven nodes of higher relationships among Peronia species are well supported. Supports are indicated here in parentheses in the following order: ML bootstrap in    mitochondrial analysis, Bayesian posterior probability in mitochondrial analysis, bootstrap in ITS2 analysis, bootstrap in ITS2 and 28S analysis (bootstrap values below 50% and posterior probabilities below 0.90 are replaced by a dash). Most basally, Peronia is always split in clades A and B. Clade A is strongly supported (99, 1.0, 100, 100) and includes P. peronii and P. okinawensis. Clade B is also strongly supported (99,1,93,99) and includes clade C and P. madagascariensis as its most basal species. Clade C, which is consistently recovered but moderately supported (-, -, 90, 87) 991 Singapore 1538, 1747, 2571, 2987, 5468, 5639, 5670, 1759, 3751, 3752, 5481 & 6202 Japan, Lombok, New Caledonia, PNG, Queensland, Sumatra, Vanuatu & Vietnam 2162, 698, 6212, 6214, 2682, 2856, 2870 The monophyly of each species recognized here is strongly supported in all analyses, except for the special case of P. sydneyensis (see below, species delineation). Within four species, some least-inclusive units are supported by the mitochondrial markers but not by comparative anatomy and nuclear markers (Figs 2-4): two units within P. peronii (one unit from Mauritius and the other from the West Pacific); two units within P. platei (one unit from Hawaii and the other from Papua New Guinea); two units within P. griffithsi (one unit from Mauritius and the other from Kei Islands and Papua New Guinea); and three units of P. verruculata from South-East Asia and the West Pacific (units #1, #2, and #3). Two least-inclusive mitochondrial units within P. verruculata from the western Indian Ocean (units #4 and #5) are also monophyletic in nuclear analyses (Figs 2-4) but are anatomically cryptic (see below). Note that populations of P. verruculata from the Red Sea are not represented in molecular analyses (see below, species delineation).
In mitochondrial analyses (Fig. 2), P. sydneyensis and P. willani form together the strongly supported clade E and the monophyly of each species is also strongly supported. In nuclear analyses (Figs 3, 4), they also form a strongly supported clade but P. sydneyensis is paraphyletic with respect to P. willani. Both species are close geographically (P. sydneyensis is distributed in New South Wales, Queensland and New Caledonia, and P. willani is distributed in the Northern Territory) and may be the result of a recent divergence. The paraphyly in nuclear analyses most likely is the result of incomplete lineage sorting (see below, species delineation).

Pairwise genetic divergences
Pairwise genetic distances were calculated for a total of 13 units (Fig. 5, Table 3): the five mitochondrial units within P. verruculata as well as the eight other species. A barcode gap is found in all cases, apart from the mitochondrial unit #1 of P. verruculata.

Comparative anatomy
All Peronia slugs are characterized by dorsal gills which are not found in other onchidiids. They are also all characterized by a unique combination of internal traits: they are the only onchidiid slugs with intestinal loops of type I or V, an accessory penial gland, and no rectal gland. The fact that any slug with this combination of traits belongs to a Peronia species is helpful to identify specimens with dorsal gills retracted inside the notum.
There are no external differences between Peronia species. In the field, it is not possible to reliably identify any of them, especially because sympatric species are often found together at the exact same sites. Individuals of very large size (longer than 100 mm) are only found in P. peronii, but smaller individuals are impossible to distinguish externally from other species. Also, tall papillae over the entire notum seem to be mostly found in P. peronii and P. madagascariensis, but that may be due to the fact that  Figure 5. Diagram to help visualize the data on pairwise genetic distances between COI sequences within and between species and mitochondrial units (P. verruculata) in Peronia (see Table 3). Ranges of minimum to maximum distances are indicated (in percentages). For instance, within P. willani, individual sequences are between 0 and 1.9% divergent; individual sequences between P. willani and the other species or units are minimally 4.3% and maximally 16.8% divergent. The colors are the same as those used in Figs 2-4, 6. slugs of both species are the largest, and it remains difficult to define exactly what a tall papilla is because papilla size is highly variable.
Internal differences help identify some species reliably, but not all (Table 4). Internal differences are almost exclusively based on combinations of traits because no Peronia species is characterized by any unique, distinctive feature, except for P. peronii (characterized by a spine of the accessory penial gland longer than 3 mm) and P. sydneyensis (characterized by strong protuberances on the spine of the accessory penial gland), and it remains difficult to identify Peronia species anatomically. For instance, where they overlap geographically (Queensland and New Caledonia), P. verruculata and P. sydneyensis can only be distinguished based on the length of the spine of the accessory penial gland, the presence of strong protuberances near the tip of the spine of the accessory penial gland, and the length of the penial hooks, which are all traits that are hardly accessible to a non-expert. However, only two Peronia species are cryptic externally and internally: P. setoensis and P. platei, which are not sister taxa (Figs 2-4) and do not overlap geographically, at least based upon current data (Fig. 6). Finally, the mitochondrial units of P. verruculata cannot be reliably distinguished anatomically.
Types of intestinal loops are useful for the identification of Peronia species (Fig. 1): species are characterized by intestinal loops of type V (P. griffithsi, P. madagascariensis, P. platei, and P. setoensis), type I with a transitional loop oriented between 12 and 3 o'clock (P. okinawensis and P. peronii), or type I with a transitional loop oriented between 3 and 6 o'clock (P. sydneyensis, P. verruculata, P. willani). Exceptions exist but are remarkably rare: only one individual in P. sydneyensis was found with a transitional loop slightly Table 3. Pairwise genetic distances between mitochondrial COI sequences in Peronia.
Ranges of minimum to maximum distances are indicated (in percentage).
For instance, the intra-specific divergences within P. madagascariensis are between 0 and 0.6%, while the inter-specific divergences between P. griffithsi and P. madagascariensis are between 9.3 and 11.3%.   Figure 6. Geographical distribution of the Peronia species A distribution of all Peronia species except for P. peronii B distribution of P. peronii. The colors are the same as those used in Figs 2-5. Colored areas correspond to hypothetical geographical ranges based on confirmed records only. Distinct colors are used for each unit of P. verruculata. The distribution of P. verruculata in the Indo-West Pacific is actually continuous. However, because it is unclear which units are present in regions from where we have no fresh material of P. verruculata (red areas), no unit of P. verruculata is shown there. For P. peronii, black dots correspond to material identified based on anatomical characters and blue dots correspond to material with DNA sequences. Details on species distribution can be found in each species description.
outside the range of that species (at 2 o'clock). Types of intestinal loops, however, can only be used in combination with other traits for the purpose of species identification.
The insertion of the retractor muscle of the penis is not very useful in identification because it mostly matches the distribution of the respective intestinal loop types (Table 4). An insertion near the heart is only found in the two species with intestinal loops of type I and transitional loops oriented between 12 and 3 o'clock (P. peronii and P. okinawensis). Within each species, all individuals share the same insertion of the retractor muscle (either near the heart or at the posterior end of the visceral cavity). However, in P. griffithsi, which is widely distributed from the West Pacific to Mauritius, individuals are characterized by both insertions. In P. peronii, the retractor muscle can exceptionally be vestigial (with no clear insertion).
The length of the muscular sac of the accessory penial gland varies depending on the size of animals, but it is useful to help identify some species. Indeed, only two species (P. peronii and P. willani) are characterized by a muscular sac which is longer than 20 mm (Table 4). The length of the spine of the accessory penial gland is helpful to distinguish closely related species which, otherwise, are very similar anatomically: P. peronii (at least 3 mm) and P. okinawensis (less than 2.3 mm); P. setoensis (more than 0.9 mm) and P. griffithsi (less than 0.62 mm); and P. sydneyensis (less than 1 mm) and P. willani (more than 1.5 mm). The diameter of the spine at its base can be used in exactly the same way. The length of penial hooks also differs between species: the longest hooks are found in P. madagascariensis (up to 100 μm), the shortest in P. setoensis and P. griffithsi (less than 25 μm).

Species delineation
The delineation of Peronia species is straightforward. They are all supported by independent data sets: they are reciprocally monophyletic with both mitochondrial and nuclear markers, and their monophyly is strongly supported; they are all separated by a large barcode gap; and they each are characterized by a unique combination of anatomical traits (with the exception of P. setoensis and P. platei, which are cryptic). Only two species need special attention: P. sydneyensis and P. verruculata.
The paraphyly of P. sydneyensis with respect to P. willani in nuclear analyses most likely is the result of incomplete lineage sorting, because lineage sorting progresses more rapidly for mitochondrial alleles than for nuclear alleles (Funk and Omland 2003). Also, P. sydneyensis and P. willani species are clearly distinct anatomically: in particular, P. sydneyensis is characterized by unique, strong protuberances near the tip of the spine of the accessory penial gland. Therefore, P. sydneyensis and P. willani are regarded as two recent but well-delineated species.
Despite some genetic structure, Peronia verruculata is regarded as a single species for various reasons. In mitochondrial analyses, P. verruculata is split in five leastinclusive mitochondrial units of which the relationships are basically unresolved due to low support (Fig. 2). In nuclear analyses, the mitochondrial units #1, #2, and #3 are not monophyletic (Figs 3, 4). Therefore, they should not be recognized as distinct taxa. In nuclear analyses, units #4 and #5 are monophyletic (Figs 3, 4). However, the mitochondrial units #1, #2, and #3 do not form a monophyletic group with respect to units #4 and #5. Recognizing mitochondrial units #4 and #5 each as a separate taxon would mean that mitochondrial units #1, #2, and #3 would also have to be recognized as separate taxa, which is unwarranted for the reasons given above.
All mitochondrial units of P. verruculata are cryptic anatomically (their anatomical traits display overlapping variation) while P. verruculata is clearly distinct from other Peronia species (Table 4). Finally, it would seem premature to recognize units #4 and #5 as independent lineages because our geographical sampling of P. verruculata is not continuous (Fig. 6). Future samples from southern India (including Sri Lanka) or the Arabian Sea (the coast of Yemen, Oman, Somalia) might show that the individuals of the western mitochondrial units #4 and #5 can still interbreed, exactly like units #1, #2, and #3. We therefore refrain from naming those five mitochondrial units within P. verruculata. They are merely regarded as mitochondrial units that indicate some genetic structure, but the current data do not suggest that they should be recognized as distinct taxa. Note that taxon names are already available for the mitochondrial units #1, #4, and #5 of P. verruculata (Table 1).
Finally, note that Peronia verruculata was described from the Red Sea, from which no fresh material could be obtained. However, the specimens examined from the Red Sea are anatomically indistinguishable from the specimens of the five mitochondrial units of P. verruculata (Table 4). Therefore, at this stage, there is no reason to think that the populations from the Red Sea belong to a distinct species and that the name P. verruculata cannot apply to the whole species from the Red Sea and South Africa all the way to the West Pacific (Japan, New Caledonia, and Queensland). At any rate, there are plenty of available names that can be used in the future if it were to be demonstrated that the populations from the Red Sea belong to a distinct species (see remarks on P. verruculata).

Species distribution
Geographic distribution is discussed in detail with each species description. The map of species distributions only illustrates the records that are regarded as correct (Fig. 6). Most of those correct records correspond to the specimens included in our molecular analyses. However, they also include types as well as historical museum specimens and records from the literature which could be positively identified using anatomical traits (e.g., intestinal loops, length of the spine of the accessory penial gland).
The secondary literature was read with great attention, especially in cases where it could provide geographical records not included in our material. Every record found in the literature is commented on (in the species remarks). Records from the literature are certainly not taken for granted because the secondary literature is plagued with two major issues. First, past authors did not always take the time to examine type specimens. For instance,  did not examine the types of Onchidium verruculatum and Onchidium peronii which are preserved at the Paris Museum (he did not list them in the material examined for these species), even though his study was almost exclusively based on material from that institution. Second, because there was no proper knowledge about intraspecific character variation, nobody knew which character could help distinguish species or not. For instance, Hoffmann's (1928: 73) record of Peronia verruculata from Hawaii was never questioned, but Peronia slugs from Hawaii are all characterized by intestinal loops of type V, which means that they cannot belong to P. verruculata (which is characterized by intestinal loops of type I).  Gistel, 1848: x. Paraperonia Labbé, 1934a: 196. Scaphis Labbé, 1934a: 203. Lessonia Labbé, 1934a: 213 [junior homonym of Lessonia Swainson, 1832, replaced by Lessonina Starobogatov, 1976. Quoya : 216. Lessonina Starobogatov, 1976: 211. Quoyella Starobogatov, 1976: 211 [unnecessary replacement name for Quoya .  (Baker 1938: 86).
Scaphis: Onchidium astridae , by subsequent designation (Starobogatov 1976 Eudrastus: Likely, although for unclear reasons, from the Greek εὖ, eu, for true, and δραστέoς, drasteos, a verbal adjective which means to be done. Paraperonia: From the Greek παρα, para, meaning beside, and Peronia. Scaphis: After the Greek ἡ σκᾰφίs, which means small boat (Labbé, 1934a: 202). Quoya: After the French naturalist Jean René Constant Quoy , a member of two circumnavigations from 1817 to 1820 with captain Freycinet and from 1826 to 1829 with captain Dumont d'Urville. Joseph Paul Gaimard [1793-1858] described several species of onchidiids based on their collections in the southern seas. Quoyella has the same etymology.
Lessonina: After the French naturalist René Primevère Lesson [1794Lesson [ -1849, a member of a circumnavigation from 1822 to 1825 with captain Duperrey. Lesson described several species of onchidiids based on his collections in the southern seas, such as the type species of Lessonina, Onchidium ferrugineum, which he collected in West Papua, Indonesia. Labbé's invalid name Lessonia was also dedicated to Lesson. Gender. Onchis: Masculine. Férussac did not specify the gender of Onchis which he did not combine with any specific name, and even the binomen Onchis peronii, which Férussac did not use per se, would not help in that respect. Because Onchis is derived from the masculine Greek noun ὁ ὂγκος, it is considered to be of masculine gender.
Peronia: Feminine. No gender was specified by Fleming, and the combination Peronia peronii does not help to determine it. Because no gender was originally specified or indicated and because Peronia ends in -a, it is treated as a name of feminine gender (ICZN 1999: Article 30.2.4). Indeed, Peronia mauritiana, an early combination used by Blainville (1824: 281), shows that Peronia has always been treated as a name of feminine gender.
Eudrastus: Masculine. No gender was originally specified or indicated. Eudrastus ends in a word derived from a word of variable gender (a verbal adjective) and should be treated as masculine (ICZN 1999: Article 30.1.4.2).
Paraperonia: Feminine. Gender of Peronia. Scaphis: Feminine. The gender was not specified by Labbé, but his original combinations S. atra, S. carbonaria, S. lata, and S. punctata indicate that he treated Scaphis as a name of feminine gender, which is correct since Scaphis is derived from the feminine Greek noun ἡ σκᾰφίs.
Quoya: Feminine. The gender was not specified by Labbé, but his original combination Q. indica indicates that he treated Quoya as a name of feminine gender, which is assumed to be the gender of Quoyella as well.
Lessonina: Feminine. The gender was not specified by Starobogatov, and no gender was specified for Lessonia by Labbé. Labbé's original combination Lessonia ferruginea indicates that he treated Lessonia as a name of feminine gender, which is assumed to be the gender of Lessonina as well.
Diagnosis. Body not flattened. Dorsal gills present. Dorsal eyes present. No retractable, central papilla present. Eyes at tip of short ocular tentacles. Male opening below right ocular tentacle and to its left. Foot wide. Pneumostome median, on ventral hyponotum. Intestinal loops of types I or V. Rectal gland absent. Accessory penial gland present, with muscular sac. Penis with hooks.
Remarks. Phylogenetic analyses show that all species of slugs with dorsal gills belong to the same clade (Figs 2-4). Seven generic names apply to that clade (excluding spelling mistakes, unjustified emendations, replaced names, and Peronia Blainville, 1824, a junior homonym of Peronia Fleming, 1822a). Note that the species name of a type species can be valid (such as Peronia peronii), synonymous (such as Onchidium tonganum, junior synonym of P. peronii), or even a nomen dubium (such as Quoya indica). Remarks on the nomenclatural history of the genus Peronia follow a chronological order. Cuvier (1804) described the first Peronia species as Onchidium peronii but did not mention the presence of dorsal gills. Nor did he illustrate them. He only described a mantle covered by small warts subdivided in even smaller warts. Dorsal gills are actually present on the dorsum of the type specimen of Onchidium peronii from Timor, but they are retracted, as most often seen in preserved specimens. Cuvier (1804: 41) also confessed that he would have believed O. peronii to be terrestrial, due to its pulmonary cavity "similar to that of reptiles", but that he regarded it as marine because Péron was certain to have collected it in seawater. But, Cuvier (1804: 41) Buchannan (1800) wrote that slugs live in Bengal on leaves of Typha reeds and are "very nearly allied" to Limax, suggesting that they are terrestrial, although he did not mention the presence of a pulmonary cavity and did not clearly state whether the slugs were terrestrial or not. At any rate, authors considered that Buchannan's (1800) O. typhae was not a marine species and Blainville (1817: 440) fig. 3.5) in the Description de l'Egypte for slugs from the Red Sea; for a collation, see Baring (1838) and Sherborn (1897). However, gills remained completely unnoticed because the explanation of Savigny's plate was published nearly ten years later by Audouin (1826: 18-20).
Onchis is not etymologically rigorous. The latinization of ὂγκος is oncos or oncus, as in the English word oncology. The Greek letter κ is "c" in Latin, while χ becomes "ch." That Férussac used onchis instead of oncos is not surprising, as naturalists often took liberties with the latinization of Greek words. A famous example being the word taxonomy, created as taxonomie by De Candolle (1813: 19) from the Greek words taxis (arrangement, order) and nomos (law, rule): taxis should have stayed as taxi-to form taxinomie, taxinomy, exactly like in the English word taxidermy (from taxis and dermis, skin). However, the Code does not require taxon names to be etymologically correct. Therefore, the intentional spelling change of Onchis to Oncus by Agassiz (1846: 259;1848: 748) is an unjustified emendation because Onchis is not the result of "inadvertent error, such as a lapsus calami or a copyist's or printer's error" (ICZN 1999: Article 32.5.1) and therefore Onchis must not be corrected. The emendation of Onchidium into Oncidium by Agassiz (1846: 259;1848: 748) also is unjustified for the same reason.
The generic name Peronia first appeared in two different venues, both published by Fleming (1822). One venue is Fleming's (1822a: 574) article "Mollusca" in the fifth volume (second part) of the Supplement to the fourth, fifth, and sixth editions of the Encyclopaedia Britannica published in May 1822 (as clearly indicated in a memorandum at the end of the sixth volume of the Supplement), even though the Supplement was only completed in 1824 (date on the title page). The other venue is Fleming's (1822b: 463) Philosophy of Zoology which, according to Feuer and Smith (1972: 55), was not published earlier than May 1822 but no later than June 1822. The mention of Peronia in the Supplement is considered here to be the earliest one because it was published in May 1822.
Peronia Fleming, 1822a is an objective junior synonym of Onchis Férussac, 1822, because Férussac's Onchis was published prior to Fleming's Peronia and both generic names share the same type species (Onchidium peronii). However, to the best of our knowledge, Onchis has only been used twice in a binomen, and both times before 1899: by Stimpson (1855) for Onchis fruticosa, a species name that has remained unnoticed until now, and by Mörch (1863) for Onchis (Peronella) armadilla Mörch, 1863, i.e., Onchidella armadilla (Mörch, 1863). Reversal of precedence applies here (ICZN 1999: Article 23.9). Onchis, the senior synonym, "has not been used as a valid name after 1899" (ICZN 1999: Article 23.9.1.1) and Peronia, the junior synonym, "has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years and encompassing a span of not less than 10 years." (ICZN 1999: Article 23.9.1.2) A chronological list of 25 works meeting the criteria of ICZN Article 23.9.1.2 is provided here, all of which mentioning Peronia, Peronia verruculata, or Peronia peronii as valid names: Marcus and Marcus (1970), Starobogatov (1976), , Biskupiak and Ireland (1985), Faulkner (1987), Pietra (1990), Arimoto et al. (1993), Davies-Coleman and Garson (1998) Xu et al. (2018). Onchis Férussac, 1822, objective senior synonym, is regarded as a nomen oblitum, and Peronia Fleming, 1822a, objective junior synonym, is regarded as a nomen protectum (ICZN 1999: Article 23.9.1.2). Fleming (1822a: 571, 574) classified Onchidium (with only the type species O. typhae) in a group of slugs that "reside constantly on the land," and transferred O. peronii to Peronia, a genus for marine slugs that have "their residence constantly in water" and look like Onchidium. However, Fleming (1822a: 574) expressed doubts that Peronia slugs are air-breathing, as Cuvier (1804) claimed in the original description of O. peronii: "This genus, which we have named in honor of M. Peron, was referred by Cuvier to the Onchidium of Buchanan (…) and the species termed O. Peronii. It was found creeping upon marine rocks, under water, at the Mauritius, by M. Peron. M. Cuvier conjectures that it breathes free air, and has accordingly inserted it among the Pulmones aquatique [Pulmonés aquatiques, i.e., aquatic pulmonates]. Some doubts, however, may reasonably be entertained about the truth of this supposition. It would certainly be an unexpected occurrence to find a marine gasteropodous mollusca obliged to come to the surface at intervals to respire. It will probably be found that it is truly branchiferous." It was Audouin (1826) who demonstrated later that both Cuvier and Fleming were correct because Peronia peronii can breathe through both its pulmonary cavity and dorsal gills. Blainville (1824: 280) created the generic name Peronia without being aware that Fleming (1822a, b) had already created exactly the same name two years before. Indeed, that Blainville (1824: 258) wrote "our genus Péronie" clearly suggests that he thought he was the author of Peronia. Also, most past authors attributed the authorship of Peronia to Blainville instead of Fleming (e.g., Stoliczka 1869: 100;Plate 1893: 102;Labbé 1934a: 189). Peronia Blainville, 1824 is a junior homonym of Peronia Fleming, 1822a and thus cannot be used as a valid name (ICZN 1999: Article 52.2). However, Peronia Blainville is also a junior objective synonym of Peronia Fleming, because they "both denote nominal taxa with name-bearing types whose own names are themselves objectively synonymous." (ICZN 1999: "objective synonym" in the glossary) Indeed, O. peronii, the type species of Peronia Fleming, and P. mauritiana, the type species of Peronia Blainville are objective synonyms because they share the same lectotype, i.e., the specimen from Mauritius which Cuvier (1804: pl. 6) illustrated (see below, the comments on the type material of O. peronii and P. mauritiana).
When he created the generic name Peronia, Blainville (1824: 280, 281) cited only one species name, Peronia mauritiana, a junior objective synonym of Onchidium peronii. Blainville (1824: 281) also claimed that he knew four or five other species of marine onchidiids from the southern hemisphere, without naming them, but Blainville (1826: 523) listed them two years later (Table 1): Peronia laevis, a junior objective synonym of Marmaronchis vaigiensis; Peronia semituberculata, a junior objective synonym of Onchidium planatum, itself a nomen dubium which may or may not refer to an onchidiid species; Peronia oniscoides, which all authors ignored except for Labbé (1934a: 243) and which clearly does not refer to a Peronia species (see general discussion). In addition, Blainville (1826: 523) also pointed out that Onchidium celticum, a name which Cuvier used for small marine slugs from the coast of Brittany, France, could also refer to a Peronia; Onchidium celticum remained a nomen nudum until 1832, when it was described by Audouin and Milne-Edwards (1832: 118).
Like Cuvier (1804), Férussac (1822), and Fleming (1822a, b), Blainville (1824) did not mention the existence of dorsal gills. Dorsal gills were first described by Audouin (1826: 18-20) in the explanation of a plate by Savigny (1817: pl. 2) from the Description de l'Egypte. Savigny's (1817: pl. 2, figs 3.1-3.8) plate displays eight drawings for two onchidiid slugs from the Red Sea, with one of them clearly representing a dorsal gill (Savigny 1817: pl. 2, fig. 3.5). According to Audouin (1826: 19), it was Cuvier himself who identified those two slugs as Onchidium peronii, although Cuvier (1830) later changed his mind and created the new name Onchidium verruculatum for them. More importantly, Audouin (1826: 19) described in great detail the "small vascular branches" at the posterior end of the dorsum, or "tubercles" that work as "true gills." And Audouin (1826: 19) even made this clever statement: "The Onchidie thus would have at the same time a pulmonary apparatus and a branchial apparatus; and that structure is in perfect agreement with what we know of the habits of that mollusk: Péron says that it is aquatic; on the contrary M. Cuvier, without the authority of this observer, would have believed it to be terrestrial. (...) We think that the Onchidie, at least the species illustrated here, enjoys the capacity to breathe under water thanks to the help of those ramified tubercles which cover the posterior end of its body, without the necessity of coming up to the surface; which is relatively difficult for an animal that slowly crawls at the bottom underwater. As for the pulmonary opening, it indicates that the onchidie breathes air as well; and we must suppose that several times in its life it finds itself in the condition to do so. " Audouin supposedly assumed that those slugs were truly aquatic.
Because Peronia was originally used as a genus for all marine onchidiids by both Fleming (1822a, b) and Blainville (1824Blainville ( , 1826, several Peronia species names already existed by 1830: Peronia mauritiana, P. peronii, P. oniscoides, P. semituberculata, and P. laevis (see above). Of those names, only the two objective synonyms P. mauritiana and P. peronii refer to true Peronia slugs, i.e., slugs with dorsal gills (Table 1). Cuvier (1830: 46) did not see the need for a genus assignment for marine onchidiid species and still only recognized Onchidium, but other naturalists started transferring species names from Onchidium to Peronia. Lesson (1833: pl. 19) transferred his own Onchidium ferrugineum Lesson, 1831a to Peronia, and clearly specified that he agreed with Blainville that marine onchidiids should be classified in a distinct genus. Dorsal gills are very clearly described by Lesson (1831a: 128-130;1831b: 300-302;1832: 36-37, fig. 32;1833: pl. 19) in O. ferrugineum, but they were not the reason why he transferred it from Onchidium to Peronia. Shortly after that, Oken (1834a) also transferred six Onchidium species names by Gaimard (1832-1833) to Peronia (P. cinerea, P. incisa, P. nigricans, P. patelloides, P. punctata, and P. tongana), with no justification but most likely because he also adopted the idea that marine onchidiids should not be classified in Onchidium.
The name Eudrastus was created by Gistel (1848: x), as a replacement name for "Peronia (Quoy, Isis 1834. 287.)." Gistel refers here to a report (Oken 1834a: 283-310) on Gaimard's (1832-1833) contribution to the Voyage de découvertes de l'Astrolabe published in Isis, the encyclopedic journal edited by Lorenz Oken from 1817 to 1848. This report was most likely written by Oken himself, as was often the case (Kertesz 1986), which would explain that the six onchidiid specific names mentioned (tongana, incisa, patelloides, nigricans, punctata, cinerea) are combined with Peronia instead of Onchidium, the generic name originally used by Gaimard (1832-1833). Regardless of who authored that Isis report, Gistel (1848) did create the new generic name Eudrastus for those six species. Baker (1938: 86) subsequently designated Onchidium tonganum Quoy & Gaimard, 1832 (Peronia tongana in Isis), as the type species of Eudrastus. Onchidium tonganum is regarded here as a junior subjective synonym of Peronia peronii, so Eudrastus is a junior subjective synonym of Peronia. Britton (1984: 182-183) suggested that Eudrastus should be regarded as a junior synonym of Peronia because it seemed to be based on "unimportant characters." John Edwards Gray (1847: 179) attributed the authorship of Peronia to Blainville (with an erroneous date of 1825) but, most importantly, gave its modern definition to Peronia by restricting it to six species of slugs with "radiating processes" on the back (JE Gray 1850: 117): P. alderi, P. ferruginea, P. mauritiana, P. peronii, P. punctata, and P. tongana. All those names refer to true Peronia slugs with dorsal gills. JE Gray (1850: 117) restricted Onchidium to Buchannan's O. typhae and included all the other marine species without dorsal gills in a new genus Onchidella. JE Gray's (1850) clarity only lasted for a few years. Indeed, Adams and Adams (1855: 234) pointed out that Peronia slugs differ from Onchidium and Onchidella because of "arbusculiform and other appendages of the mantle, which have sometimes been mistaken for gills." Because they did not believe that gills were distinct from other dorsal papillae, Adams and Adams (1855: 234) classified in Peronia some names that belong to both Peronia (P. ferruginea, P. mauritiana, P. peronii, P. punctata, P. tongana) and to Onchidella (O. celtica, O. indolens, O. marginata, and O. parthenopeia). JE Gray's (1850) classification was adopted by Keferstein (1865a) but, until  work, all authors have ignored the genus Peronia and simply used the genus Onchidium for slugs with and without dorsal gills (Stoliczka 1869;Semper 1880Semper -1885Plate 1893;Bretnall 1919;. Stoliczka (1869: 100-102), who was the first one to re-examine live slugs of O. typhae since Buchannan (1800), firmly argued that slugs with "dorsal tufts" were anatomically so similar to Onchidium and Onchidella that only one name, Onchidium, was needed. Stoliczka (1869: 98) also clarified that O. typhae is not a terrestrial species but that, instead, it lives in "damp places, generally close to tanks or ditches, especially those which are supplied during high tide with brackish water." Stoliczka's (1869) strong influence can be seen in Semper's (1880Semper's ( -1885 study of the onchidiids from the Philippines (and other parts of the Indo-West Pacific) in which all onchidiids are in Onchidium, with the exception of a single species in his new genus Onchidina Semper, 1882; for a collation of Semper's work, see Johnson (1969). Plate (1893) adopted a classification with five genera, but the four species of slugs with dorsal gills recognized by Plate are classified in Onchidium with thirteen species of slugs without dorsal gills. Hoffmann (1928) adopted a classification with six genera, six species of slugs with dorsal gills being classified in Onchidium with 34 species without dorsal gills.
Then, suddenly, in 1934, the number of onchidiid taxon names for slugs with dorsal gills dramatically increased. Based on the onchidiid collection at the Paris Museum,  created fourteen new species-group names for slugs with dorsal gills (all but one name are species names) and four new generic names: Lessonia (later replaced by Lessonina), Paraperonia, Quoya, and Scaphis. Below, the nomenclatural status of Labbé's generic names is justified first (they all are junior synonyms of Peronia), followed by opinions in the secondary literature.
The generic name Paraperonia was created by Labbé (1934a: 196) for four species similar to Peronia but with intestinal loops of type V (instead of type I). The type species is Paraperonia gondwanae , by subsequent designation (Starobogatov 1976: 211). Labbé's description of Paraperonia gondwanae was based on 38 individuals with intestinal loops of types I and V which belong to different species. The application of the name P. gondwanae is clarified through the designation of a lectotype (see P. verruculata): Paraperonia gondwanae is a junior synonym of Peronia verruculata, and Paraperonia is a junior synonym of Peronia.
The generic name Scaphis was created by Labbé (1934a: 203) for nine species similar to Peronia but supposedly with an oblique, almost vertical hyponotum. The type species is Onchidium astridae , by subsequent designation (Starobogatov 1976: 211). Onchidium astridae is a junior synonym of Peronia verruculata, and Scaphis is a junior synonym of Peronia. Lessonia Labbé, 1934a is objectively invalid because it is the junior homonym of Lessonia Swainson, 1832 [Aves]. Starobogatov (1976: 211) replaced it by Lessonina. Labbé (1934a: 213-216, figs 48-50) described Lessonia based on a single species, Onchidium ferrugineum Lesson, 1831a, of which he examined no other material than the four syntypes (MNHN-IM-2000-22951). The examination of the three remaining syntypes (one syntype was lost by or after Labbé) revealed that the lectotype (Goulding et al. 2018b: 75) belongs to a Peronia species and that the two paralectotypes belong to Wallaconchis ater (Lesson, 1831a). Both Lesson's original description of Onchidium ferrugineum and Labbé's re-description of Lessonina ferruginea are a confusing combination of traits that characterize species from two distinct genera. For instance, the dorsal gills mentioned by both authors, are characteristic of Peronia, while the absence of an accessory penial gland mentioned by Labbé (even though there is a penial gland in the lectotype) is characteristic of Wallaconchis. Thanks to the designation of a lectotype with dorsal gills, the name Onchidium ferrugineum clearly applies to a Peronia species and Lessonina becomes a junior synonym of Peronia. Starobogatov (1976: 211) created Quoyella as a replacement name of Quoya , which he treated as a junior homonym of "Quoya Deshayes, 1843" [Mollusca, Gastropoda, Planaxidae]. In the second edition of Lamarck's Histoire naturelle des animaux sans vertèbres, Deshayes indicates that he originally thought of creating a new genus Quoya but that, after all, he decided not to (Deshayes and Milne-Edwards 1845: 236). Deshayes still used the binomen "Planaxis decollata Quoy" (Deshayes and Milne-Edwards 1845: 238). However, in the Explication des planches of his Traité élémentaire de conchyliologie, Deshayes (1853: 50) used Quoya for two valid species names: Quoya decollata and Quoya grateloupi. Regardless, according to Gray (1847: 138), the generic name Quoya by Deshayes is an incorrect subsequent spelling of his Quoyia JE Gray, 1839. As an incorrect subsequent spelling, Quoya Deshayes is not available (ICZN 1999: Article 33.3) and, as a result, Quoyella is an unnecessary replacement name. Ironically, Gray (1847: 138) indicated that he originally found the generic name Quoyia in a manuscript by Deshayes in 1830 ("Quoyia, Desh. MSS. 1830; Gray, 1839 (...) Quoya, Desh. 1843"). According to Baker (1938: 87), Quoya Agassiz, 1862 [Coelenterata] is another homonym of Quoya Labbé, 1834a. However, the spelling of that generic name is not Quoya but Quoyia (Agassiz, 1862: 173). So, Quoyia Agassiz, 1862 is a junior homonym of Quoyia Gray, 1839, but Quoya Labbé, 1934a is not a junior homonym of Quoyia. Quoya indica , type species of Quoya by monotypy, is regarded here as a nomen dubium even though it applies to a species with dorsal gills and thus belongs to Peronia (see general discussion).
Nothing is ever simple in onchidiid taxonomy. Indeed, Labbé (1935a, b) also described what he called "microgills" in Elophilus Labbé, 1935a, a name preoccupied by Elophilus Meigen, 1803 (Diptera) and replaced by Labbella Starobogatov, 1970. Labbé's (1935a microgills consolidated the old idea of a gradual continuum between regular dorsal papillae and dorsal gills. So, for instance, Marcus and Marcus (1960: 875) argued that one cannot say for sure whether a papilla is a dorsal gill or not. However, Dayrat et al. (2016Dayrat et al. ( , 2019d demonstrated that there are no gills at all (not even microgills) on the notum of the type material of the type species of Labbella which actually belongs to Onchidium stuxbergi (Westerlund, 1883). Labbella is a junior synonym of Onchidium. Contrary to regular papillae, dorsal gills are distinctively branched, which is striking if specimens are fully relaxed before preservation but otherwise difficult to see. Finally, note that Labbé (1935b: 320) claimed that he observed rudimentary eyes on dorsal gills, which, to our knowledge, has never been confirmed. Labbé (1934a: 187, 188) rightly recognized the importance of dorsal gills for classification and he separated all five genera of slugs with dorsal gills from all other onchidiids. According to Labbé, onchidiids deserved their own order, the Silicodermatae, composed of two suborders: Dendrobranchiatae (onchidiids with dorsal gills) and Abranchiatae (onchidiids without dorsal gills). Our phylogenetic analyses clearly demonstrate that all species of slugs with dorsal gills belong to a single clade, and that only one generic name (Peronia) is necessary (Figs 2-4). However, the species of slugs with no dorsal gills do not form a natural group (Figs 2-4). In other words, the absence of dorsal gills is a plesiomorphic trait for the onchidiids and the presence of dorsal gills is a synapomorphy for the genus Peronia. Labbé's (1934a: 187) distinction between the tribes Peroniidae (Peronia and Paraperonia) and Scaphidae (Scaphis, Lessonina, Quoya) based on the orientation of the hyponotum (horizontal versus oblique) is meaningless. This trait obviously varies depending on preservation, and Labbé exclusively studied preserved material from the collections of the MNHN without access to live animals. Labbé's (1934a: 187) distinction between Peronia and Paraperonia based on the intestinal types (type I in Peronia and type V in Paraperonia) is unwarranted because Peronia species with intestinal loops of type V are not more closely-related to each other (Table 4, Figs 2-4). Also, Labbé often made mistakes with respect to intestinal types: for instance, the type material of Paraperonia gondwanae includes individuals with loops of both types I and V, even though Labbé described it as a species with loops of type V. Labbé asserted that the position of the pneumostome and the size of the muscular sac differ between Peronia and Paraperonia. However, the position of the pneumostome varies between individuals and is not consistently on the right side of the median axis in species he classified as Paraperonia. Labbé's (1934a: 187) distinction between Scaphis, Quoya, and Lessonina, is also unwarranted. Again, the position of the pneumostome (on the right of a median line in Scaphis according to Labbé) varies between individuals.  re-description of Lessonina ferruginea (the type species of Lessonina, by monotypy) was based on individuals of two different species (see above). The male opening of the lectotype, which bears dorsal gills, is on the left of the right ocular tentacle, exactly as in all Peronia species, while the male opening of the two paralectotypes, which belong to Wallaconchis ater, is under the right ocular tentacle (Goulding et al. 2018: 75). Labbé (1934a: 216, fig. 51) described a double male opening in Quoya indica (the openings of the penis and of the accessory penial gland being supposedly separated), but this could not be confirmed in the type material. Regardless, male openings occasionally appear separated due to preservation (when the vestibule is everted) and that is by no means a trait of generic value.
Authors completely rejected  idea that the presence or absence of dorsal gills could be of any use in onchidiid classification (e.g., Marcus and Marcus 1960;Starobogatov 1976). Britton (1984: 180) even asserted that "the division of the group into two subordinate taxa based on this character is no longer admissible." As for the status of Labbé's ( , 1935a generic names for slugs with dorsal gills, authors were not in agreement. Marcus and Marcus (1970: 213) regarded Peronia and Paraperonia "at most as subgenera." Starobogatov (1976) regarded all names as valid: Lessonina, Paraperonia, Peronia, Quoyella (unnecessary replacement name for Quoya), Scaphis, and Labbella (supposedly with micro-gills). Britton (1984: 182-183) suggested that Paraperonia, Eudrastus and Scaphis should be regarded as junior synonyms of Peronia because they seemed to be based on "unimportant characters," but treated Labbella (supposedly with micro-gills), Lessonina, and Quoyella (for Quoya) as valid. In a recent review of the application of onchidiid generic names, Dayrat et al. ( : 1861 made it clear that all slugs with dorsal gills belong to one clade and that Eudrastus, Lessonina, Onchis, Paraperonia, Peronia, Quoyella (for Quoya), and Scaphis all refer to that clade. Note that the application of Lessonina was fully clarified when a lectotype was designated for its type species Onchidium ferrugineum (Goulding et al. 2018: 75). ( Labbé, 1934a: 197-198 Cuvier's (1804: pl. 6) illustrations are truly remarkable, they are flipped at 180° because, for instance, the heart and the male anterior parts are on the left. Something must have happened during the engraving or the printing. Hoffmann (1928: 71) referred to Mauritius as the "Typ-Lokalität" of Onchidium peronii but did not formally designate a lectotype for O. peronii. In case of syntypes, "the place of origin of the lectotype becomes the type locality of the nominal species-group taxon, despite any previously published statement of the type locality." (ICZN 1999: Article 76.

Peronia peronii
2) The original description of Onchidium peronii was based on two specimens collected by Péron: the lectotype from Mauritius, of which the internal anatomy was illustrated in detail by Cuvier (1804: pl. 6), could not be located and is likely lost; the paralectotype from Timor (MNHN-IM-2000-22938) was very briefly mentioned by Cuvier (1804: 39) who merely wrote that another specimen was brought from Timor by Péron and that Onchidium peronii is present "at the two extreme ends of the Indian Ocean." The paralectotype (60/40 mm) is well preserved even though dorsal papillae with eyes cannot be counted because their color faded. It is obvious that Cuvier did not actually use it for his detailed anatomical description and illustrations on plate 6, because it was never opened prior to the present study, except for a tiny cut near the lung. It was carefully opened on its side to draw a dorsal view of its intestinal loops of type I (Fig. 9A) and measure the length (4.5 mm) of the spine of the accessory penial gland (by transparency, so that the male copulatory apparatus was not dissected).
Blainville also mentioned the name Peronia mauritiana in his Manuel de Malacologie et de Conchyliologie (Blainville 1825: 490) and in the article "Péronie" of the Dictionnaire des Sciences Naturelles (Blainville 1826: 523). The illustration published by Blainville (1827: pl. 46, fig. 7) in the Atlas of the Manuel differs from that published by Cuvier (1804: pl. 6, fig. 1). The specimen used by Blainville for that illustration could not be located, which does not matter much since it does not have any name-bearing function. However, it also means that, because there are two species of Peronia in Mauritius, Blainville's (1827: pl. 46  It is unclear how many specimens Quoy and Gaimard (1832: 210-211, pl. 15, figs 17, 18) examined for the original description of Onchidium tonganum. They may have examined more than one individual. Regardless, it is clear that Onchidium tonganum applies to a Peronia species because the notum of the lectotype bears gills which were also illustrated in the original description. Its notum also bears fifteen dorsal papillae with eyes but others probably faded. The lectotype was dissected prior to the present study. The accessory penial gland and the penial apparatus are missing (pieces of the deferent duct remain). The intestinal loops are of type I with a transitional loop between 2 and 3 o'clock ( Fig. 9B). Quoy and Gaimard (1832: 216) briefly mentioned the presence of O. tonganum in Manokwari, West Papua, Indonesia, but that record could not be confirmed (although P. peronii is known to be present there because Manokwari is the type locality of O. punctatum).
Lectotype and paralectotypes (Onchidium punctatum). Indonesia • lectotype, hereby designated, 70/60 mm; dans le port de Dorey " That old label does not say "Dorey" (for the locality), which is only mentioned in the original description, but it clearly indicates that the lectotype was part of the type series of Onchidium punctatum. The lectotype bears dorsal gills, as illustrated by Quoy and Gaimard (1832: pl. 15, figs 27, 28). It was dissected prior to the present study, likely by Labbé (1934a: 203-204) and its penis is missing but its intestinal loops are of type I with a transitional loop at 3 o'clock ( Fig. 9C). Its spine of the accessory penial gland, still in place in the animal, is 3.7 mm long.
A second jar was found with two paralectotypes (MNHN-IM-2000-33701). An old label for that second jar says "Onchidium piquetée, Q G. MM Quoy Gaimard, 1829" with no locality data. The name "Peronia" was added on the label. The number "51" also appears on another old label, which corresponds to an unknown numbering system. There also is a more recent label saying "Peronia picta QG, M. Quoy et Gaimard, 1829." Quoy and Gaimard did not describe any onchidiid species with the specific name picta. However, the French vernacular name of Onchidium punctatum in Quoy and Gaimard's (1832: 215) original description is "Onchidium piquetée." So, it is likely that these two additional specimens were part of the type series of Onchidium punctatum. Both paralectotypes (35/25 and 32/30 mm) bear dorsal gills. The largest paralectotype was dissected prior to the present study, possibly by Labbé (1934a: 203, 204), and its penis is missing but its accessory penial gland remains. The small paralectotype was not dissected. Labbé (1934a: 203) listed three individuals from Port-Dorey which he (implicitly) regarded as part of the original series of Onchidium punctatum. Labbé gave the measurements for only two individuals: "a" (35/25 mm), likely the largest paralectotype; "b" (77/56 mm), likely the lectotype. In addition, in his redescription of Scaphis punctata, Labbé (1934a: 204-205) mentioned two individuals identified as Peronia and collected by Quoy and Gaimard in 1829, from an unknown locality. Those two individuals are likely within another jar found at the MNHN with the old number "48" and a label saying "Peronia M. Quoy et Gaimard 1829." There is no reason to consider that those two unidentified individuals from the collection  (Goulding et al. 2018: 63), and one specimen collected by Raffray in 1878 (with numbers "22" and "75" on the label).
Lectotype and paralectotypes (Paraperonia fidjiensis). Fiji • lectotype, hereby designated, 60/50 mm; 1876; Filhol leg.; MNHN-IM-2000-33692. No jar clearly labeled as the type material of Paraperonia fidjiensis was found at the MNHN, but the lectotype could be traced, and six paralectotypes could not be found at the MNHN. Labbé (1934a: 197-198, figs 9-11) described Paraperonia fidjiensis based on seven individuals from Fiji ("Iles Fidji") collected by Filhol (Henri Filhol [1843-1902) in 1876 and with the following sizes: 75/50 mm for six "a" individuals and 70/50 mm for a seventh "b" individual. Two jars of material collected in Fiji by Filhol in 1876 were found at the MNHN. The first jar, labeled as "Peronia [written over Oncidium] I. Fidji M r . Filhol n°11 1876" and "71," contains a single Peronia specimen which, given its size (60/50 mm), very likely is part of the type series of P. fidjiensis, and which is designated as the lectotype (MNHN-IM-2000-33692). Its radula and all reproductive parts are missing. Its intestinal loops are clearly of type I, with a transitional loop at ~ 1 o'clock ( Fig. 9E). The second jar, labeled as "Oncidiella I. Fidji M r . Filhol n°11 1876" and "101," contains four poorly-preserved specimens which do not even appear to belong to Peronia, with a size (less than 30 mm) not compatible with the original description of P. fidjiensis, and which, therefore, cannot be regarded as part of the type series. Additional material examined (historical museum collections). Chagos Archipelago • 1 specimen 95/65 mm; Ye Ye, Peros Banhos atoll; 24 Feb 1996; M Spalding (from N Yonow's personal collection) leg.; exposed on shallow reef flat on rocks; MNHN-IM-2014-7992.
Fiji    GenBank sequence. One COI sequence was obtained from GenBank (LC390402) for an individual identified as Peronia sp. and collected from Okinawa, Japan (Takagi et al. 2019), which is the northernmost confirmed locality for Peronia peronii. Distribution (Fig. 6). Given that our fresh molecular samples of P. peronii from the West Pacific (Guam, Papua New Guinea) are conspecific with those from Mauritius, it is assumed here that all individuals with a long spine of the accessory penial gland belong to the same species. Strictly speaking, however, the presence of P. peronii from places like Zanzibar, the Maldives, Nicobar Islands, West Papua, Timor, Palau, New Caledonia, and Tonga, would still need to be validated with fresh material.
Interestingly, but for unclear reasons, Peronia peronii seems to be only recorded from relatively small islands, the largest ones being Timor, New Caledonia, and Fiji. Even in Papua New Guinea, it was found on small islands close to the mainland but not on the mainland. Peronia peronii seems to be transported across vast distances from the western Pacific Ocean to the western Indian Ocean, but which does not seem to settle on the coasts of large land masses. We did not find it in any of the many localities we visited in the Philippines, Vietnam, Malaysia, Borneo, Sulawesi, Halmahera, Sumatra, etc. It is possible that we occasionally missed it in a few places (obviously we missed it in Timor and New Caledonia where it is present), but it is unlikely that we missed it everywhere.
Habitat (Fig. 7). Live slugs of Peronia peronii are found in the rocky intertidal, like most other Peronia slugs. Many of our specimens were collected at night or just before sunrise, suggesting that P. peronii is, at least partly, a nocturnal species. This could explain why we missed it at some localities where we only collected during the day. Peronia peronii is not rare, but it is definitely not as common as some other species. The fact that collecting it at night seems necessary, at least in some localities, might explain why collections of P. peronii are not as abundant as collections of P. verruculata.
Color and morphology of live animals (Fig. 8). No picture of live animals was available for individuals from the West Pacific (Guam and Papua New Guinea). The description of the color of live animals is based on the Mauritius individuals. The dorsal notum is brown, with a greenish hue, light to dark, mottled with darker and lighter areas. The color of the dorsal papillae varies a s that of the background itself. The ventral surface (foot and hyponotum) is yellowish-greenish and can change rapidly in any given individual. The ocular tentacles are brown-grey, like the head. The dorsal notum of live animals is covered by dozens of papillae of various sizes. Dorsal papillae can be particularly tall (easily up to 4 mm), even in preserved specimens, and are evenly distributed over the entire notum. Preserved, they are difficult to distinguish from retracted dorsal gills in the posterior half of the notum. Some papillae bear black dorsal eyes at their tip. The number of papillae with dorsal eyes is variable (15)(16)(17)(18)(19)(20). The longest animals are 140 mm long in Mauritius and 115 mm long in the West Pacific.
Digestive system (Figs 9-12). Examples of radular formulae are presented in Table 5. The median cusp of the rachidian teeth is approximately 75 μm long. The hook of the lateral teeth is approximately 160-200 μm long. The intestinal loops are of type I, with a transitional loop oriented between 12 to 3 o'clock.
Reproductive system (Figs 13-16). In the anterior (male) parts, the muscular sac of the accessory penial gland is at least 30 mm long in specimens from Mauritius and at least 25 mm long in specimens from the West Pacific (Guam & Papua New Guinea). Note that, in some additional museum specimens, the muscular sac was only 20 mm long, and, even exceptionally 17 mm long (see remarks below). The hollow spine of MNHN-IM-2013-12500) in the West Pacific (Guam and Papua New Guinea). Its diameter at the conical base is approximately 400 μm in specimens from Mauritius and between 400 and 500 μm in specimens from the West Pacific (Guam and Papua New Guinea). Its diameter at the tip measures 160-170 μm in specimens from the West Pacific, and from 180 to 200 μm in specimens from Mauritius. Note that, in some additional museum specimens, the spine was only 3 mm long (see remarks below). The retractor muscle is shorter or longer than the penial sheath and inserts near the heart. Exceptionally, the retractor muscle can even be vestigial ([5472] MNHN-IM-2013-14052). Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 50 μm long.
Diagnostic features (Table 4). Peronia peronii is the only Peronia species which is easy to identify anatomically. Indeed, it is characterized by a very long spine (at least 3 mm) of the accessory penial gland, which is distinctive and easily accessible (one just needs to pull on the flagellum of the penial gland or, even, in some cases, measure the spine by transparency). The two longest spines were found in the lectotype of P. fidjiensis (MNHN-IM-2000-33692) from Fiji (5 mm), and in an old historical specimen (ANSP 304860) from the Maldives (4.8 mm).
Peronia peronii is additionally characterized by a unique combination of anatomical traits: muscular sac longer than 20 mm, intestinal loops of type I (with a transitional loop oriented between 12 and 3 o'clock), retractor muscle inserting near the heart. Also, no individual larger than 80 mm was found in any other Peronia species so far. Animal size can be useful when several Peronia species are found at the same site. For instance, the two individuals of P. verruculata (unit #1) found at the station PM 12 (near Madang, Papua New Guinea) are 35 and 38 mm long while the individual of P. peronii from the same station is 80 mm long. The type I of its intestinal loops (with a transitional loop oriented between 12 and 3 o'clock) is only shared by P. okinawensis, a species endemic to Japan with which it is most closely related.
Remarks. Synonymies. There is no doubt that Cuvier's (1804) Onchidium peronii applies to the species described here, just based on animal size alone. According to Cu- vier, the lectotype from Mauritius measured approximately 140 mm long and our molecular data show that all individuals of that size from Mauritius belong to a single species (Table 4). Cuvier's (1804: pl. 6) detailed anatomical description and drawings are exclusively based on the lectotype (he did not dissect the paralectotype from Timor). Cuvier (1804: 48, pl. 6, fig. 8) described the spine of the accessory penial gland as a "very sharp, brown spike" but unfortunately did not provide its length. However, Cuvier's (1804: pl. 6, fig. 4) illustration of the intestinal loops is identical to some of our Mauritius individuals here: intestinal loops of type I with a transitional loop at 3 o'clock. The paralectotype of Onchidium peronii from Timor (MNHN-IM-2000-22938) is only briefly mentioned by Cuvier in the original description. The length (4.5 mm) of the spine of its accessory penial gland (checked for the present study) indicates that it also belongs to P. peronii. Its intestinal loops are also identical to those of the lectotype (Fig. 9A).
Peronia mauritiana is a junior objective synonym of Onchidium peronii because they share the same name-bearing type.
The lectotype was dissected prior to the present study and most of the male copulatory parts are missing (only the deferent duct remains). As a result, the length of the spine of the accessory penial gland, which is diagnostic of P. peronii, cannot be checked. Labbé (1934a: 191) listed the lectotype in the material he examined for his re-description of P. tongana, but he did not point out that it was part of the type material of O. tonganum and he did not describe it anatomically. It is possible but not certain that Labbé dissected the lectotype. At any rate, its intestinal loops are of type I with a transitional loop between 2 and 3 o'clock ( Fig. 9B). Both the length (100 mm) of the lectotype as well as its intestinal loops indicate that Peronia tongana is a junior synonym of P. peronii (Table 4). Onchidium punctatum is regarded here as a junior synonym of P. peronii because the length (3.7 mm) of the spine of the accessory penial gland of the lectotype (MNHN-IM-2000-22966) is only compatible with P. peronii (Table 4). The length of the lectotype (70 mm, preserved) is also far more compatible with P. peronii than with P. verruculata, another species found in West Papua. Our many individuals of P. verruculata are all less than 60 mm long (alive), except a single individual from New Caledonia (73 mm alive). Given their small size, the two paralectotypes (MNHN-IM-2000-33701) likely belong to P. verruculata (unit #1) instead of P. peronii, which would not be surprising at all because the type locality of O. ferrugineum (a junior synonym of P. verruculata) is the same as that of O. punctatum (Manokwari, West Papua, Indonesia). At the end of the description of O. punctatum, Quoy & Gaimard (1832: 216) also mention in passing that they also found Onchidium tonganum in Port Dorey (i.e., Manokwari, West Papua, Indonesia) and they even point out that local inhabitants know how to distinguish both species. Both O. punctatum and O. tonganum are regarded here as junior synonyms of P. peronii. However, it remains true that there are two sympatric Peronia species in West Papua, P. verruculata and P. peronii, which can be distinguished in the field based on animal length (except, of course, for individuals measuring less than 60 mm long).
Bergh (1884a: 129-142, pl. IV, figs 25-27, pl. V, figs 1-27, pl. VI, figs 5-18, 20, 21) described Onchidium melanopneumon from a single individual (65/40 mm) from Fiji. This specimen was completely dissected by Bergh and is now empty (NHMUK 1888.5.30.39). Onchidium melanopneumon applies to a Peronia species due to the presence of dorsal gills, and the length (4 mm) of the spine of the accessory penial gland indicates that it applies to P. peronii (Table 4). Its intestinal loops (Bergh 1884a: pl. V, fig. 27) are also similar to those found in P. peronii, although the transitional loop is slightly past the 3 o'clock limit. As a result, O. melanopneumon is regarded as a junior synonym of P. peronii. Bergh (1884b: 263;1885: 176) briefly mentioned again O. melanopneumon in a comparative study on the affinities of onchidiids. Labbé (1934a: 197-198, figs 9-11) described Paraperonia fidjiensis based on seven individuals from Fiji, one of which could be found and is designated as the lectotype (MNHN-IM-2000-33692). Because all reproductive parts are missing, the length of the spine of the accessory penial gland cannot be checked. However, according to Labbé (1934a: 197, fig. 10), the spine of the accessory penial gland is 5 mm long, which is only compatible with P. peronii (Table 4), and is the longest spine known in P. peronii. The intestinal loops of the lectotype of P. fidjiensis are clearly of type I, with a transitional loop oriented at ~ 1 o'clock ( Fig. 9E), even though Labbé (1934a: 197) erroneously described them a type V, which is a mistake he often made. Given the length of the lectotype (60 mm) and, most importantly, the length of the spine of the accessory penial gland, P. fidjiensis is regarded as a junior synonym of P. In the seventh volume of the second edition of Lamarck's Histoire naturelle des animaux sans vertèbres, which was revised by Deshayes and Milne-Edwards (1836), P. mauritiana is proposed as a synonym of Onchidium peronii. However, as a reference for P. mauritiana, the authors mentioned the illustration published by Blainville (1827: pl. 46, fig. 7) in the Atlas of his Manuel which differs from that published by Cuvier (1804: pl. 6, fig. 1) and may or may not refer to Peronia mauritiana.
Adams and Adams (1855: 235) merely listed Peronia mauritiana, P. peronii, P. punctata, and P. tongana as Peronia species names. Note that for P. peronii, they refer to Savigny's illustrations of individuals from the Red Sea misidentified as P. peronii by Audouin instead of Cuvier's original description of P. peronii, which means that Adams and Adams refer to P. verruculata instead of P. peronii (see remarks on P. verruculata). Adams and Adams (1855: pl. LXXXI, fig. 3) also reproduced the original illustration of O. tonganum by Quoy and Gaimard (1832: pl. 15, fig. 17 fig. 2) mentioned by Berge was actually published after Cuvier's death in the Disciples' edition of the Règne Animal which was accompanied by beautiful illustrations. According to , the authorship for the mollusks should be attributed to Deshayes (1836Deshayes ( -1845  The record of Onchidium peronii from Natal, South Africa (Krauss 1848: 72) likely is a record of P. madagascariensis, the only Peronia species known in South Africa so far (see remarks on P. madagascariensis). However, P. verruculata (unit #5) could also be present in northeastern South Africa because its southernmost known locality is in Maputo, Mozambique (ca. 26°S). This record by Krauss was mentioned again by a few authors (Sturany 1898: 73;Collinge 1910: 171;Connolly 1912: 224-225;Connolly 1939: 454).
The records of Onchidium peronii from Mozambique by Martens (1879: 735) in Ibo Island (ca. 12°21'S) and Inhambane (ca. 23°52'S) are within the geographical range of both P. verruculata (unit #5) and P. madagascariensis (Fig. 6). It is not possible to know to what species Martens was referring; this record by Martens was mentioned twice by Connolly (1912: 225;1939: 454). Semper (1880: 258-260, pl. XIX, figs 2, 9, pl. XXII, figs 1, 2, 10) referred to huge onchidiid slugs (from 50 to 105 mm, preserved) as Quoy and Gaimard's (1832) Onchidium tonganum and merely suggested, with a question mark, that O. peronii could refer to the same species. Semper (1880: 258) listed five geographical records for O. tonganum: Tonga and West Papua (as Port Dorey), from Quoy and Gaimard (1832); Mauritius, based on some material from the Vienna and Kiel museums; Samoa, based on some material from the Museum Godeffroy; and Bohol, Philippines, based on his own collections. Semper (1880: 258) indicated that the specimens he examined were from 50 to 105 mm long, preserved, and that the smallest individual was found in Mauritius. His anatomical description perfectly matches the anatomy of P. peronii. In particular, a spine of an accessory penial gland measuring 4 mm long is only compatible with P. peronii (Table 4). However, he did not clearly indicate whether he observed a long spine in every specimen. It cannot be excluded that he measured the length of the accessory penial gland spine only in a specimen from Mauritius. Therefore, the records of P. peronii in Bohol and in Samoa are regarded here as questionable, even though it is very possible that P. peronii lives in Samoa, given that it is so close to Tonga (800 km) and Fiji (1100 km). Semper (1882: 290) thought that Cuvier's (1804) original description of P. peronii was problematic because his drawing of the dorsal notum did not match the internal anatomy. Because Semper was convinced that Cuvier used specimens that did not belong to the same species, he thought that the name P. peronii should not be used. Plate (1893: 172-173) disagreed with Semper even without examining the type material of P. peronii. It is demonstrated here that the two type specimens described by Cuvier as P. peronii both belong to the same species (see above). Semper (1882: 268) was undecided about the nomenclatural status of what he called Onchidium mauritianum (then a new combination), which he listed as one of the names for which a "closer inspection of the originals" was needed. Like most authors, he cited Blainville's (1827: pl. 46, fig. 7) illustration (which is not part of Blainville's original description) as a reference without realizing that it may or may not correspond to P. mauritiana, a junior objective synonym of P. peronii. Semper (1882: 289) was also undecided about the status of Onchidium punctatum, for which he erroneously thought that the type locality was unknown. He suggested that it might refer to the same species as Onchidium tumidum, which is not possible because O. tumidum was recently transferred to Paromoionchis (Dayrat et al. 2019a).
Tapparone Canefri (1883: 214) listed all of Semper's geographic records for Peronia tongana with no new material or anatomical observations (see above). Tapparone Canefri (1883: 214) also regarded Peronia punctata as a valid species name, but with no other reference or material than the original description by Quoy and Gaimard (1832). Tapparone Canefri's suggestion that Peronia punctata could refer to the same species as Onchidium tumidum must be rejected because O. tumidum was recently transferred to Paromoionchis (Dayrat et al. 2019a). Smith (1884: 92) mentioned Onchidium (Peronia) punctatum from Albany Island and Thursday Island, in the Torres Strait, without any description. This is likely a record of Peronia verruculata (unit #1), the only species thought to be present in the Torres Strait, although our study does not include any fresh material from the Torres Strait and P. peronii could also live there (Fig. 6). Note that Thursday Island also happens to be the type locality of Scaphis viridis, a junior synonym of Peronia verruculata. Bergh (1884a: 142-148, pl. VI, fig. 19, pl. VII, figs 1-6) described as Onchidium tonganum a specimen from the collections of the Copenhagen Museum which was collected in the Nicobar Islands during the Galathea Expedition (station 305). That specimen (85/55 mm), dissected by Bergh, is in a jar (NHMD 613753) with a second specimen (70/50 mm) which is still entire and not dissected by Bergh. Both specimens were re-examined for the present study, although Bergh's measurement of the penial gland spine in the largest specimen (4.25 mm) could not be checked because internal organs are missing. Given the specimen sizes, their intestinal loops (type I with a transitional loop at 3 o'clock in the second specimen), and the spine of their accessory penial glands (4.25 mm in the largest specimen according to Bergh, and 4 mm in the second specimen), those two specimens belong to P. peronii.
Plate (1893: 172-173, pl. 12, figs 85, 87, 91) re-described Onchidium peronii based on at least one specimen from Mauritius for which he did not provide any size. However, given the length of the spine of the accessory penial gland (7 mm long), there is no doubt that he examined P. peronii. It is possible that he included a part of the duct of the accessory penial gland in that measurement because the longest spine observed in the present study was 5 mm, in the lectotype of P. fidjiensis (MNHN-IM-2000-33692). According to Plate, the retractor muscle inserts near the central nervous system, which does fit in the variation observed here (Table 4). Plate listed several synonyms: Peronia mauritiana, Onchidium tonganum, O. melanopneumon, and possibly (with a question mark) P. corpulenta. Note that Plate (1893: 172) rightly regarded O. melanopneumon as a junior synonym of O. peronii but for a weak reason (a similar pigmentation of the lung). These synonymies are all accepted here, except for P. corpulenta which is regarded as a nomen dubium (see general discussion). Plate (1893) did not comment on O. punctatum.
Godwin-Austen (1895: 443) listed Onchidium mauritianum in Little Nicobar. It is impossible to know what species was referred to. However, P. peronii (of which P mauritiana is a junior synonym), is known to be present in Nicobar Islands.
Onchidium punctatum is one of the eight onchidiid species mentioned by Hedley (1909: 369) from Queensland, Australia, without any reference to any material. It is impossible to know what species Hedley refers to. Our data show that there are two Peronia species in Queensland (Fig. 6).
The record of Onchidium peronii from Durban, Natal, South Africa by Collinge (1910: 171) likely is a record of P. madagascariensis, the only Peronia species known in South Africa so far (see remarks on P. madagascariensis). However, P. verruculata (unit #5) could also be present in northeastern South Africa because its southernmost known locality is in Maputo, Mozambique (ca. 26°S). This record was mentioned again by Connolly (1912: 225;1939: 454).
According to Connolly (1912: 224-225), Onchidium peronii is a valid name and Peronia mauritiana (as spelling mistake mauritziana) and O. tonganum are its synonyms. The references listed by Connolly are all commented on above. Let us briefly emphasize here, however, that the localities of Onchidium peronii mentioned by Connolly in South Africa and Mozambique are problematic. Connolly (1939: 454) later admitted that "it is open to question (...) whether the true O. peronii Cuv. really exists in South Africa." Connolly (1939:453), who did not cite  work, considered that Peronia was a subgenus of Onchidium and should include onchidiid slugs with dorsal gills. Vayssière (1912: 125-129) recorded seven individuals of Peronia peronii shipped to him from Moucha Islands (Djibouti) by Charles Gravier and Félix Pierre Jousseaume, two of the people who also collected many specimens studied by . Vayssière mostly focused on the description of the radula, which is not useful to identify species. Vayssière reported a wide range of animal sizes (from 10 to 80 mm long and from 6 to 60 mm wide). Thus, it is very possible that he examined more than one species. Instead of P. peronii, which has never been positively recorded from Djibouti, Vayssière likely examined P. verruculata, P. madagascariensis, or both (Fig. 6). His specimens of large size most likely were P. madagascariensis because P. verruculata individuals rarely are longer than 60 mm (Table 4). Note that the number of rows of teeth and the number of teeth per half row mentioned by Vayssière (95 to 100 rows on average) are higher than what was observed here, although they are more compatible with P. madagascariensis than P. verruculata (Table 5), acknowledging that radular formulae are expected to vary.
It is not possible to determine to what species Odhner (1919: 42) was referring solely based on his brief, external description of Onchidium peronii from Toliara, Madagascar. However, his material, dissected here, clearly belongs to P. peronii: a single individual (65/50 mm) is characterized by intestinal loops of type I with a transitional loop at 3 o'clock, a spine of the accessory penial gland of 3 mm long, and a muscular sac of 25 mm long (SMNH 180381). Bretnall (1919) uncritically took for granted every species record ever published, without considering that species often are misidentified. Bretnall (1919) accepted O. peronii as a valid name, with Onchidium tonganum and Peronia mauritiana as synonyms, and P. corpulenta as a potential synonym (with a question mark). The references listed by Bretnall (1919: 311-312) for O. peronii are all commented on above already. However, Bretnall's (1919: 313) list of geographic records needs to be discussed, especially because Bretnall did not mention the key characters supporting a proper identification of P. peronii (Table 4). The presence of P. peronii in Samoa, which Bretnall obtained from Semper (see above), should not be taken for granted, even if it is quite possible. The presence of P. peronii in the Buccaneer Archipelago, northern Western Australia (16S, 123E), based on specimens from the Australian Museum, should not be taken for granted, even though it is quite possible. The identification of P. peronii in the Santa Cruz Islands, Solomon Islands, based on specimens from the Australian Museum, should not be taken for granted (specimens may have been misidentified), even if the Santa Cruz Islands are within the known geographical range of P. peronii (Fig. 6). Bretnall (1919: 315-316) also regarded O. melanopneumon as a valid name, for which he cited Bergh's (1884a) original description and its French summary by Joyeux-Laffuie's (1885), and indicated Plate's (1893) proposed synonymy (with O. peronii) with a question mark. Bretnall (1919: 316) listed Lord Howe Island, off southeastern Australia (based on specimens from the Australian Museum), as a locality for O. melanopneumon, but without description of key characters. Thus, the presence of P. peronii in Lord Howe Island, which is 1350 km south of the southernmost known locality of P. peronii (New Caledonia), is not taken for granted here. As for Onchidium punctatum, Bretnall (1919: 316-317) followed Semper (1882: 289) and Tapparone Canefri (1883: 214) who both thought that it could be a synonym of Onchidium tumidum (see above), which is not possible because O. tumidum refers to a Paromoionchis species (Dayrat et al. 2019a). Hoffmann (1928: 71), following most of Plate's (1893) nomenclatural decisions, accepted Peronia mauritiana, P. corpulenta, Onchidium tonganum and O. melanopneumon as junior synonyms of O. peronii. Hoffmann, like other authors, did not mention the key anatomical characters that allow a reliable identification of P. peronii and uncritically accepted most geographical records published before him. As a result, his proposed distribution for O. peronii should not be taken for granted. For instance, the presence of P. peronii in Lord Howe Island, off southeastern Australia, obtained from Bretnall (1919) is questionable. Hoffmann (1928: 44) examined a specimen from the Nicobar Islands (NHMD 613753) which was originally mentioned by Mörch (1872a: 28; 1872b: 325; see above). Hoffmann (1928: 44-45) also provided several geographical records (Sumatra, Java, Marshall Islands, Kiribati, Fiji) for O. peronii based on material preserved at the SMNH in Stockholm. His material was re-examined and all records are confirmed. Hoffmann only dissected two individuals, one from Sumatra (SMNH 180354) and one from Kiribati (SMNH 180379). The other eighteen specimens were dissected for the present study.
Eight large specimens (longer than 65 mm) examined by Hoffmann from Sumatra (SMNH 180354), Java (SMNH 180355), Kiribati (SMNH 180376, 180377, 180380, 180382, 180475), and Fiji (SMNH 180373) share the diagnostic characteristics of P. peronii: a spine of the accessory penial gland between 3 and 4 mm long, intestinal loops of type I with a transitional loop at 3 o'clock, and a muscular sac between 20 and 25 mm long (exceptionally 17 mm, SMNH 180354). Seven smaller specimens (between 15 and 37 mm long) examined by Hoffmann from the Marshall Islands (SMNH 180356), Fiji (SMNH 180374), and Kiribati (SMNH 180353, 180383, 180384) are immature: the anterior male reproductive parts are barely developed, and, if present, the spine of the accessory penial gland is still soft (SMNH 180353). Given their intestinal loops (type I with a transitional loop at 3 o'clock), they are regarded as individuals of P. peronii. In other species, individuals of that size are already fully mature. Two smaller specimens (between 28 and 37 mm long) examined by Hoffmann from Fiji (SMNH 180357, 180375) belong to P. peronii because of several characteristics (retractor muscle inserting near the heart, intestinal loops of type I with a transitional loop at 3 o'clock, a spine of 3 mm long). Their muscular sacs (11 and 15 mm) are shorter than in other specimens, suggesting that they likely are not fully mature. Two specimens from Kiribati (SMNH 180378, 180478), poorly preserved, could not be confidently identified. Finally, the male reproductive parts are missing in a specimen from Kiribati dissected by Hoffmann (SMNH 180379), but its intestinal loops (type I with a transitional loop oriented between 1 and 2 o'clock) confirm that it belongs to P. peronii.
Note that the locality of the specimen from Sumatra (SMNH 180354) is problematic. The label and Hoffmann's publication both say "Pulu Pasu, west coast of Sumatra," but there is no such place on the west coast of Sumatra. There are two small islands off the west coast of Sumatra called Pulau Asu (Hinako Islands) and Pulau Pasumpahan (south of Padang). There also is a small island called Pulau Pasu in the Riau Islands, but that archipelago is located north of Sumatra, in the South China Sea. So, it is unclear where that specimen was collected exactly in Sumatra. O'Donoghue (1929: 833) reported one specimen (30/21 mm) of Peronia peronii from Port Taufiq, Suez, Egypt. A radular formula (65 × 72-1-72) is not enough to identify a Peronia species, and he most likely examined P. verruculata or P. madagascariensis (Fig. 6).
Two names accepted as valid by  are regarded as synonyms of Peronia peronii: P. tongana, and P. fidjiensis. Labbé (1934a: 191) himself acknowledged that differences between P. peronii and P. tongana were weak. The traits that he mentioned (position of the pneumostome with respect to the anus, head longer than the foot) vary greatly due to preservation. Labbé (1934a: 197-198) did not compare Paraperonia fidjiensis with Peronia peronii probably because he classified them in two distinct genera.
However, there are no differences between the type material of P. fidjiensis and the type material of P. peronii. Labbé (1934a: 190) agreed with most authors that P. mauritiana and O. melanopneumon were synonyms of P. peronii. Like Plate (1893), Labbé (1934a: 190) thought that P. corpulenta was simply a potential synonym of P. peronii but in fact it is a nomen dubium (see general discussion).
All references cited by Labbé for P. peronii and P. tongana have been commented on above, but  proposed distribution ranges need additional clarification. Labbé's (1934a: 190-191) re-description of P. peronii was based on one individual (100/70 mm) from Sumatra (MNHN-IM-2012-25150), one individual (90/70 mm) from the Seychelles (MNHN-IM-2012-25149), and ten individuals from the Red Sea (not found in the MNHN collections). At least one of those specimens belongs to P. peronii because of the length of the spine of the accessory penial gland mentioned by Labbé as 6 to 7 mm. The specimens from Sumatra and the Seychelles were fully dissected by Labbé (the Sumatra individual is basically empty): the male parts are missing, and it is not possible to determine the type of intestinal loops. However, given their huge size, they most likely belong to P. peronii. The presence of P. peronii in the Red Sea is possible but, at this stage, questionable: the size mentioned by Labbé for those specimens (17/12 mm) strongly suggests that he did not examine P. peronii from the Red Sea. Those specimens from the Red Sea identified as P. peronii by Labbé could not be located at the MNHN (there are no specimens collected by Clot-Bey in the collections, and there are too many jars of specimens collected by Jousseaume to determine which jar corresponds to the species description in Labbé's monograph). Labbé's (1934a: 191-192, figs 4-7) re-description of P. tongana was based on one individual from Djibouti (Obock), one individual (85/60 mm) from the Seychelles (MNHN-IM-2012-25148), two individuals from New Ireland, and one individual from Tonga which happens to be part of the type series by Quoy and Gaimard (MNHN-IM-2000-22937) even though Labbé does not mention it. The specimen from the Seychelles was re-examined for the present study and, given its huge size, it is confirmed that it belongs to P. peronii: its intestinal loops are of type I, with a transitional loop at 3 o'clock; the male parts are missing. There are two specimens (60/50 mm) from New Ireland at the MNHN which could potentially be the two specimens mentioned by Labbé, but the collecting dates do not match. At any rate, it does not matter much since our fresh specimens demonstrate that P. peronii is present in New Ireland (Fig. 6). The specimen from Obock could not be traced back at the MNHN; there is a specimen (80/60 mm) which could possibly correspond to it but it is a problematic specimen as it could also be a type specimen for P. gaimardi, and is now an empty notum (see below, remarks on the type material of P. gaimardi in P. verruculata). Thus, the presence of P. peronii in Djibouti is not accepted here and would need to be supported by positive evidence. Risbec (1935: 415) illustrated the eggs of an onchidiid individual from New Caledonia which he called "Oncidium tonga Q et G," clearly a spelling mistake for Onchidium tonganum Quoy & Gaimard, 1832. It is not possible to know what species Risbec was referring to because there are three Peronia species in New Caledonia (Fig. 6). White (1951: 241) reported a single specimen (53/38 mm) of Onchidium peronii from the Persian Gulf. The radular formula (88 × 88-1-88) is not enough to identify a Peronia species. White's record referred either to P. verruculata (unit #4) or P. madagascariensis (Fig. 6).
In Japan, Baba (1958: 144) indicated that some specimens of Onchidium verruculatum from Tokara Islands, south of Kyushu (ca. 30°N) are very large (up to 120 mm long), suggesting that P. peronii is found there, which would be its most northern record. Macnae and Kalk (1958: 34, 44, 128) mentioned Onchidium peronii from Inhaca Island, Mozambique (ca. 26°S). Given that no information is provided for species identification, this record is not taken for granted. Onchidium peronii was likely confused with P. verruculata (unit #5), which our material indicates is present in Inhaca, or even P. madagascariensis, known from South Africa to western India (Fig. 6). The fact that the slugs were found on sand (Macnae and Kalk 1958: 128) could suggest that they saw P. verruculata (unit #5). Solem (1959: 39) did not report any new material or localities for P. peronii. The references that he mentioned (e.g., Bretnall 1919; are already commented on above. His proposed distribution ("from the Red Sea and Mauritius to New Caledonia, Samoa, and the Marshall Islands") is not fully accurate because it is based on the assumption that people never made any mistakes when identifying P. peronii, which is unfortunately not true. Solem (1959: 38) mentioned what he thought were the three "most obvious" of the "numerous differences" between O. peronii and O. verruculatum: distribution of branchial plumes (dorsal gills) on the notum, relative position of the pneumostome and the anus, and relative width of the hyponotum and pedal sole. But those features vary among individuals and should not be used for species identification.
Marcus and Marcus (1960: 877) described Peronia peronii from the Maldives based on eight specimens. Given that they report a maximum animal length of 155 mm, a long (4.5 mm) spine of the accessory penial gland, as well as a retractor muscle inserting near the heart, there is little doubt that they did examine P. peronii (Table 4). Later, Marcus and Marcus (1970: 213) added that they observed a retractor muscle inserting near the nerve ring in another of their specimens from the Maldives, which also is compatible with our present observations: a vestigial retractor muscle was even observed here in P. peronii (Table 4). Some of the material examined from historical museum collections for the present work also came from the Maldives (ANSP 304860). Webb et al. (1969: 107-112) described copulatory mechanisms in specimens they identified as O. peronii. It is unclear from where those specimens were, possibly South Africa. At any rate, given that they illustrate a spine of the accessory penial gland which is only 1.4 mm long (Webb et al. 1969: 110, fig. 3), they did not examine individuals of P. peronii.
It is not possible to determine whether Marcus and Marcus (1970: 213) examined an individual of Peronia peronii from Madagascar because they do not provide the key features that characterize it. They could have seen a large individual of P. madagascariensis instead. Britton (1984: 183) merely mentioned the fact that Marcus and Marcus (1970) accepted only two valid species names (P. peronii and P. verruculata), which is not strictly exact because Marcus and Marcus (1970) did not address the nomenclatural status of P. tongana and did say that P. branchifera was close to P. verruculata but not that it was its synonym. Patil and Kulkarni (2013) reported Onchidium peronii from Uran City, near Mumbai, India, but it is impossible to determine what species they saw (most likely it was P. madgascariensis or the unit #4 of P. verruculata, or both).
Many chemical studies have mentioned P. peronii in the past few decades. However, the name P. peronii was used arbitrarily. The individuals used for the extraction of natural products may not have been properly identified. Biskupiak and Ireland (1985) extracted peroniatriols from specimens identified as P. peronii from Guam. Peronia peronii is undeniably present in Guam. However, it is possible that P. verruculata (unit #1) could be present there as well. Pietra (1990: 145) mentioned peroniatriols in Peronia peronii from Micronesia where more than one species may be found. Arimoto et al. (1993) did not indicate where specimens of P. peronii and O. verruculatum were collected. In Japan, where the individuals used by Arimoto et al. (1993) possibly came from, there are four Peronia species which are all cryptic externally. Pietra (2002: 290) briefly cited peroniatriols in P. peronii based on the work by Arimoto et al. (1993). Finally, the antibacterial peptide extracted from individuals of Peronia peronii from the Persian Gulf (Bitaab et al. 2015) was most likely extracted from individuals of either P. verruculata (unit #4), or P. madagascariensis, or both (Fig. 6). The same general remark applies to ecological studies: Morrisey et al. (2010: 72) listed (with no justification for species identification) the presence of Peronia peronii in mangroves of the estuary of the Mtata River (31°57'S), South Africa; most likely, Morrisey et al. (2010: 72) encountered P. madagascariensis instead.
Finally, a few last words on P. peronii in phylogenetic studies. Dayrat et al. (2011: 428) and White et al. (2011: 4) identified a specimen from Guam (CASIZ 180486) as Peronia peronii, which is specimen [443] in the present study (Fig. 2). The specimen tentatively identified as Peronia cf. peronii from Mozambique (NHMUK 20060414) by Dayrat et al. (2011: 428) belongs to P. madagascariensis, which is specimen [735] in the present study (Fig. 2). The DNA sequences of the specimen from Guam were used again in several studies (e.g., Gaitán-Espitia et al. 2013;Harasewych et al. 2015).  , hermaphroditic (female) reproductive system C anterior, male, copulatory apparatus. Scale bars: 3 mm (A-C). Abbreviations: ag accessory penial gland, dd deferent duct, ddg dorsal digestive gland, fgm female gland mass, hg hermaphroditic gland, i intestine, ms muscular sac, ov oviduct, pdg posterior digestive gland, ps penial sheath, rm retractor muscle, rs receptaculum seminis, sp spermatheca, st stomach, v vestibule. Distribution (Fig. 6). Endemic to Okinawa, Japan. Etymology. Peronia okinawensis is named after its type locality: okinawensis is a latinized adjective that agrees in gender (feminine) with the generic name (ICZN 1999: Article 31.2). Habitat. The only specimens known were found on a reef flat. Peronia okinawensis seems to be rare compared to P. verruculata (unit #1) but may be more abundant at some other sites in Okinawa. It would be interesting, in the future, to map in detail at what exact sites the three Peronia species that are sympatric in Okinawa (P. okinawensis, P. peronii, and P. verruculata) overlap or not, in Okinawa and possibly in the rest of the Ryukyu Islands.

Peronia okinawensis
Color and morphology. No picture of live animals is available. The color of preserved specimens is beige mottled with darker areas dorsally and whitish ventrally. The dorsal notum of live animals is covered by dozens of papillae of various sizes. Some papillae bear black dorsal eyes at their tip. The number of papillae with dorsal eyes is variable (8-15). The largest specimens are 27 mm long.
Digestive system (Figs 17A, 18). Examples of radular formulae are presented in Table 5. The median cusp of the rachidian teeth is approximately 45 μm long. The  hook of the lateral teeth is approximately 110 μm long. The intestinal loops are of type I, with a transitional loop oriented between 12 to 3 o'clock.
Reproductive system (Figs 17B,C,19,20). In the anterior (male) parts, the muscular sac of the accessory penial gland is less than 15 mm long. The hollow spine of the accessory penial gland is narrow, elongated, and straight or slightly curved, and its shape (including at its tip) varies between individuals. Its length ranges from 1.8 mm ([696-3] UF 352288) to 2.3 mm ([696-4 H] UF 352288). Its diameter at the conical base ranges from 240 to 300 μm. Its diameter at the tip ranges from 115 to 150 μm. The retractor muscle is shorter or longer than the penial sheath and inserts near the heart. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 35 μm long.
Diagnostic features (Table 4). Peronia okinawensis is characterized by a unique combination of anatomical traits: muscular sac shorter than 15 mm, intestinal loops of type I (with a transitional loop oriented between 12 and 3 o'clock), retractor muscle inserting near the heart.

Remarks.
A new species name is needed because no existing name applies to the species described here. The specimen [696-2] was tentatively identified as Peronia cf. verruculata by Dayrat et al. (2011). This identification should be disregarded because the specimen [696-2] belongs to the species described here (Figs 2-4). Peronia okinawensis is one of the four Peronia species in Japanese waters (Fig. 6). For a comparison of their geographic range, see remarks on P. setoensis. For their identification, see the identification key as well as Table 4. It is possible that P. okinawensis is not strictly endemic to Okinawa.

Peronia madagascariensis (Labbé, 1934a) Figs 21-25
Paraperonia madagascariensis Labbé, 1934a: 199, fig. 15. Paraperonia jousseaumei Labbé, 1934a: 198 [1891-1973]) in 1932. Only one old jar was found at the MNHN with a specimen collected from Fort-Dauphin (MNHN-IM-2000-33680). The information on the label (specimen collected by Décary in 1932) matches the information provided in Labbé's original description of P. madagascariensis, and even the specimen size matches. Therefore, that specimen is considered to be the holotype by monotypy of P. madagascariensis. The holotype was dissected by Labbé. The radula, the posterior (hermaphroditic) reproductive parts, and the male parts are all missing. The intestinal loops are of type V (Fig. 21A).  that collecting information. One of them contains specimens that are part of the type series of P. gondwanae because the specific name "gondwanae" is written on an old label (MNHN-IM-2000-33683). The three labels of the other jar (MNHN-IM-2014-7993) say: "Peronia Mer Rouge Mr Jousseaume n°15, 1892," "Oncidium [written over "Oncidiella"] peronii Cuvier Mer Rouge M. Jousseaume n°15-1892," and, for unknown reasons, "60." This jar contains six specimens of Peronia, from 60/45 to 25/15 mm, two of which were dissected, possibly by Labbé. The intestinal loops of the two dissected specimens are of type I and thus are not in agreement with   fig. 12) original illustration of the intestinal loops of type V in P. jousseaumei. Also, the sizes and the number of individuals do not match the original description of P. jousseaumei. Those specimens could possibly be some of the eight non-type specimens that Labbé (1934a: 190) mentioned in his re-description of Peronia peronii collected by Jousseaume from the Red Sea ("Mer Rouge") in "1852"  Oman GenBank and BOLD sequences. One COI sequence was obtained from BOLD (LGEN099-14) for an individual identified as Onchidium verruculatum and collected from Dwarka, Gujarat, on the western coast of India (ca. 22°N), which is the easternmost known locality for P. madagascariensis. A second COI sequence was obtained from GenBank (LC027608) for an individual identified as Peronia sp. and collected from the coast of Iran in the Persian Gulf. Both sequences were unpublished. Distribution (Fig. 6). From South Africa to the Red Sea and western India (ca. 22°N): South Africa, Mozambique, Madagascar (type locality of P. madagascariensis), Gulf of Oman, Iran (Strait of Hormuz), Yemen (Socotra), India (Mumbai, Gujarat), Red Sea (type locality of P. jousseaumei). All records are new except for the type locality in Madagascar. Peronia madagascariensis is, so far, not present in Mauritius.
Etymology. Peronia madagascariensis was named after its type locality, Madagascar. Peronia jousseaumei was named after Félix Pierre Jousseaume , a medical doctor and malacologist who collected many specimens from the Red Sea preserved at the MNHN and which  studied for his monograph on onchidiids.
Habitat. Peronia madagascariensis is found in the rocky intertidal, like most other Peronia slugs.
Color and morphology. No picture of live animals was available. The color of preserved specimens is not different from other species (greyish brown and mottled with darker and lighter areas dorsally, and light brown greyish ventrally). The dorsal notum of live animals is covered by dozens of papillae of various sizes. In large individuals, dorsal papillae can be particularly tall (easily up to 4 mm), even in preserved specimens, and are evenly distributed over the entire notum. Preserved, they are very difficult to distinguish from retracted dorsal gills in the posterior half of the notum, but they are regular papillae with or without eyes. Some papillae bear black dorsal eyes at their tip. The number of papillae with dorsal eyes is variable (from 12 to 18). Dorsal gills seem taller and denser than in other species. The largest specimens in our fresh material are 55 mm long but two additional museum specimens are much longer (80 mm).
Digestive system (Figs 21A-D, 22). Examples of radular formulae are presented in Table 5. The median cusp of the rachidian teeth is approximately 55 μm long. The hook of the lateral teeth is approximately 100 to 130 μm long. The intestinal loops are of type V.
Reproductive system (Figs 21E,(23)(24)(25). In the anterior (male) parts, the muscular sac of the accessory penial gland is less than 15 mm long. The hollow spine of the accessory penial gland is narrow, elongated, and straight or slightly curved, and its shape (including at its tip) varies between individuals. Its length ranges from 2 mm ([5502] MNHN-IM-2009-16393) to 2.4 mm ([5500] MNHN-IM-2009-16391). Its diameter at the conical base ranges from 200 to 230 μm. Its diameter at the tip ranges from 70 to 80 μm. The retractor muscle is shorter or longer than the penial sheath and inserts near the heart. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 100 μm long.
Diagnostic features (Table 4). Peronia madagascariensis is characterized by a unique combination of two anatomical traits: intestinal loops of type V and a spine of the accessory penial gland longer than 2 mm.
Remarks. The name Paraperonia madagascariensis clearly applies to a Peronia species because of the dorsal gills on the notum of the holotype. The holotype was entirely dissected by Labbé. The radula, the posterior (hermaphroditic) reproductive parts, and the anterior copulatory apparatus are missing. The intestinal loops are of type V (Fig. 21A), as illustrated by   fig. 17). The name Peronia madagascariensis applies to the species described here because it is, according to our molecular data, the only Peronia species with intestinal loops of type V along the eastern African coast, from South Africa to the Persian Gulf and western India, including Madagascar. Note that some of our fresh material was collected only 150 km east of the type locality in southern Madagascar. Some internal characters described by Labbé (1934a: 199) could not be verified on the holotype because most internal parts are missing, but they are similar to the species described here. In particular, the length of the spine of the accessory penial gland (2 mm) is compatible with what was observed in our material.
Additional, non-type specimens were found in historical museum collections which could be identified as P. madagascariensis due to the presence of intestinal loops of type V, from Oman (UF 368019), the Strait of Hormuz (NHMD 635302), and Socotra (SMF 358305). Those localities, however, are all already included within the known distribution of P. madagascariensis based on our DNA sequences, as the Strait of Hormuz is very close to the Gulf of Oman. Finally, one of the "a" paralectotypes of Labbé's (1934a: 199) Paraperonia gondwanae from Bombay (MNHN-IM-2000-33682), with intestinal loops of type V (Fig. 21B), belongs to P. madagascariensis. Note that two of those museum specimens are longer (80 mm) than our fresh material (less than 55 mm).
Peronia slugs with intestinal loops of type V are without doubt present in the Red Sea. For instance, one of the "c" paralectotypes of Labbé's (1934a: 200) Paraperonia gondwanae from Suez (MNHN-IM-2000-33683) is characterized by intestinal loops of type V (Fig. 21C), which means that it does not belong to P. verruculata (characterized by intestinal loops of type I).  Paraperonia jousseaumei, with the Red Sea as type locality, is also characterized by intestinal loops of type V. Even though the type material of P. jousseaumei could not be located at the MNHN, Labbé's (1934a: fig. 12) drawing of the internal anatomy of P. jousseaumei clearly illustrates intestinal loops of type V. Given that P. madagascariensis is widespread from South Africa all the way to western India, including the Strait of Hormuz, it is accepted here that it also is distributed in the Red Sea. That, however, will still need to be confirmed with fresh material from both the Red Sea and the Gulf of Aden. If it appears that the populations of Peronia slugs with intestinal loops of type V from the Red Sea are a distinct species, then the name P. jousseaumei could apply to them and be valid. Finally, given that P. madagascariensis is present in the Strait of Hormuz, it most likely also is distributed in the rest of the Persian Gulf, which hopefully will be confirmed at some point with fresh material.
Even though the names Peronia madagascariensis and Peronia jousseaumei were never used prior to the present contribution, they are not regarded as new combinations because Paraperonia has already been regarded as a synonym of Peronia by Britton (1984: 182) and because it has also been made clear that the genus Peronia included all species of slugs with dorsal gills (e.g., Dayrat et al. : 1861. The specimen [703] from Oman was tentatively identified as Peronia sp. 2 by Dayrat et al. (2011) but it clearly belongs to P. madagascariensis (Fig. 2). Also, note that its COI sequence was resubmitted to GenBank because the old one (GenBank HQ660044) was inaccurate. The specimen [735] from Mozambique was tentatively identified as Peronia cf. peronii by Dayrat et al. (2011). This identification should be disregarded because the specimen [735] belongs to P. madagascariensis (Fig. 2).
Etymology. Peronia platei was named after German zoologist Ludwig Hermann Plate , professor of zoology at the University of Jena and author of a monograph on onchidiids (Plate 1893).
Habitat. Peronia platei is found primarily in the rocky intertidal. According to the label, specimens from Kiribati were collected on sand inside a lagoon (P. sydneyensis and P. willani are also known to be found on sand).
Color and morphology of live animals (Fig. 26). No picture of live animals was available for specimens from the West Pacific. The description of the color of live animals is based on Hawaii individuals. The dorsal notum is uniformly very dark grey, almost black, including papillae. The hyponotum is light yellowish. The foot is light yellowish to orange. The ocular tentacles are grey, like the head. The dorsal notum of live animals is covered by dozens of papillae of various sizes. Some papillae bear black dorsal eyes at their tip. The number of papillae with dorsal eyes is variable (from 7 to 10). The papillae with dorsal eyes cannot be counted in specimens from Hawaii because the no-tum is too dark and because eye pigmentation tends to fade in preservation. The largest specimens are 30 mm long in Hawaii and 20 mm in Papua New Guinea.
Digestive system (Figs 27, 28). Examples of radular formulae are presented in Table 5. The median cusp of the rachidian teeth is approximately 30 to 35 μm long. The hook of the lateral teeth is approximately 60 to 90 μm long. The intestinal loops are of type V.
Reproductive system (Figs 29-32). In the posterior (hermaphroditic) parts, the deferent duct and the oviduct are straight. In the anterior (male) parts, the muscular sac of the accessory penial gland is less than 5 mm long. The hollow spine of the accessory penial gland is narrow, elongated, and straight or slightly curved, and its shape (including at its tip) varies between individuals. Its length ranges from 0. Guinea. Its diameter at the conical base ranges from 95 to 100 μm (Hawaii) and from 65 to 80 μm (Papua New Guinea). Its diameter at the tip ranges from 25 to 30 μm (Hawaii) and from 20 to 30 μm (Papua New Guinea). The retractor muscle is shorter or longer than the penial sheath and inserts at the posterior end of the visceral cavity. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 60 μm long in Hawaii and less than 20 μm long in Papua New Guinea.
Diagnostic features (Table 4). Peronia platei is cryptic with P. setoensis. Both species share the same combination of anatomical traits: intestinal loops of type V, retractor muscle inserting at the posterior end of the visceral cavity, a spine of the accessory  penial gland from 0.8 to 1 mm long (P. platei) and from 0.9 to 1.2 mm long (P. setoensis). The diameter of the spine of the accessory penial gland at its tip is larger in P. platei (25 to 30 μm) than in P. setoensis (less than 25 μm) but that may be simply due to limited sampling. Peronia platei and P. setoensis are both distributed in the West Pacific but they are not sympatric based on current data (Fig. 6).
Remarks. Onchidium platei applies to the species described here because the anatomy of the lectotype is identical to the anatomy of our material (Table 4): gills on the dorsal notum; muscular sac of the accessory penial gland less than 5 mm long; spine of the accessory penial gland 0.9 mm long (observed by transparency); intestinal loops of type V; seven dorsal papillae with eyes. Our molecular analyses show that the species described here is widespread across the West Pacific, from Papua New Guinea to Hawaii. There is no reason to think that the populations in French Polynesia (type locality of O. platei) are a distinct species. This, however, will have to be confirmed with fresh material from French Polynesia, preferably from Moorea, the type locality. All eight paralectotypes (also from Tahiti) also belong to the same species.
Hoffmann's (1928: 51-53, figs 9, 10, pl. 3, figs 11, 12) original description, which is quite detailed, needs to be briefly commented on. Hoffmann mentions that dorsal gills are lacking but they are undoubtedly present in the lectotype and all paralectotypes (dorsal gills are often hard to see in preserved animals). The anatomical traits he describes agree with our observations on the type material. The intestinal loops, Hoffmann says, are of type I but slightly different from the regular type I due to the absence of a loop. Hoffmann calls it a type Ia. His illustration of it clearly represents a type V (Hoffmann 1928: pl . 3, fig. 11). The spine of the accessory penial gland is 1 mm long and the retractor muscle attaches to the posterior end of the visceral cavity. According to Hoffmann (1928: 53), O. platei is most closely related to O. tumidum Semper, 1880 and O. nebulosum Semper, 1880 but differs from them based on the penis size. Onchidium tumidum was recently transferred to Paromoionchis (Dayrat et al. 2019a), and O. nebulosum (type locality in Palau) applies to a Peronia species but is regarded here as a nomen dubium (see general discussion).
Additional specimens were found in historical museum collections which could be identified as P. platei mostly based on the intestinal loops of type V, the specimen size, and their geographic origin. Specimens from Kiribati (SMNH 106488) are especially interesting because they confirm the presence of specimens similar to P. platei far from Hawaii and Papua New Guinea, which strongly supports the assumption that P. platei is widespread across the entire West Pacific. Note that those specimens from Kiribati are not identified as P. setoensis (which is anatomically cryptic with P. platei) because P. setoensis is found in much colder waters (33°N) in Japan (Fig. 6). Labbé (1934a: 224) merely mentioned Onchidium platei as one of the valid Onchidium species names. Ruthensteiner (1997) briefly commented on the anatomy of the lung of Onchidium cf. branchiferum, based on specimens from Hawaii. Those were most likely specimens of Peronia platei, the only Peronia species found in Hawaii. Finally, note that the specimen [706] (UF 303653) was tentatively referred to as Peronia sp. 1 by Dayrat et al. (2011).
No Peronia slug from Hawaii was positively demonstrated to belong to P. verruculata (unit #1), which is characterized by intestinal loops of type I. Therefore, Hoffmann's (1928: 44, 73) record of O. verruculatum from Hawaii is interpreted here as a misidentification of P. platei. Labbé (1934a: 193), Solem (1959: 39), and Marcus and Marcus (1970: 213) all assumed that P. verruculata was present in Hawaii based on  study, without collecting or examining any new material.
Onchidella evelinae Marcus & Burch, 1965 was described based on small specimens (average length 6 mm) from Eniwetok Atoll, Marshall Islands (ca. 11°N, 162°E). The type material was deposited at the Museum of Zoology, University of Michigan, but could not be located there (personal communication from the collection manager, Dr. Taehwan Lee). Onchidella evelinae is a misidentification for one of the onchidiid species present in the Marshall Islands: it cannot refer to Onchidella slugs because an accessory penial gland is mentioned in the original description and because Onchidella is not present in the middle of the West Pacific. The Marshall Islands are within the distribution range of P. platei (Fig. 6), but a detail from the original description (the internal organs can be seen through the dorsal notum) suggests that O. evelinae does not refer to Peronia slugs because their notum is too thick for internal organs to be seen through it. Peronia peronii is also present in the Marshall Islands (Fig. 6), but, given the very small size of the specimens and that they were sexually mature, it is most unlikely that O. evelinae is a junior synonym of P. peronii (Fig. 6). The size of the spine of the accessory penial gland (1.3 mm) reported in the original description of O. evelinae is compatible with what is currently known (< 1 mm) for P. platei (Table 4). Onchidella evelinae is regarded here as a new junior subjective synonym of Marmaronchis vaigiensis (Quoy & Gaimard, 1825): first, because internal organs can occasionally be seen through its thin notum (e.g., Dayrat et al. 2018: fig. 5E); second, because there are known records (Dayrat et al. 2018: fig. 9) of M. vaigiensis in Pohnpei, Micronesia (ca . 6°N, 158°E), just a few degrees west of the Marshall Islands, and it is very possible that M. vaigiensis also is in the Marshall Islands. The size of the spine of the accessory penial gland (1.3 mm) reported in the original description of O. evelinae is higher than what is currently known for M. vaigiensis (< 1 mm), but that trait does vary intra-specifically.  Distribution (Fig. 6). Endemic to subtropical waters of Japan: Honshu, Nishimuro, near Seto Marine Biological Laboratory (33N, type locality), Sagami Bay (35°N), and possibly Boso Peninsula, near Sagami Bay (35°N); Kyushu, Nagasaki, 32N (Keferstein 1865a, b, as P. verruculata).
Etymology. Peronia setoensis is named after its type locality, near the Seto Marine Biological Laboratory: setoensis is a latinized adjective that agrees in gender (feminine) with the generic name (ICZN 1999: Article 31.2).
Habitat (Fig. 33). Peronia setoensis is found in the rocky intertidal. Few individuals are currently known but it may be discovered in additional localities in the future.
Color and morphology of live animals (Fig. 34). The dorsal notum is greenish brown, light to dark, mottled with darker and lighter areas, occasionally with yellowish sides. The color of the dorsal papillae varies as that of the background itself. The ventral surface (foot and hyponotum) is yellowish or greyish and can change rapidly in any given individual. The ocular tentacles are brown-grey, like the head. The dorsal notum of live animals is covered by dozens of papillae of various sizes. Some papillae bear black dorsal eyes at their tip. The number of papillae with dorsal eyes is variable (from 8 to 12). The largest specimens are 20 mm long.
Digestive system (Figs 35A, B, 36). Examples of radular formulae are presented in Table 5. The median cusp of the rachidian teeth is approximately 35 μm long. The hook of the lateral teeth is approximately 90 μm long. The intestinal loops are of type V.
Reproductive system (Figs 35C,D,37,38). In the anterior (male) parts, the muscular sac of the accessory penial gland is less than 5 mm long. The hollow spine of the accessory penial gland is narrow, elongated, and straight or slightly curved, and its shape (including at its tip) varies between individuals. Its length ranges from 0.9 mm ([3754] NSMT-Mo 78987) to 1.2 mm ([3753] NSMT-Mo 78987). Its diameter at the conical base ranges from 80 to 85 μm. Its diameter at the tip ranges from 15 to 25 μm. The retractor muscle is shorter or longer than the penial sheath and inserts near the  heart. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 25 μm long.
Diagnostic features (Table 4). Peronia setoensis is cryptic with P. platei. Both species share the same combination of anatomical traits: intestinal loops of type V, retractor muscle inserting at the posterior end of the visceral cavity, a spine of the accessory penial gland from 0.9 to 1.2 mm long (P. setoensis) and from 0.7 to 1 mm long (P. platei). Peronia setoensis and P. platei are anatomically very similar to P. griffithsi, in which, however, the spine of the accessory penial gland is slightly shorter (less than  0.62 mm long). All three species are distributed in the West Pacific but Peronia setoensis is adapted to much colder waters than P. platei and P. griffithsi (Fig. 6).

Remarks.
A new species name is needed because no existing name applies to the species described here. A specimen from Sagami Bay (35°N), preserved in Stockholm (SMNH 180725), not included by Hoffmann (1928: 73) in his list of material for O. verruculatum, is identified here as P. setoensis because of its intestinal loops of type V (Table 4). This specimen indicates that P. setoensis is distributed on the eastern Pacific coast of Japan north of the type locality.
Keferstein (1865b) described as P. verruculata three slugs from Nagasaki, Kyushu, Japan (ca. 32°44'N). His written description (Keferstein 1865b) was also based on an individual from Java but his figure captions clearly indicate that his drawings illustrated an individual from Nagasaki (Keferstein 1865b: pl. VI, figs 14-16): Keferstein's (1865b: pl. VI, fig. 16) drawing of the internal anatomy unmistakably illustrates intestinal loops of type V. Therefore, it is very likely that P. setoensis, the only one species of Peronia slugs with intestinal loops of type V in Japan, is also distributed in Kyushu. It is unclear whether Keferstein's (1865a: pl. CII, figs 20*, 20**, pl. CV, figs 1, 2) drawings illustrate the same Nagasaki individual as the one with intestinal loops of type V (Keferstein 1865b: pl. VI, fig. 16). It cannot be excluded that Keferstein examined several species found in Japan (Fig. 6). The Java individual cannot be identified. The molecular data presented here indicate that there are four Peronia species in Japanese waters, but their geographic ranges need to be explored in better detail (Fig. 6). Peronia setoensis is definitely (our DNA sequences) present in southern Honshu (Wakayama Prefecture) and very likely in Kyushu based on Keferstein's (1865b: pl. VI, fig. 16) drawing of intestinal loops of type V. Peronia verruculata (unit #1) is definitely (our DNA sequences) present in Wakayama Prefecture (ca. 33°N), southern Honshu, and is thus expected to be present in all Japanese waters south of Wakayama Prefecture. Also, Peronia verruculata is present in Sakurajima, Kyushu (ca. 31°N) and Okinawa (ca. 26°N) based on sequences that Takagi et al. (2019) recently published (see remarks on P. verruculata). Peronia peronii is also present in Okinawa based on COI sequences that Takagi et al. (2019) recently published (see remarks on P. peronii). And, finally, our new species P. okinawensis is only known from Okinawa so far.
Besides Keferstein (1865a, b), several authors mentioned onchidiids from Japan but, in most cases, species cannot be identified based on the limited information provided. Stimpson (1855: 380) described Onchis fruticosa based on slugs with dorsal gills from Kikaijima (28°30'N), between Kyushu and Okinawa, which could potentially belong to any of the four species present in Japanese waters. As a result, Onchis fruticosa is regarded as a nomen dubium (see general discussion). Baba (1958) illustrated onchidiid slugs from three different places: Tokara Islands, just south of Kyushu (ca. 30°N); Amakusa, near Nagasaki, Kyushu (ca. 32°30'N); and Misaki, Osaka, Honshu (ca. 34°N). Baba (1958: 144) indicates that some specimens of Onchidium verruculatum from Tokara Islands were very large (up to 120 mm long), suggesting that P. peronii is found there, which would be its northernmost record (see remarks on P. peronii). The smaller specimens that Baba (1958: 144) mentions from Tokara Islands could be a combination of P. verruculata (unit #1) and possibly P. setoensis. The two species which Baba (1958: 21) seems to distinguish (as Onchidium and Onchidium verruculatum) in Misaki, near Osaka, could be P. verruculata (unit #1) and P. setoensis, which, based on our DNA sequences, are sympatric near the Seto Marine Laboratory, which is close to Osaka. And, finally, the slugs crawling on mud in Amakusa, near Nagasaki, are not Peronia slugs (Baba 1958: 51) but most likely belong to Paromoionchis tumidus, a species which is present nearby, in Kumamoto Uki, as the COI sequences from the slugs of "Group I" in Takagi et al. (2019) cluster with our sequences of P. tumidus (Dayrat et al. 2019a). Katagiri and Katagiri (2007) distinguished two Peronia species (both as Onchidium verruculatum) in the waters of the Boso Peninsula (near Sagami Bay, Honshu, ca. 35°N) based on external appearance and development. One species, called Isowamochi, is characterized by planktotrophic development, and the other, called Minneawamochi, by direct development. Most likely, these slugs belong to P. verruculata (unit #1) and P. setoensis, which are the only two Peronia species found north of 30N. However, this assumption would have to be confirmed with fresh collections and DNA sequences. Ueshima (2007) commented that the external distinction between the two species recognized by Katagiri and Katagiri (2007) is far more subtle and problematic, and he rightly suggested that molecular data could determine the relationships between those two species and P. verruculata (erroneously said to be from the Mediterranean). Note that Ueshima's (2007) material, which covered a broad latitudinal range from the Kanagawa Prefecture (near Sagami Bay, ca. 35°N) all the way to Ishigaki Island (Okinawa, ca. 24°N), potentially included slugs from all four Peronia species found in Japan. Distribution (Fig. 6). Indo-West Pacific: Mauritius (type locality), Indonesia (Kei Islands), and Papua New Guinea (New Ireland).

Etymology. Peronia griffithsi is named after Owen Griffiths, who kindly and generously hosted and guided one of us (Tricia Goulding) in Mauritius.
Habitat (Fig. 39). Peronia griffithsi is found in the rocky intertidal, like most other Peronia slugs. Our specimens from Mauritius were collected just before sunrise, suggesting that P. griffithsi is, at least partly, a nocturnal species.
Color and morphology of live animals (Figs 40,41). No picture of live animals was available for specimens from Kavieng. The description of the color of live animals is based on Mauritius and Kei individuals. The dorsal notum is greenish brown, light to dark, mottled with darker and lighter areas. The color of the dorsal papillae varies as that of the background itself, but dorsal papillae can also be yellowish-greenish. The ventral surface (foot and hyponotum) varies from whitish to yellowish and can change rapidly in any given individual. The ocular tentacles are brown-grey, like the head. The       dorsal notum of live animals is covered by dozens of papillae of various sizes. Some papillae bear black dorsal eyes at their tip. The number of papillae with dorsal eyes is variable (from 6 to 10). The largest specimens are 25 mm long.
Digestive system (Figs 42-44). Examples of radular formulae are presented in Table 5. The median cusp of the rachidian teeth is approximately 35 μm long. The hook of the lateral teeth is approximately 70 μm long. The intestinal loops are of type V.
Reproductive system (Figs 45-48). In the anterior (male) parts, the muscular sac of the accessory penial gland is less than 5 mm long. The hollow spine of the accessory penial gland is narrow, elongated, and straight or slightly curved, and its shape (including at its tip) varies between individuals. Its length is 0.  retractor muscle is shorter or longer than the penial sheath and inserts near the heart. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 25 μm long.
Diagnostic features (Table 4). Peronia griffithsi is characterized by a unique combination of anatomical traits: intestinal loops of type V, muscular sac of the accessory penial gland less than 5 mm long, spine of the accessory penial gland less than 0.62 mm long. In P. platei and P. setoensis, which are anatomically similar to P. griffithsi, the spine of the accessory penial gland is longer than 0.7 mm (P. platei) and 0.9 mm (P. setoensis).

Remarks.
A new species name is needed because no existing name applies to the species described here. A large population (161 specimens) from Kei Islands and identified by Hoffmann as Onchidium verruculatum was found in the collections of the Copenhagen Museum (NHMD 635303). Those specimens most likely belong to P. griffithsi because their intestinal loops are of type V (only a few individuals were dissected). Also, the retractor muscle of the few individuals dissected inserts near the end of the visceral cavity, as in specimens from Mauritius, suggesting that an insertion near the heart is not as common. Interestingly, Hoffmann (1928: 44)         Distribution (Fig. 6). Southern West Pacific: New South Wales (type locality) and Queensland (up to 20°S), Australia, and New Caledonia.
Etymology. Peronia sydneyensis is named after its type locality in Sydney, New South Wales, Australia: sydneyensis is a latinized adjective that agrees in gender (feminine) with the generic name (ICZN 1999: Article 31.2).  A, B). Abbreviations: dd deferent duct, fgm female gland mass, hg hermaphroditic gland, ov oviduct, rs receptaculum seminis, sp spermatheca Habitat (Fig. 49). Unlike most other Peronia species, which are found in the rocky intertidal, P. sydneyensis is primarily found on muddy or coarse sand.
Color and morphology of live animals (Figs 50,51). The dorsal notum is greenish brown, light to dark, mottled with darker and lighter areas. The color of the dorsal papillae varies as that of the background itself. The ventral surface (foot and hyponotum) varies from whitish to dark grey, including yellowish, bluish, and greenish, and can change rapidly in any given individual. The ocular tentacles are brown-grey, like the head. The dorsal notum of live animals is covered by dozens of papillae of various sizes. Some papillae bear black dorsal eyes at their tip. The number of papillae with dorsal eyes is variable (from 8 to 16). The largest specimens are 30 mm long (New South Wales), 50 mm long (Queensland), and 41 mm long (New Caledonia).
Digestive system (Figs 52-54). Examples of radular formulae are presented in Table 5. The median cusp of the rachidian teeth is approximately 40 μm long. The hook of the lateral teeth is approximately 80 μm long. The intestinal loops are of type I, with a transitional loop oriented between 3 and 6 o'clock; exceptionally, the transitional loop is oriented at 2 o'clock.
Reproductive system (Figs 55-58). In the anterior (male) parts, the muscular sac of the accessory penial gland is less than 10 mm long. The hollow spine of the accessory penial gland is narrow, elongated, and straight or slightly curved, and its shape (includ- ing at its tip) varies between individuals. Its length ranges from 0.6 mm ([2680] MTQ) to 1 mm ([2661] MTQ). Its diameter at the conical base ranges from 90 to 100 μm. Its diameter at the tip measures 20-50 μm. The retractor muscle is shorter or longer than the penial sheath and inserts near the heart. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 30 μm long.
Diagnostic features (Table 4). Peronia sydneyensis is characterized by unique and distinctive protuberances on the spine of the accessory penial gland (Fig. 58). These strong protuberances were observed in all individuals. Protuberances can also be observed (as exceptional cases) in other species but they are always much smaller in size (Figs 37D, 104B, 105F). In addition, Peronia sydneyensis is characterized by a  Peronia sydneyensis is distinct anatomically from P. willani, with which it is most closely related (Figs 2-4), and from P. verruculata, with which it overlaps geographically in Queensland and New Caledonia (Fig. 6).

Remarks.
A new species name is needed because no existing name applies to the species described here. The records of Onchidium verruculatum from New South Wales (Bretnall 1919: 310;Dakin 1947: 144;Smith and Kershaw 1979: 92;Hutchings and Recher 1982: 119;Hyman 1999) are most likely records of Peronia sydneyensis, the only Peronia species known in New South Wales based on current data (Fig. 6). Some of these records (or even all of them) could be a combination of both P. sydneyensis and P. verruculata: the southernmost locality of P. verruculata (unit #1) is in MacKay, Queensland (21°22'S), but given that P. verruculata tolerates colder waters in Japan (up to at least 33°40'N), it is possible that it is also present in New South Wales. Peronia sydneyensis was collected only in Sydney (33°39'S), but it is not excluded that both species are sympatric as far south as Sydney. Additional fresh material between southern Queensland and New South Wales is needed to determine more precisely the geographic range of each species. Note that the intestinal loops of type II by Hyman (1999: fig. 7B) illustrate the digestive system of a misidentified individual (most likely Paromoionchis daemelii, easily confused in the field with Peronia sydneyensis). Finally, note that the specimen [734] (AM C.459511) was tentatively referred to as Peronia sp. 3 by Dayrat et al. (2011). Distribution (Fig. 6). Endemic to Darwin, Northern Territory, Australia.

Peronia willani
Etymology. Peronia willani is named after Richard Willan, senior curator of mollusks at the Museum and Art Gallery of the Northern Territory, Darwin, Australia, who kindly and generously helped us during our field expedition around Darwin.
Habitat (Fig. 59). Unlike most other Peronia species, which are usually found in the rocky intertidal, P. willani is primarily found on sandy mud or even directly on mud. Color and morphology of live animals (Fig. 60). The color of the dorsal notum is highly variable, from nearly whitish to dark brown and greenish, most often mottled with darker and lighter areas. The color of the dorsal papillae varies as that of the background itself, but dorsal papillae can also be lighter (yellowish-greenish) than the background. The ventral surface (foot and hyponotum) varies from whitish (almost transparent) to yellowish and can change rapidly in any given individual. Occasionally, a black ring is present on the hyponotum around the pedal sole. The ocular tentacles are browngrey, like the head. The dorsal notum of live animals is covered by dozens of papillae of various sizes. Some papillae bear black dorsal eyes at their tip. The number of papillae with dorsal eyes is variable (from 10 to 25). The largest specimens are 65 mm long.      (Figs 61A, 62). Examples of radular formulae are presented in Table 5. The median cusp of the rachidian teeth is approximately 30 μm long. The hook of the lateral teeth is approximately 100 μm long. The intestinal loops are of type I, with the transitional loop oriented between 3 to 6 o'clock.

Digestive system
Reproductive system (Figs 61B, C, 63, 64). In the anterior (male) parts, the muscular sac of the accessory penial gland is less than 25 mm long. The hollow spine of the accessory penial gland is narrow, elongated, and straight or slightly curved, and its shape (including at its tip) varies between individuals. Its length ranges from 1.5 mm ([1620] NTM P.57626) to 1.9 mm ([1628 H] NTM P.57625). Its diameter at the conical base ranges from 240 to 250 μm. Its diameter at the tip ranges from 80 to 100 μm. The retractor muscle is shorter or longer than the penial sheath and inserts near the heart. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 37 μm long.
Diagnostic features (Table 4). Peronia willani is characterized by a unique combination of anatomical traits: intestinal loops of type I (with a transitional loop oriented between 3 and 6 o'clock), retractor muscle inserting at the posterior end of the visceral cavity, muscular sac up to 25 mm, spine of the accessory penial gland between 1.5 and 1.9 mm long. Peronia willani is anatomically distinct from P. sydneyensis, with which it is most closely related (Figs 2-4), and from P. verruculata, from which it is close geographically (Fig. 6).

Remarks.
A new species name is needed because no existing name applies to the species described here. A specimen from Darwin, Northern Territory, preserved in Stockholm (SMNH 180715) identified as O. verruculatum by Hoffmann (1928: 73) is identified here as P. willani because of its massive (18 mm long) muscular sac (Table 4). Also, to our knowledge, P. verruculata is not present in Northern Territory (Fig. 6).

Peronia verruculata (Cuvier, 1830) Figs 65-109
Onchidium verruculatum Cuvier, 1830: 281;Semper 1880: 255-257, pl. 22, figs 3, 4; 1882: pl. 21, fig. 1 . 3.2) looks exactly like the MNHN specimen (without the male parts outside, which were subsequently removed). The figure 3.3 illustrates a much larger individual which could not be located. No information was provided on sizes, except that the illustrations were of "natural length" (figures 3.1, 3.2) and "likely of natural length" (figure 3.3) according to Audouin (1826: 19). Given that it is unclear whether Savigny (unknowingly) illustrated one or two species, it is appropriate to designate the specimen preserved at the MNHN as the lectotype (MNHN-IM-2000-22941). The animals illustrated by Savigny (1817) were not accompanied by any species name, but they were named and described ten years later by Audouin (1826:  Cuvier likely changed his mind and later decided that, for some reason, the specimens illustrated by Savigny were a distinct species he called Onchidium verruculatum. The lectotype is still well preserved, considering how old it is. The radula and the posterior (female) reproductive parts are still inside but only the deferens duct remains for the male copulatory parts. Its intestinal loops are of type I (Fig. 86A)  and 26/18 mm; same collection data as for the lectotype; MNHN-IM-2000-22951. The lectotype was designated by Goulding et al. (2018: 75) to clarify the application of Onchidium ferrugineum. The two paralectotypes belong to Wallaconchis ater (Lesson, 1831a) because they lack dorsal gills, lack an accessory penial gland, and are characterized by a highly coiled penis. Labbé re-examined four specimens from the original type series but there are only three specimens left in the jar, so one specimen was lost by or after Labbé. The lectotype is well preserved. Its dorsal notum bears obvious gills. Its male opening is located below and to the left of the right ocular tentacle. Pieces of the deferent duct and of the flagellum of the accessory penial gland remain, but the muscular sac and the spine of the accessory gland are missing. The posterior (female) part of the reproductive system is still in place inside the lectotype. Its radula is missing. Its intestinal loops of type I (with a transitional loop at 4 o'clock) are illustrated here (Fig. 80A).
Lectotype ( (figs 3.1-3.8). There is no easy explanation for the exclusion of         (Fig. 80B). Dorsal gills are present on the notum. The holotype, clearly labeled as "Oncidium Astridae Labbé," was dissected by Labbé for the original description but is relatively well preserved. The radula, the posterior (female) reproductive parts, and the intestinal loops of type I (Fig. 80C) are still in place inside the specimen. Male parts are missing. Dorsal gills are present on the notum (partly cut by Labbé). Note that the locality on the label of the holotype is indicated as Sorong, but with a question mark.
Lectotype and paralectotypes (Peronia gaimardi). Solomon Islands • lectotype, hereby designated, 44/27 mm; Vanikoro; 1829; JRC Quoy & JP Gaimard leg.; MNHN-IM-2000-33705. • 1 paralectotype, 35/30 mm; same collection data as for the lectotype; MNHN-IM-2000-33705. The type material also includes a paralectotype from Djibouti which could not be located with certainty (see below). Originally, no jar clearly labeled as the type material of Peronia gaimardi was found at the MNHN. The original description of P. gaimardi is based on three individuals, two individuals identified as Onchidium, from "Vanikoro (Quoy and Gaimard 1829)," and one individual identified as "Oncidium Peronii," from "Obock, Récif de Clochettins (Gravier 1904)." The two specimens from Vanikoro were found at the MNHN in a jar with three labels. One old label says "Onchidium [subsequently replaced by Peronia] de Vanikoro, mm Quoy et Gaimard 1829." Another label only says "44" for unknown reasons. And  two individuals from Vanikoro were originally used by Labbé to describe P. gaimardi. The lectotype was dissected by Labbé. Its radula and male apparatus are missing. The female parts are still inside the animal. Its intestinal loops of type I are illustrated here (Fig. 80E). Dorsal gills are present on the notum. The paralectotype from Vanikoro was not dissected by Labbé. As for the paralectotype from Obock, Djibouti, it could not be traced with certainty, which does not matter given that it has no name-bearing function. Based on the original description (Labbé 1934a: 194), the paralectotype from Djibouti was collected by Gravier in 1904 at the "Récif de Clochettins, Obock," that it measured 80/57 mm, and that its body was "very flattened." There is a jar at the MNHN with a label saying "Oncidium Peronii, Cuv. Obock M. Gravier 1904 -A Labbé, dét [for "déterminé," i.e., identified] 1933." Another label says "F" for unknown reasons. All the informa- tion on the label matches the information provided by Labbé in the original description of P. gaimardi, and the size (80/60 mm) of the specimen perfectly matches the size of the paralectotype of P. gaimardi. That specimen is just an empty notum with dorsal gills (all internal organs are missing). However, for two reasons, it is extremely unclear Abbreviations: ag accessory penial gland, dd deferent duct, ddg dorsal digestive gland, fgm female gland mass, hg hermaphroditic gland, i intestine, ms muscular sac, ov oviduct, pdg posterior digestive gland, ps penial sheath, rm retractor muscle, rs receptaculum seminis, sp spermatheca, st stomach, v vestibule.
in 1904 from Obock (there are other specimens from Obock at the MNHN, but not collected in 1904 by Gravier), and that the specimen may not even have been collected by Gravier, it is not possible to know whether that specimen is the paralectotype of P. gaimardi, a non-type material used by Labbé for a re-description of Peronia tongana, or even something completely different.
Lectotype and paralectotype (Peronia anomala). Red Sea • lectotype, hereby designated, 10/8 mm; 1893; Jousseaume leg.; MNHN-IM-2000-33678. • 1 paralectotype, 6/3 mm; same collection data as for the lectotype; MNHN-IM-2000-33678. Originally, no jar clearly labeled as the type material of Peronia anomala was found at the MNHN, but it could be traced back. The original description of P. anomala is based on two individuals (10/9 and 5/5 mm) from the Red Sea ("Mer Rouge") collected by Jousseaume in 1893. Several old jars were found at the MNHN with material collected from the Red Sea by Jousseaume. Most jars are labeled as "1892" for collecting date, one jar is labeled as "1893" (MNHN-IM-2000-33678), and another as "1823" (MNHN-IM-2000-33698). The jar with the (erroneous) collecting date of 1823 is the type series of Onchidium durum (see below). The jar with a collecting date of 1893 matches perfectly the information provided in Labbé's original description of P. anomala and even the animal sizes match (MNHN-IM-2000-33678): these two specimens are considered to be the type series of P. anomala, and the largest specimen is designated as the lectotype. Both the lectotype and the paralectotype were dissected by Labbé. The radula and female and male reproductive parts of the lectotype are missing (the lack of penis and accessory penial gland, mentioned by Labbé, but likely due to the lectotype being not fully mature, cannot be checked). Dorsal gills are present on the notum. Its intestinal loops are not of type II (Labbé 1934a: 195), but of type I instead (Fig. 86B). The paralectotype is largely destroyed but bears dorsal gills on the notum. The type material mentioned in the original description also includes a paralectotype from Mauritius which could not be located with certainty at the MNHN, a paralectotype from the Red Sea which could not be located at the MNHN, and another individual missing from one of the jars from the Red Sea (see below). Most importantly, the type specimens belong to more than one species, so a lectotype is designated to clarify the application of the name P. gondwanae.
Originally, no jar clearly labeled as the type material of P. gondwanae was found at the MNHN, but most of the type material could be traced back. The original description of P. gondwanae is based on 38 individuals which Labbé, as often, listed in his article using italicized letters: a) three individuals from Bombay and one individual from The specific name "gondwanae" was written in pencil only on two old jars at the MNHN. One jar contains four of the five "c" individuals collected from the Red Sea by Jousseaume in 1892 (MNHN-IM-2000-33683); the name "gondwanae" is written on the small label with the number "59;" the size of the four specimens (40/30 mm) matches the size provided by Labbé. Another jar contains the 13 "e" individuals collected from the Red Sea by Jousseaume in 1892 (MNHN-IM-2000-33688); this jar was found only labeled as "57 gondwanae," i.e., with no locality, collector name, or collecting year, but the number of individuals and their size (32/25 to 25/20 mm) matches the size provided by Labbé (35/25 mm).
No other jar labeled as P. gondwanae was found at the MNHN, but most of the remaining type material could be traced back thanks to the matching of collector's name, collecting date, specimen sizes, and the number of old jars from any given locality at the MNHN. There are only three old jars with specimens from Bombay at the MNHN. One jar contains seven Platevindex individuals collected by Roux in 1826. The two other jars contain the three "a" individuals from Bombay: one jar contains two individuals (  Both specimens match the size provided by Labbé for the "b" individual (60/50 mm).  listed only once a specimen from Mauritius by Mathieu in his entire work, and that specimen could be the one in either jar (i.e., MNHN-IM-2000-33686 or MNHN-IM-2000. Finally, the "a" individual identified from the Red Sea could not be located. The 29 mm long "a" individual from Bombay, dissected by Labbé, is designated here as the lectotype of Paraperonia gondwanae (MNHN-IM-2000-33681). Its radula and male parts are missing. Its intestinal loops are clearly of type I (Fig. 84A) even though Labbé described loops of type V. The 50 mm long "a" individual from Bombay was also dissected by Labbé (MNHN-IM-2000-33682). Its radula and male parts are missing but its intestinal loops are of type V (Fig. 21B), as in the original description, so it does not belong to P. verruculata but P. madagascariensis instead. Labbé dissected only two of the 15 specimens from Suez (MNHN-IM-2000-33684): the radula and the male parts are missing from both specimens (38/32 and 35/28 mm) but their intestinal loops are both of type I (Fig. 86D), suggesting that they belong to P. verruculata, even though Labbé described loops of type V. Labbé dissected only one (40/30 mm) of the four specimens from Suez (MNHN-IM-2000-33683), acknowledging that maybe one specimen was lost: the radula and the male parts are missing, but its intestinal loops are of type V (Fig. 21C), as in the original description, suggesting that it belongs to P. madagascariensis. Labbé dissected seven of the 13 specimens (assumed to be) from the Red Sea (MNHN-IM-2000-33688). Those specimens are all completely destroyed and extremely poorly preserved. An undissected individual (35/25 mm) from the same lot was dissected for the present study and its intestinal loops are of type I, suggesting that it belongs to P. verruculata (Fig. 86E). Finally, according to Labbé, the intestinal loops of the specimen from Mauritius (collected by Mathieu) are of type V. One specimen collected by Mathieu from Mauritius is completely empty inside (MNHN-IM-2000-33687). The loops of the other specimen are of type I (Fig. 9D), suggesting that it belongs to P. peronii (MNHN-IM-2000-33686).
Lectotype  labeled as the type material of Scaphis viridis was found at the MNHN. However, only one old jar was found at the MNHN with specimens collected from Thursday Island, and the collecting information on the label (specimens collected by M. Lix in 1892) matches the information provided in Labbé's original description of S. viridis (even though, according to Labbé, the specimens were collected in 1890). The sizes provided by Labbé (48/20,47/30,and 42/25 mm) match the sizes of the three specimens here and their notum clearly bears dorsal gills, as in the original description of S. viridis. Labbé mentioned four specimens but, given that he provided measurements for only three specimens, it is possible that he only examined three specimens. Or he examined four specimens and one is now missing. The three type specimens are largely destroyed inside (due to Labbé's dissections). The male parts and radula are missing in both paralectotypes but are still inside the lectotype. The intestinal loops of the lectotype are of type I, with a transitional loop at 5 o'clock (Fig. 80F). The three types are green (hence the specific name chosen by Labbé) but that color is clearly due to preservation.
Holotype (Scaphis carbonaria). New Caledonia • holotype, by monotypy, 40/26 mm; 1880; Réveillère leg.; MNHN-IM-2000-33708. Originally, no jar clearly labeled as the type material of Scaphis carbonaria was found at the MNHN. However, of the several old jars found at the MNHN with specimens collected from New Caledonia, only one matches perfectly the information provided in Labbé's original Other jars with specimens from New Caledonia were collected by Fisher in 1878 or by François in 1894. Therefore, it is extremely likely that the specimen collected by Réveillère in 1880 and identified as "Peronia" is the holotype, by monotypy, of Scaphis carbonaria. The size of the holotype (40/26 mm) matches the size provided by Labbé in the original description of S. carbonaria (36/25 mm). Its notum is not well preserved. Dorsal papillae are quite flattened (as pointed out by Labbé) and dorsal eyes cannot be seen, likely because their black color faded. However, dorsal gills are clearly present on the notum. Its intestinal loops are of type I (Fig. 80D) but its radula is missing.  . Abbreviations: ag accessory penial gland, dd deferent duct, fgm female gland mass, hg hermaphroditic gland, ms muscular sac, ov oviduct, ps penial sheath, rm retractor muscle, rs receptaculum seminis, sp spermatheca, v vestibule.
The posterior (female) reproductive parts are still present but poorly preserved. The copulatory parts are missing, except for the muscular sac of the accessory penial gland (approximately 10 mm long) and so the length of the spine of the accessory penial gland cannot be checked (it was not mentioned by Labbé in the original description).
The original description of S. gravieri is based on seven individuals: two individuals (10/7.5 and 8/6.5 mm) from Djibouti collected by Gravier in 1904; four individuals (32/29 and 30/25 mm) from Zanzibar collected by Grandidier (the French naturalist and explorer Alfred Gandidier ) in 1865; and one individual (28/19 mm) from Mayotte collected by Ach. Vimont in 1883.
One old jar was found at the MNHN with a specimen from Mayotte (MNHN-IM-2000-33695). The information on the label (specimen collected from Mayotte by Vimont in 1883) matches the information provided in Labbé's original description of S. gravieri, and the specimen size also matches. Therefore, that specimen from Mayotte is here considered to form part of the type series of S. gravieri and designated as the lectotype (MNHN-IM-2000-33695). This lectotype was dissected by Labbé: the radula and the posterior (hermaphroditic) reproductive parts are still in place but the male parts are missing. The intestinal loops are of type I with a transitional loop at 6 o'clock (Fig. 85A).
Another old jar was found at the MNHN with specimens from Zanzibar (MNHN-IM-2000-33693). The information on the label (specimens collected from Zanzibar by Grandidier in 1865) matches the information provided in Labbé's original description of S. gravieri, and the specimen size also matches (Labbé likely provided the size of the largest two specimens). Therefore, those four specimens from Zanzibar are considered to form part of the type series of S. gravieri and are now paralectotypes (MNHN-IM-2000-33693). Only one paralectotype (30/28 mm) from Zanzibar was dissected by Labbé: the radula and the posterior (female) reproductive parts are still in place but the male parts are missing. The intestinal loops are of type I.
The two paralectotypes from Djibouti could not be traced with certainty. There are two old jars of specimens collected by Gravier in 1904 at the MNHN. One jar is labeled with Obock as locality (not Djibouti, even though Obock is in Djibouti) and contains one Peronia specimen of which the size (80/60 mm) does not match Labbé's original description of S. gravieri. Also, that specimen from Obock is more likely to be a paralectotype of P. gaimardi or a non-type specimen used by Labbé for the re-description of Peronia tongana. The three specimens (70/60, 70/65, and 65/65 mm) of the second jar collected by Gravier in 1904 are from Djibouti (MNHN-IM-2000-33696), which matches perfectly the original description of S. gravieri by Labbé. The problem is that the specimen sizes do not match because Labbé described two individuals of only 10/7.5 and 8/6.5 mm. It is likely that Labbé meant centimeters instead of millimeters (even though he wrote "mm") because he described a muscular sac of 8 mm in the specimens from Djibouti, which is impossible in individuals that are only 8 and 10 mm Figure 96. Reproductive system, Peronia verruculata, Red Sea, spm #1 (ZMH 27472/4) A posterior, hermaphroditic (female) reproductive system B anterior, male, copulatory apparatus. Scale bars: 2 mm (A), 4 mm (B). Abbreviations: Abbreviations: ag accessory penial gland, dd deferent duct, fgm female gland mass, hg hermaphroditic gland, ms muscular sac, ov oviduct, p penis, ps penial sheath, rm retractor muscle, rs receptaculum seminis, sp spermatheca. long. One of three specimens, possibly dissected by Labbé, possibly is part of the type series of S. gravieri, but it remains questionable. In addition, a specific name was added in pencil on an old label with the number "69" but that name, which is impossible to read, seems to start with a J, and not a G. In summary, it remains unclear whether those three specimens from Djibouti can be regarded as part of the type series of S. gravieri; however, it ultimately does not matter because a lectotype is designated here.
Syntypes (Scaphis tonkinensis). The type material of Scaphis tonkinensis (ten syntypes up to 20/18 mm, according to the original description) could not be located with certainty at the MNHN. Only one old jar was found at the MNHN (MNHN-IM-2000-33700) with specimens collected from Vietnam (as "Tonkin"), and the information on the label (material collected by M. Julien in 1874) matches the informa- tion provided in Labbé's original description of S. tonkinensis. Therefore, it is possible that the jar mentioned here contains the type material of S. tonkinensis. Unfortunately, the jar only contains three pieces of unidentifiable and poorly preserved tissue (each piece measuring approximately 20/10 mm). Two pieces are likely not even part of an onchidiid slug, and it is unclear whether the third piece is part of an onchidiid dorsal notum or not. So, regardless of whether this material is regarded as part of the type material of S. tonkinensis, it is basically useless.
Syntypes (Scaphis lata). The type material of Scaphis lata (four syntypes up to 28/28 mm, from Vietnam) could not be located at the MNHN. Only one old jar was found at the MNHN (MNHN-IM-2000-33700) with specimens collected from Vietnam (as "Tonkin"), but the information on the label (specimens collected by M. Julien, in 1874) does not match exactly the information provided in Labbé's original description of S. lata (specimens collected by M. Julien in 1878), and, instead, matches the information provided in Labbé's original description of S. tonkinensis (see above). Several old jars were found at the MNHN with material collected from the Red Sea by Jousseaume. Most jars are labeled with 1892 as collecting date, one jar is labeled with 1893, and another with 1823. The jar with a collecting date of 1893 (MNHN-IM-2000-33678) contains the type series of P. anomala (see above). On the jar with the collecting date of 1823, there is another tiny label with the number "61" (for an unknown numbering system) on which O. durum is clearly written in pencil. It is one of the very few cases in which a species name is indicated for some MNHN mate- rial studied by Labbé and there is little doubt that the specimens are the type series of O. durum, especially because the number of individuals and their sizes perfectly match with Labbé's original description. Clearly, 1823 is a mistake for 1893. Most importantly, contrary to what was described by Labbé, gills are present on the dorsal notum of those individuals. All specimens are poorly preserved. They likely dried at some point and their body is hard. Three specimens were opened by Labbé and are now largely destroyed with only the digestive system inside. A lectotype is designated here in order to clarify the application of O. durum (specimens in the type series could belong to more than one species). Its intestinal loops are not of type II (Labbé 1934a: 221): they clearly are of type I (Fig. 86C).
Holotype and paratypes (Peronia persiae). The original description of P. persiae is based on a total of 14 individuals (from 13 to 37 mm): the four types (see above) and ten other specimens from the same two localities as the types. DNA sequences (COI and 16S) are provided for 11 of those 14 individuals, including all four type specimens. However, it is unclear which GenBank sequences correspond exactly to the holotype because this information is missing in GenBank as well as in Maniei et al. (2020a: table 2). It is assumed that the holotype, called "specimen LA7" in Maniei et al. (2020a: table 1), corresponds to the individual called "voucher LaFM7S" in GenBank. Ultimately, it does not matter at all because all mitochondrial sequences of P. persiae cluster together within the unit #4 of P. verruculata: only the COI (MK993404) and the 16S (MK993392) sequences of the "voucher LaFM7S" are included in our phylogenetic analyses to represent P. persiae (Fig. 2). Finally, note that the COI and 16S GenBank accession numbers are switched for P. persiae in Maniei et al. (2020a: [1795]; Sumatra, (MH002570) for an individual identified as Peronia sp. and collected from Singapore . This individual as well as others referred to as "Peronia sp. 2" by Chang et al. (2018), following Dayrat et al. (2011), clearly belong to the mitochondrial unit #1 of Peronia verruculata (Fig. 2). A third COI sequence was obtained from  GenBank (LC390389) for an individual identified as Peronia sp. and collected from Sakurajima, Kagoshima, Japan (Tagaki et al. 2019), which is south to the northernmost known locality near the Seto Marine Biological Laboratory (see material examined). This individual as well as others from "Group V" were referred to as "Peronia sp." by Takagi et al. (2019) and clearly belong to the mitochondrial unit #1 of Peronia verruculata (Fig. 2). Four COI sequences were obtained from GenBank (JN543152, JN543153, JN543154, JN543165) for individuals from the coast of China, from Hainan (18°N) to Fujian (26°N) . These individuals were referred to as "Peronia verruculata" by Sun et al. (2014) and clearly belong to the mitochondrial unit #1 of Peronia verruculata (Fig. 2). Finally, the COI (MK993404) and 16S (MK993392) sequences of the "voucher LaFM7S" represent P. persiae (Fig. 2): all published mitochondrial sequences of P. persiae cluster together within the unit #4 of P. verruculata so only one individual is needed to represent P. persiae. Distribution (Fig. 6). Peronia verruculata is the most widespread of all onchidiid species. Its most western records are known from the Red Sea and southern Mozambique (26°S). Its most eastern records are in Japan, Wakayama (33°N), Vanuatu,and Queensland (21°S). It is unclear how far south it is distributed in southeastern Australia, although we did not find it in Sydney, New South Wales (see remarks below as well as remarks on P. sydneyensis). Undoubtedly, the delineation and distribution of the mitochondrial units of P. verruculata will change as new DNA sequences are added, especially from the Arabian Sea, the Red Sea, southern India, as well as southeastern Australia (see species remarks). Note that the range of P. verruculata is continuous. Even though our molecular analyses do not include specimens of P. verruculata from places like southern India, the Persian Gulf, or the northwestern corner of the Indian Ocean (coasts of Somalia, Yemen, and Oman), P. verruculata must be present there (red areas in Fig. 6). As of today, units #1 and #2 are sympatric in southeastern Sumatra (we found them both together at our stations 78 and 82), and units #1 and #3 are sympatric in Singapore.
Peronia verruculata also is very abundant and has been very often recorded in the past. However, Peronia species are externally cryptic and can be easily misidentified and confused. Here the records that are positively confirmed are distinguished from the records that cannot be confirmed. Erroneous applications of the name P. verruculata (or some of its synonyms) are also listed. All the details can be found in the species remarks (see below).
Onchidium durum was named after the hard (durum in Latin) notum of the preserved type specimens.
Onchidium elberti was named after Dr. J. Elbert, who collected the holotype in 1909.
Onchidium ferrugineum was named after the rusty (ferrugineum in Latin) color of the live individuals collected by Lesson which belong to two different species: the lectotype belongs to Peronia verruculata (unit #1) and the paralectotypes to Wallaconchis ater. The dorsal notum of some individuals of W. ater can be homogenously of rusty color (e.g., Goulding et al. 2018b: fig. 36F) but individuals of P. verruculata (unit #1) are not typically of rusty color, although their notum commonly displays red patches. Lesson's (1833: pl. 19, figs 1, 2) illustrations of Peronia ferruginea in his Illustrations de Zoologie represent a Peronia slug with a dorsal notum that is homogenously of rusty color: it almost looks like an individual of Wallaconchis ater to which dorsal gills were artificially added.
Peronia gaimardi was named after Joseph Paul Gaimard , who collected (with Jean René Constant Quoy) the type material in Vanikoro in 1829 during a voyage of the Astrolabe.
Paraperonia gondwanae was named after its supposedly Gondwanan distribution (Red Sea, Mauritius, western India, and Torres Strait).
Scaphis gravieri was named after Charles Joseph Gravier , professor of zoology (worms and crustaceans) at the MNHN, who collected two paralectotypes from Djibouti.
Scaphis lata was named after the broad (lata in Latin) and circular shape of preserved type specimens.
Peronia persiae was named after the Persian Gulf. Peronia savignii was named after Marie Jules César Lelorgne de Savigny [1777Savigny [ -1851, a French zoologist who participated in Napoleon's expedition to Egypt and published a plate of illustrations for gastropods (including onchidiids) in the Description de l'Egypte (Savigny 1817: pl. 2).
Scaphis tonkinensis was named after its type locality in Tonkin, i.e., Vietnam.
Onchidium verruculatum was named after the dorsal notum covered with warts (verruculatum in Latin).
Scaphis viridis was named after the (artificial) green color of the preserved type specimens.
Habitat (Figs 65-69). Unit #1 is found in a large variety of habitats. It is predominantly found on rocks in the rocky intertidal (including man-made structures). It can also be found on huge and isolated boulders on a sandy beach or in coral rubble mixed or not with sand. The rocks on which the unit #1 is found can be associated or not with sparse mangrove trees. It is also found on sandy mud inside or nearby mangroves. Exceptionally, it can be found on old logs inside muddy mangroves. Unit #2 is found on coral rubble and rocks on sandy beaches. Unit #3 is found on rocks on a beach and in the rocky intertidal. Unit #4 is found in the rocky intertidal. Unit #5 is found in the rocky intertidal as well as on mud, sandy or not. There was no habitat data on the labels of the material studied here for unit #6 but it is most likely found in the rocky intertidal, like the other units of Peronia verruculata. Peronia verruculata is extremely common across its entire distribution. In localities where they overlap geographically, the different mitochondrial units are found more or less in equal abundance (units #1 and #3 in Singapore,and units #1 and #2 in southeastern Sumatra). Peronia verruculata is commonly found during the day, even though a few individuals were also collected at night.
Color and morphology of live animals (Figs 70-77). In unit #1, live animals are not covered with mud, but they can often bear tiny pieces of various materials, such as sand and broken shells (Figs 70-73). The background color of the dorsal notum is highly variable, most often brown (light to dark), or greenish, and occasionally even black. The background is mottled with darker areas, occasionally with red areas. In most animals, the color of the dorsal papillae varies as that of the background itself. In some animals, however, the tip of the dorsal papillae (with and without dorsal eyes) can be bright yellow. The color of the foot is the same as that of the hyponotum, which varies greatly from pure white to dark blue-green. In most animals, the ventral surface is yellowish-greenish or yellowish-bluish. The ventral color (foot and hyponotum) of an individual can change rapidly, especially when disturbed. The ocular tentacles are brown-grey (variable from light to dark), like the head. The ocular tentacles are short (just a few millimeters long). Preserved specimens no longer display the colors of live animals. Colors tend to fade rapidly with preservation.
The dorsal notum of live animals is covered by dozens of papillae of various sizes. Those papillae do not retract within the notum, whether animals are disturbed or not, and so the dorsal notum is never smooth. Larger papillae are not arranged in two longitudinal and lateral ridges (on either side of the median line), even though larger papillae are mostly concentrated in the central area of the dorsal notum. Some papillae bear from one to five black dorsal eyes at their tip (most papillae bear three eyes). The number of papillae with dorsal eyes is variable (from 10 to 22) and papillae in the central area of the dorsum tend to bear more eyes than those on the side. Occasionally, papillae can bear more than five eyes: a central, large papilla can bear up to eight eyes but, like other papillae, is not fully retractable within the notum. The exact number of papillae with eyes can be difficult to count because papillae are often dark, and because the eyes, which are located at the tip of the papillae, can be seen only if papillae are relaxed. Dorsal gills are present on the posterior third of the dorsal notum. Dorsal gills are most easily observed when animals are relaxed under water. When slugs are not under water, dorsal gills are retracted and hard to see. If animals were not relaxed before preservation, gills can be retracted and hard to see in preserved specimens (the best relaxation method is to immerse live specimens in a solution of magnesium chloride).
The color variation in unit #2 (Fig. 74) and unit #3 (Fig. 75) is similar to the color variation in unit #1, and specimens cannot be separated where units overlap geographically (in Singapore for units #1 and #2, and in southeastern Sumatra for units #1 and #3). The number of papillae with dorsal eyes observed in unit #2 (from 14 to 22) and in unit #3 (from 10 to 18) is within the range observed in unit #1. Slight differences may be due to a more limited sampling.
In unit #4, the color of the dorsal notum is brown, mottled with darker and lighter areas (Fig. 76). The ventral surface (foot and hyponotum) is brown-greyish. The number of papillae with dorsal eyes varies from 10 to 18.
In unit #5, the dorsal notum is brown, light to dark, mottled with darker areas (Fig. 77). The ventral surface (foot and hyponotum) is yellowish, greenish, or bluish, and can change rapidly in any given individual. The number of papillae with dorsal eyes varies from 10 to 20.
Pictures of live animals were not available for unit #6 (Red Sea). The dorsal color of preserved specimens is beige with faded darker areas. The ventrum is beige. The number of papillae with dorsal eyes varies from 10 to 18, but the black eye color possibly faded in some of them.
The largest specimens are 60 mm long in unit #1, 55 mm long in unit #2, 40 mm long in unit #3, 60 mm long in unit #4, 50 mm long in unit #5, and 40 mm long in unit #6. Exceptionally, one individual in New Caledonia was 73 mm long (unit #1).
External morphology (Fig. 78A-C). The body is not flattened. The notum is oval. The hyponotum is horizontal in live animals. The orientation of the hyponotum as well as the shape of the dorsal notum of preserved animals greatly vary depending on preservation. The width of the hyponotum relative to the total width of the ventral surface (pedal sole and hyponotum) varies among individuals but is approximately one third. In the anterior region, the left and right ocular tentacles are superior to the mouth. Eyes are located at the tip of the two ocular tentacles. Inferior to the ocular tentacles, superior to the mouth, the head bears a pair of oral lobes. The latter are smooth, with no transversal protuberance. The male opening (of the copulatory complex) is below and to the left of the right ocular tentacle (i.e., between the two ocular tentacles, but closer to the right than to the left tentacle). The anus is posterior, median, close to the edge of the pedal sole. On the right side (to the left in ventral view), a peripodial groove is present at the junction between the foot and the hyponotum, running longitudinally all the way from the head to the posterior end. The female pore, which marks the posterior end of the peripodial groove, is located a few millimeters from the anus and the pneumostome, which does not vary much among individuals. The pneumostome is median. Its position on the hyponotum relative to the notum margin and the edge of the pedal sole varies among individuals but averages in the middle.
Visceral cavity and pallial complex. The anterior pedal gland is small, more or less round, and flattened, lying on the floor of the visceral cavity below the buccal mass and below a thin layer of connective tissue (it can be hard to detect). The heart, enclosed in the pericardium, is on the right side of the visceral cavity, slightly posterior to the middle. An anterior vessel supports several anterior organs such as the buccal mass, the nervous system, and the copulatory complex. The kidney is nearly symmetrical, the right and left parts being equally developed. The kidney is intricately attached to the respiratory complex. The lung is posterior in two more or less symmetrical parts, left and right, which are joined in the middle.
Nervous system (Fig. 78D). The circum-esophageal nerve ring is post-pharyngeal and pre-esophageal. The paired cerebral ganglia are separated by a short cerebral commissure of which the length varies among individuals. Paired pleural and pedal ganglia are also all distinct. The visceral commissure is short but distinctly present and the visceral ganglion tends to be slightly to the left. Cerebro-pleural and pleuro-pedal connectives are short and pleural and cerebral ganglia touch each other on either side. Nerves from the cerebral ganglia innervate the buccal area and the ocular tentacles and, on the right side, the penial complex. Nerves from the pedal ganglia innervate the foot. Nerves from the pleural ganglia innervate the lateral and dorsal regions of the mantle. Nerves from the visceral ganglia innervate the visceral organs. Ganglia are commonly surrounded by almost transparent connective tissue through which they can be observed.
Digestive system 82A,83A,84A,B,(85)(86)(87)(88)(89)(90)(91)(92)(93). There are no jaws. The left and right salivary glands, heavily branched, join the buccal mass dorsally, on either side of the esophagus. The esophagus is narrow and straight, with thin internal folds. The esophagus enters the stomach anteriorly (Fig. 79). Only a portion of the posterior aspect of the stomach can be seen in dorsal view because it is partly covered by the lobes of the digestive gland. The dorsal lobe is mainly on the right. The left, lateral lobe is mainly ventral. The posterior lobe covers the posterior aspect of the stomach. The stomach is a U-shaped sac divided into four chambers (Fig. 79). The first chamber, which receives the esophagus, is delimited by thin tissue, and receives the ducts of the dorsal and lateral lobes of the digestive gland. It is internally smooth (with no ridges). The second, posterior chamber, delimited by thick muscular tissue (which takes most of the space inside), receives the duct of the posterior lobe of the digestive gland. The third, funnel-shaped chamber is delimited by thin tissue with high leaflet-like ridges internally. The fourth chamber is continuous and externally similar to the third, but it bears only low, thin ridges internally.
The intestine is long and narrow. Intestinal loops were checked in every specimen listed in the material examined: the intestinal loops are of type I with a transitional loop oriented between 3 and 6 o'clock 82A,83A,84A,B,85,86). There is no rectal gland.
The radula is in between two large postero-lateral muscular masses (Figs 87-93). Each radular row contains a rachidian tooth and two half rows of lateral teeth of similar size and shape. Examples of radular formulae are presented in Table 5. The rachidian teeth are unicuspid (Fig. 87A): the median cusp is always present; there are no conspicuous cusps on the lateral sides of the base of the rachidian tooth. The median cusp of the rachidian teeth is approximately 40 μm long. The lateral aspect of the base of the rachidian teeth is straight. The half rows of lateral teeth form an angle of 45° with the rachidian axis. Except for the few innermost and few outermost lateral teeth, the size and shape of the lateral teeth do not vary along the half row, nor do they vary among half rows. The lateral teeth are unicuspid with a flattened and curved hook (approximately from 80 to 120 μm long) with a rounded tip, but there is also a pointed spine on the outer lateral expansion of the base, or basal lateral spine (Fig. 87D). In most cases, the basal lateral spine cannot be observed because it is hidden below the hook of the next, outer lateral tooth. It can only be observed when the teeth are not too close (such as in the innermost and outermost regions) or when teeth are placed in an unusual position. The inner and outer lateral aspects of the hook of the lateral teeth are straight (i.e., not wavy and not with any protuberance).
The female organs are located (with some male parts) at the posterior end of the visceral cavity (Figs 82B, 83B, 84C, 94A-C, 95A, B, 96A). The hermaphroditic gland is a single mass, joining the spermoviduct through the hermaphroditic duct (which conveys the eggs and the autosperm). There is a narrow, elongated receptaculum seminalis (caecum) along the hermaphroditic duct. The female gland mass contains various glands (mucus and albumen) which can hardly be separated by dissection and of which the exact connections remain uncertain. The hermaphroditic duct becomes the spermoviduct (which conveys eggs, exosperm, and autosperm). Proximally, the spermoviduct is not divided (at least externally) and is embedded within the female gland mass. Distally, the spermoviduct branches into the straight deferent duct (which conveys the autosperm up to the anterior region, running through the body wall) and the oviduct. The free oviduct conveys the eggs up to the female opening and the exosperm from the female opening up to the fertilization chamber. The large, spherical-ovate spermatheca connects to the oviduct through a short duct. The oviduct is narrow and straight. There is no vaginal gland.
The penial sheath is narrow and elongated. The penial sheath protects the penis for its entire length. The beginning of the retractor muscle marks the separation between the penial sheath (and the penis inside) and the deferent duct, which is highly coiled. The retractor muscle, which can be shorter or longer than the penial sheath, inserts at the posterior end of the visceral cavity. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube. Its distal end bears conical hooks which are less than 50 μm long in units #1 and #2, less than 55 μm long in units #5 and #6, and less than 60 μm in units #3 and #4 (Figs 97-102). When the penis is retracted inside the penial sheath, the hooks are densely packed inside the tube-like penis; during copulation, the penis is evaginated like a glove and the hooks are outside, not as densely packed. In some individuals of unit #4, a few penial hooks are exceptionally double, or twopronged (Fig. 100C).
The accessory penial gland is a long, tube-like flagellum with a proximal dead end. The length of the flagellum of the penial gland varies among individuals but it is [991] would be assigned to the mitochondrial unit #3 because the spine is longer than 200 μm in unit #3 while it usually is less than 200 μm in unit #1, but it belongs to the mitochondrial unit #1 (Fig. 2). The diameter of the tip of the spine only partly overlaps between unit #1 (from 35 to 50 μm) and unit #3 (from 40 to 80 μm), but that trait is hardly practical when it comes to identification (it requires SEM). The units #1 and #2 are sympatric in Sumatra (we found them both together at the stations 78 and 82) but they cannot be separated because they are completely cryptic anatomically ( Table 4). All that is not to say that there are no anatomical differences between units of P. verruculata. On average, the diameter of the spine of the accessory penial gland tends to be larger both at the base and at the tip in unit #3. However, because ranges of variation overlap, anatomical traits cannot be used to reliably assign individuals to any particular unit.
Peronia verruculata is close anatomically to P. sydneyensis and P. willani. They all share intestinal loops of type I with a transitional loop oriented between 3 and 6 o'clock. There are, however, important differences. The muscular sac of the accessory penial gland is significantly longer in P. willani (up to 25 mm) than in P. verruculata (up to 15 mm); the spine of the accessory penial gland is significantly shorter in P. sydneyensis (less than 1 mm) than in P. verruculata (at least 1.3 mm); strong, hemispherical protuberances cover the spine in all individuals of P. sydneyensis and are absent in all other species. Peronia sydneyensis and P. verruculata cannot be confused even where they are sympatric (Queensland and New Caledonia) and Peronia verruculata and P. willani are not sympatric based on current data.
Remarks. Species delineation. Our decision of recognizing a single species with high population structure and several mitochondrial units is explained in the results (see species delineation). Fresh material from the Red Sea, Somalia, Yemen, Oman, and the Persian Gulf is needed to determine the relationships between the populations of P. verruculata from the Red Sea and the remainder of the species. Similarly, fresh material is needed from southwestern and southeastern India, including Sri Lanka, to determine the relationships between the western (Indian Ocean) and eastern (South-East Asia and West Pacific) populations. Most likely, additional populations will show that mitochondrial units are even more mixed than what is already shown here, and new units may be found. Nuclear markers will remain indispensable as the current data show that populations that seem divergent using mitochondrial markers are not reproductively isolated. It is not excluded that populations from the Red Sea belong to two distinct species (both with intestinal loops of type I): P. verruculata and another species endemic to the Red Sea. The Peronia diversity in the Red Sea would thus be similar to what is found in Japan, which is also at the periphery of the distribution of Peronia (Fig. 6).
Synonymy. The application of all the species names regarded as junior synonyms of P. verruculata is addressed here, following a chronological order starting with P. verruculata (Tables 1, 6).
Based on our data, there are two Peronia species in the Red Sea, one characterized by intestinal loops of type I (with a transitional loop oriented between 3 and 6 o'clock) and  Simroth, 1920 Sulawesi, Indonesia Onchidium astridae  West Papua, Indonesia Peronia gaimardi  Vanikoro, Solomon Islands Scaphis viridis  Torres Strait, Australia Scaphis carbonaria  New Caledonia Scaphis tonkinensis  Vietnam Scaphis lata  Vietnam Persian Gulf, Pakistan & western India Paraperonia gondwanae  Mumbai, western India Peronia persiae Maniei et al., 2020a Persian Gulf, Iran #5 Mozambique & Madagascar Scaphis gravieri Mayotte Red Sea Onchidium verruculatum Cuvier, 1830Red Sea Peronia savignii Récluz, 1869 Red Sea Peronia anomala  Red Sea Onchidium durum  Red Sea the other characterized by intestinal loops of type V (see remarks on P. madagascariensis).
Because the intestinal loops of the lectotype of O. verruculatum are of type I (Fig. 86A), P. verruculata applies to the species described here with intestinal loops of type I. The original description of Onchidium ferrugineum was published four times in different venues by Lesson, twice in 1831 (first in the Bulletin des sciences naturelles and then in the zoology section of the Coquille voyage), once in February 1832 (in the Mémorial encyclopédique), and once again in 1833 (in his Illustrations de Zoologie). According to Cretella (2010), the date of publication for the description of O. ferrugineum in the Coquille voyage is November 15, 1831. Therefore, the oldest and original description of O. ferrugineum is the one published in the Bulletin des sciences naturelles in April 1831. Both descriptions from 1831 did not include any illustration. An illustration of an animal ventral view was published by Lesson (1832: 36-37, fig. 32) in the Mémorial encyclopédique. Two beautiful, colored pictures were published in Lesson's (1833: pl. 19) Illustrations de Zoologie.
The type locality (of the lectotype) of Onchidium ferrugineum is Manokwari, West Papua, Indonesia, where at least three Peronia species are known to be present (Fig. 6). Based on the length of the lectotype (35 mm) and its intestinal loops of type I with a transitional loop at 4 o'clock ( Fig. 80A), Onchidium ferrugineum applies to the species described here (P. verruculata) and not to P. griffithsi or P. peronii (Table 4). Unfortunately, this identification cannot be confirmed by the muscular sac or the spine of the accessory penial gland, which are missing in the lectotype. Labbé (1934a: 213-216) claims that there is no accessory penial gland in Onchidium ferrugineum and thus does not comment on the spine and the muscular sac of the accessory penial gland of the lectotype. It is unclear whether Labbé dissected the lectotype or if he found it already dissected by Lesson (who commented on the penis of the paralectotypes and thus might have dissected the lectotype as well). Eleven dorsal papillae with eyes were counted on the lectotype, but it is possible that others faded with time. Lesson (1833: pl. 19) transferred Onchidium ferrugineum to Peronia. In the written description, Lesson (1833: unnumbered page) considered Peronia ferruginea the type of a genus which he decided to call Peronia, following Blainville, but the type species of Peronia is O. peronii, by monotypy, and the author of Peronia is Fleming (1822a, b). Oken (1834b: 269-270) reported Peronia ferruginea from Lesson's (1833: pl. 19) Illustrations de Zoologie. Van der Hoeven (1850: 786;1856: 817) suggested, based on Lesson's (1833: pl. 19) own illustration, that Peronia ferruginea may be a nudibranch instead of an onchidiid, but there is no question that Peronia ferruginea applies to an onchidiid species. Gray (1850: 117), Adams and Adams (1855: 235), and Tapparone Canefri (1883: 214) classified O. ferrugineum in Peronia but other authors preferred the original combination with the generic name Onchidium. Semper (1882: 268) kept O. ferrugineum in Onchidium and regarded it as a questionable name because he (erroneously) thought that its original locality was unknown. Plate (1893) did not comment on it. Bretnall (1919: 326-327) thought that O. ferrugineum referred to a species insufficiently known and merely repeated Lesson's original description. Bretnall (1919: 326) also suggested that O. ferrugineum seemed "closely related to that of M. de Blainville," i.e., Peronia mauritiana, a synonym of P. peronii. Solely based on information from the original description, Hoffmann (1928: 71, 74) regarded O. ferrugineum as a junior synonym of O. verruculatum and disagreed with Bretnall that it could refer to O. peronii. However, the application of O. ferrugineum cannot be deduced from the original description, especially because it is based on Peronia and Wallaconchis specimens (see above the remarks on the type material). Finally, Labbé (1934a: 213-216), who re-examined the type specimens of O. ferrugineum, created the generic name Lessonia (later replaced by Lessonina) for O. ferrugineum, for a genus characterized by a unique combination of traits (large and coiled penis, dorsal gills, etc.), without realizing that the types of O. ferrugineum were part of two species from two different genera.
The two syntypes of Onchidium branchiferum are from Manila, Luzon, Philippines. Anatomical traits described by Plate (insertion of the retractor muscle of the penis at the end of the visceral cavity, spine of the accessory penial gland 1 mm long) indicate that O. branchiferum applies to P. verruculata, even though they cannot be confirmed on the syntypes in which all internal organs are either missing or destroyed (Table 4). Plate did not draw the intestinal loops but he describes them as being type I (the orientation of the transitional loop is unknown). The number of radular teeth per half row (88) also matches what is known in P. verruculata (Table 5). According to Plate (1893: 184), O. branchiferum is easily recognizable because its branchial plumes are only present on the posterior end of the dorsum (posterior sixth). However, this trait is not distinct from other species and varies depending on preservation (gills are often retracted in preserved specimens and can only be observed if specimens were carefully relaxed before preservation). Plate (1893) did not provide any other feature supporting O. branchiferum as a distinct species, and he did not compare it with any other existing species. Onchidium branchiferum is regarded here as a new junior synonym of P. verruculata (Tables 1, 6). Hoffmann (1928: 75) listed Onchidium branchiferum as a valid name (solely based on information from the original description) but considered it to refer to a "local form" of O. verruculatum. Labbé (1934a: 194) transferred Onchidium branchiferum to Peronia and regarded P. branchifera as a valid name "out of deference to the eminent zoologist Ludwig Plate" even though he agreed with Hoffmann that P. branchifera most likely was just a local form of P. verruculata. Labbé's re-description of P. branchifera was based on a specimen (30/23 mm) collected by Ach. Cuming in 1844 from an unknown locality in the Philippines. There are two jars preserved at the MNHN with Peronia specimens collected by Ach. Cuming in 1844. Labbé (1934a: 192-194) also re-described a specimen collected from the Philippines by Ach. Cuming in 1844 as P. verruculata. It is not possible to determine which jar corresponds to what species in  monograph because Labbé did not indicate species identifications for any of the MNHN specimens he examined. Labbé's description of a "short penial gland" indicate that he most likely examined P. verruculata (unit #1). Finally, Marcus and Marcus (1970: 213) wrote that P. branchifera was close to P. verruculata but with no explanation.
Onchidium elberti was described by Simroth (1920) from Muna Island, southeastern Sulawesi, Indonesia, where only Peronia verruculata is known to be present (Fig. 6). Internal features of the holotype (24 mm long) are fully compatible with the anatomy of P. verruculata: intestinal loops are of type I with a transitional loop oriented at 5 o'clock ( Fig. 80B) and the muscular sac of the accessory penial gland is 8 mm long (Table 4). Eleven papillae with dorsal eyes were counted (which fits within the range of the species) but some may have faded with time. As a result, Onchidium elberti is regarded here as a junior synonym of Peronia verruculata (Tables 1, 6). Hoffmann (1928: 71, 75) thought Onchidium elberti was a junior synonym of O. verruculatum, based on information from Simroth's original description.
Onchidium astridae, the type species of Labbé's genus Scaphis, was originally described by  within the genus Onchidium. Only one specimen is known, the holotype (20/18 mm) by monotypy, from Sorong, West Papua, Indonesia. There is no doubt that Onchidium astridae applies to a Peronia species because the dorsum of the holotype bears gills. All copulatory parts are missing and Labbé did not describe the length of the muscular sac or the length of the spine of the accessory penial gland. Labbé (1934a: 213, fig. 46) described two muscular sacs instead of just one, but that could not be confirmed here. At least three Peronia species are present in West Papua (Fig. 6). However, given the size of the holotype (20 mm long) and, importantly, its intestinal loops of type I with a transitional loop at 4 o'clock ( Fig. 80C), Onchidium astridae is regarded as a junior synonym of P. verruculata (Tables 1,4,6). Note that the number of papillae with dorsal eyes could not be counted on the preserved holotype. According to , Onchidium astridae is close to Onchidium vaigiense and O. steenstrupi, but both names refer to Marmaronchis vaigiensis, a species which belongs to a distinct genus (Dayrat et al. 2018).
The original description of Peronia gaimardi was based on two specimens from Vanikoro, Solomon Islands, which were found at the MNHN, and one specimen from Djibouti, which could not be located. The type locality is Vanikoro, locality of the lectotype designated in the present study. Our molecular data demonstrate that Peronia verruculata (unit #1) is present in Vanikoro, but P. peronii and P. platei could also be found there (Fig. 6). Given the intestinal loops of type I (with a transitional loop at 5 o'clock) observed in the lectotype (Fig. 80F), P. gaimardi is regarded as a synonym of P. verruculata (Tables 1, 4, 6). The male parts of the lectotype are missing and Labbé's description of the copulatory apparatus is confusing because it is based indiscriminately on individuals from both Vanikoro and Djibouti. His measurement of the spine of the accessory gland (8 mm long) is most likely a mistake. In the present study, the longest spine (5 mm long) was found in the lectotype of P. fidjiensis (a synonym of P. peronii) from Fiji. Also, the lectotype of P. gaimardi only is 44 mm long, which would make it a very small individual of P. peronii. Given the large size (80 mm long, according to Labbé) of the paralectotype from Djibouti, it most likely belongs to P. madagascariensis, a species present there, and for which large specimens are known (Table 4). It would imply that Labbé confused its intestinal loops of type V for a type I, which is a mistake he often made. Marcus and Marcus (1970: 214) wrote that P. gaimardi might be a junior synonym of P. verruculata based on information from the original description.
Peronia anomala, originally described from the Red Sea, is regarded as a junior synonym of P. verruculata because, contrary to what Labbé indicated in the original description, Peronia anomala is characterized by intestinal loops of type I (Fig. 86B). It is assumed in this work that there is only one species of Peronia slugs with intestinal loops of type I in the Red Sea, although fresh material from the Red Sea may show that there is more than one species. Marcus and Marcus (1960: 881) suggested that P. anomala could be a synonym of P. verruculata and that intestinal loops of both types I and II are found in P. verruculata, but intestinal loops are only of type I in P. verruculata and there are no intestinal loops of type II in Peronia. Maniei et al. (2020a: table S1) took Labbé's description for granted and considered that P. anomala was characterized by intestinal loops of type II.
The type specimens used by Labbé for the original description of Paraperonia gondwanae belong to several species, because our data show that slugs with intestinal loops of types I and V necessarily belong to distinct species. The application of the name Paraperonia gondwanae is determined by the lectotype from Bombay (MNHN-IM-2000-33681) with intestinal loops of type I (Fig. 84A). Paraperonia gondwanae applies to P. verruculata, and, more specifically, to the populations of the mitochondrial unit #4 from western India and Pakistan (Fig. 6, Tables 1, 6). The paralectotypes from the Red Sea with intestinal loops of type I also belong to P. verruculata: one "e" paralectotype from the Red Sea (MNHN-IM-2000-33688), and two "d" paralectotypes from Suez (MNHN-IM-2000-33684). The paralectotypes with intestinal loops of type V belong to P. madagascariensis: one of the "a" paralectotypes from Bombay (MNHN-IM-2000-33682) and one of the "c" paralectotypes from Suez (MNHN-IM-2000-33683). The large specimen with intestinal loops of type I from Mauritius (MNHN-IM-2000-33686), which may or may not be part of the type material of P. gondwanae, likely belongs to P. peronii.
Scaphis viridis was described by Labbé based on three syntypes (four according to the original description) from Thursday Island, in the Torres Strait, Australia. The presence of P. verruculata in the Torres Strait is not demonstrated positively with fresh material. However, P. verruculata is the only species we found in northeastern Queensland (up to Cairns,16°S). None of the Peronia slugs we collected north of Bowen (20°S) were individuals of P. sydneyensis which is thought to be only distributed from southern Queensland down to New South Wales (Sydney) and eastwards to New Caledonia. More importantly, both the original description (Labbé 1934a: 207-208, figs 31-34) and the traits examined in the lectotype here confirm that S. viridis applies to P. verruculata (Table 4) fig. 32), muscular sac of the accessory penial gland 14 mm long (Labbé) and 15 mm long (lectotype), spine of the accessory penial gland 1 mm long (Labbé) and 1.7 mm long (lectotype), retractor muscle attaching at the posterior end of the visceral cavity. Because those traits are only compatible with the anatomy of P. verruculata, S. viridis is regarded here as a junior synonym of P. verruculata and which applies to the unit #1 (Tables 1, 6). Finally, a total of 13 dorsal papillae with eyes was observed in the lectotype; more may have faded with time. Labbé only compared S. viridis with Peronia acinosa, a nomen dubium which may or may not refer to an onchidiid species (see general discussion).
There are three Peronia species in New Caledonia, the type locality of Scaphis carbonaria (Fig. 6). DNA sequences of individuals from New Caledonia belong to two species in our molecular data set (P. verruculata and P. sydneyensis). Although our molecular data do not include any specimen of P. peronii from New Caledonia, it is present there based on the rest of its distribution (it is found all the way to Fiji and Tonga; Fig. 6) and on an old specimen from a historical museum collection (ANSP 203028). Two characters in Labbé's original description are problematic. The penis, described as "wide and short, without hooks" (Labbé 1934a: 209, our translation), is absolutely incompatible with Peronia, in which the penis is thin, elongated, and always with hooks in the distal region. The absence of dorsal eyes on the notum is also quite perplexing. The notum of the holotype is in poor condition and its dorsal eyes cannot be seen, likely because their black color faded. However, dorsal gills are clearly present on the notum and there is no doubt that S. carbonaria applies to a Peronia species. Based on the length of the muscular sac of the penial accessory gland (10 mm), S. carbonaria is not a junior synonym of P. peronii. However, its muscular sac and its intestinal loops of type I with a transitional loop oriented at 4 o'clock are compatible with both P. verruculata and P. sydneyensis (Table 4). The length of the spine helps distinguish both species but Labbé did not mention it and it is missing in the holotype. Therefore, strictly speaking, S. carbonaria should be regarded as a nomen dubium. However, because there are many older names available for the unit #1 of P. verruculata (Table 6), S. carbonaria can be regarded as another junior synonym of P. verruculata. It would make no sense to apply it to P. sydneyensis because several important organs (the penis, the spine of the penial accessory gland, the radula) are missing in the holotype and because Labbé's original description is problematic and incomplete.
Scaphis gravieri was described originally based on types from Mayotte, Zanzibar, and Djibouti. The application of Scaphis gravieri is now based on the lectotype from Mayotte (MNHN-IM-2000-33695), with intestinal loops of type I (Fig. 85A). Our data do not include fresh material from Mayotte, but Mayotte is located between Madagascar and Mozambique where P. verruculata (unit #5) is present. Therefore, S. gravieri is regarded as a junior synonym of P. verruculata (Tables 1,4,6). Note that P. madagascariensis, a distinct species with intestinal loops of type V, also is expected to be present in Mayotte, even though it has not been recorded there so far (Fig. 6). The presence of P. verruculata in Zanzibar (locality of some paralectotypes of S. gravieri) is possible but needs to be confirmed with fresh material. Additional, non-type specimens from Zanzibar were examined (MNHN-IM-2014-7989, MNHN-IM-2014-7990): their intestinal loops are of type I with a transitional loop at 5 o'clock. Therefore, those specimens cannot belong to P. madagascariensis (intestinal loops of type V) or P. peronii (intestinal loops of type I with a transitional loop oriented between 12 and 3 o'clock), and thus likely belong to P. verruculata. Peronia verruculata is expected to be present in Djibouti (locality of some paralectotypes of S. gravieri), but that still needs to be demonstrated with fresh material from the northwestern Indian Ocean (Somalia, Yemen, Oman) as well as from the Red Sea and the Persian Gulf.
Pieces of possibly up to three syntypes of Scaphis tonkinensis were located at the MNHN (MNHN-IM-2000-33700) but they are useless, poorly preserved, unidentifiable pieces of tissues. Determining the status of S. tonkinensis thus relies entirely on Labbé's original description. Given that P. verruculata (unit #1) is the only species known in Vietnam, and that several characters provided by Labbé (1934a: 213) match its anatomy (muscular sac 12 mm long, intestinal loops of type I), S. tonkinensis is regarded as a junior synonym of P. verruculata (Tables 1,4,6).
No type material could be located for Scaphis lata. Determining the status of S. lata thus relies entirely on Labbé's original description. Labbé's original description to determine its status. Given that P. verruculata (unit #1) is the only species known in Vietnam, and that several characters provided by Labbé (1934a: 213) match its anatomy (muscular sac 8 mm long, intestinal loops of type I), S. lata is regarded as a junior synonym of P. verruculata. Labbé mentioned the presence of dorsal gills and so at least some of the syntypes of S. lata were Peronia slugs. The fact that he also described intestinal loops of type II (which is absent in Peronia) means that he either made a mistake (all loops were of type I) or that some syntypes were not Peronia slugs.
Onchidium durum, originally described from the Red Sea, is regarded as a junior synonym of Peronia verruculata because, contrary to what was indicated in the original description, Onchidium durum is characterized by dorsal gills and intestinal loops of type I. It is presumed here that there is only one species of Peronia with intestinal loops of type I in the Red Sea. Labbé frequently confused types of intestinal loops; there are no well-documented cases of Peronia slugs with intestinal loops of type II. Peronia persiae, originally described from the Persian Gulf, is regarded as a new junior subjective synonym of P. verruculata because its mitochondrial DNA sequences, represented by the GenBank "voucher LaFM7S" in our analyses, all cluster together within the unit #4 of P. verruculata (Fig. 2). An older name, P. gondwanae , already refers to the unit #4 of P. verruculata (Tables 1, 6). So, even in the hypothetical event that unit #4 would later need to be named as a distinct taxon (of subspecific or specific rank), P. persiae would still remain invalid because P. gondwanae would always take priority over it.
The description of P. persiae by Maniei et al. (2020a) is an example of the common but regrettable practice that consists in creating new species names without a comprehensive revision, which almost inevitably leads to increasing the number of unnecessary synonyms (Dayrat 2005). Here are a few of the major methodological issues in the study by Maniei et al. (2020a). First, Maniei et al. (2020a) ignored the existence of many available Peronia species names, which is especially problematic in the case of names with type localities near the Persian Gulf (Table 1), such as Onchidium durum and Paraperonia jousseaumei with a type locality in the Red Sea, and Scaphis gravieri with a type locality in Mayotte. Second, Maniei et al. (2020a) decided to create a new name before the nomenclatural status of the other Peronia names was addressed. For instance, Maniei et al. (2020a: table S1) compared P. persiae with P. branchifera, P. ferruginea, P. gaimardi, and P. lata as if they were all valid names, but these names all refer to the unit #1 of P. verruculata (Tables 1, 6). Third, Maniei et al. (2020a) only examined specimens of P. persiae from the Persian Gulf, which means that, for comparison, they relied exclusively on the literature which, as the present work shows, is plagued with taxonomic and anatomical errors. For instance, Maniei et al. (2020a: table S1) assumed that the intestinal loops of P. verruculata were of types I and II, but it is positively demonstrated here that the intestinal loops of P. verruculata are all of type I and that there are no loops of type II in Peronia. Fourth, apart from P. persiae, only P. verruculata and P. peronii are represented in the phylogenetic trees by Maniei et al. (2020a: figs 11, 12), exclusively based on sequences obtained from GenBank (many of which were misidentified). Most specimens in their phylogenetic trees are not even identified at the species level. Using DNA sequences to create a new species name while most species are not being included in phylogenetic analyses is highly problematic. Maniei et al. (2020b) used the same mitochondrial COI sequences as in Maniei et al. (2020a) to compare metabolites between the Peronia slugs they called P. persiae and one Peronia individual from Bangka Island, near Sumatra, Indonesia. That specimen from Bangka Island, identified as Peronia sp. 7 by Maniei et al. (2020a) and as P. verruculata by Maniei et al. (2020b), belongs to the unit #1 of P. verruculata: its COI (MK993397) and 16S (MK993396) sequences cluster within unit #1. Note that the GenBank accession numbers for COI and 16S are switched in Maniei et al.'s (2020a) Table 2. Maniei et al. (2020b) summarized their rationale for creating the name P. persiae as follows: "The ABGD test revealed that specimens of P. persiae form a separate clade (clade 2). Thus, the specimens from two localities of the Persian Gulf (Iran), i.e. Bandar Lengeh and Lavan Island, were considered as a distinct new species." Mitochondrial loci alone are not sufficient evidence to delineate species: molecular delimitation analyses can over-split species based on population structure, particularly when these are based on a single locus (Sukumaran and Knowles 2017). More importantly, very high intra-specific mitochondrial divergence has been repeatedly documented in several onchidiid genera (e.g. Goulding et al. 2018c;Dayrat et al. 2019a). Maniei et al. (2020b) argue that research on metabolites requires sound taxonomic knowledge. That certainly is a commendable goal: indeed, any comparative work in any biological field should be based on correct taxonomy. Unfortunately, P. persiae is a junior synonym of both P. gondwanae and P. verruculata (Tables 1, 6). So, the metabolites compared between "P. persiae" and "P. verruculata" merely are intra-specific differences (within P. verruculata) due to the long geographic distance (between the Persian Gulf and Bangka Island) as well as, most likely, different diets: in fact, Maniei et al. (2020b) acknowledged in their introduction that numerous biotic and abiotic factors influence the chemical composition. To conclude anything about specific differences in metabolites among Peronia based on specimens from only two regions, one of which being represented by a single individual is, to say the least, premature. In order to demonstrate that distinct metabolites are found in distinct species, one needs to study actually distinct species, i.e., species that were reliably identified, and one also needs specimens of the same species from different habitats and from different locations. It is our hope that the present, comprehensive, taxonomic revision will help physiologists, biochemists, ecologists, etc., to identify Peronia slugs correctly.
Some comments are also needed regarding the original anatomical description of P. persiae by Maniei et al. (2020a). According to Maniei et al. (2020a: 510, fig. 6, table S1), the intestinal loops of P. persiae are of type II, but they are without doubt of type I: the transitional loop is oriented at ~ 5 o'clock, as in intestinal loops of type I (Fig. 1). The radular formulae provided by Maniei et al. (2020a: 509) fit well with what was observed here for the unit #4 of P. verruculata (Table 5), acknowledging individual variation: from 49 × 47.1.47 (in a live specimen 22 mm long) up to 71 × 87.1.87 (in a live specimen 65 mm long). The length of the spine of the accessory penial gland ("around 1.3 mm") reported by Maniei et al. (2020a: 513) is shorter than what was observed here (from 2.2 to 2.8 mm) but this trait is known to vary between individuals (Table 4). Maniei et al. (2020a: table S1) compared the shape of the tip of the spine of the accessory penial gland between species, but that trait varies greatly intra-specifically and is useless to distinguish species. Finally, Maniei et al. (2020a: 513, fig. 8B) reported some "fork-shaped" penial hooks, which were also observed here in the unit #4 of P. verruculata (Fig. 100C).
Additional material (historical museum collections). A specimen from Tanimbar, Indonesia (WAM S26630) is identified as P. verruculata because of its accessory gland spine (1.5 mm long), its intestinal loops of type I (with a transitional loop at 3 o'clock), and its muscular sac (10 mm). Seven specimens from Zanzibar (MNHN-IM-2014-7989 and MNHN-IM-2014-7990) are also identified as P. verruculata because their internal anatomy is only compatible with that species (Table 4). Finally, specimens from the Persian Gulf (NHMD 635301) with intestinal loops of type I (with a transitional loop at 6 o'clock) demonstrate that there is more than one Peronia species in the Persian Gulf (Fig. 6). Indeed, based on our DNA sequences, P. madagascariensis (with intestinal loops of type V) is present in the Persian Gulf, and individuals with intestinal loops of type I must belong to a different species. Given that P. verruculata is known from Pakistan and western India (unit #4), eastern Africa (unit #5), and the Red Sea, it most likely lives in the Persian Gulf too. The fresh material recently described as P. persiae by Maniei et al. (2020a) confirms with molecular data the presence of the unit #4 of P. verruculata in the Persian Gulf (Fig. 2). In addition, several historical specimens preserved at various institutions were examined for the present study. They are discussed below in the secondary literature section because they were studied by previous authors.
Secondary literature. JE Gray (1850: 117) and Adams and Adams (1855: 235) did not mention Onchidium verruculatum in their list of Peronia species names. That might seem surprising because they transferred to Peronia all slugs with "radiating processes" (Gray 1850: 117) or "arbusculiform tufts" (Adams and Adams 1855: 234) on the dor-(NHMD 635300). Those specimens are important historically because they were mentioned by several authors (see below). Given their size (35/28 to 20/15 mm), their digestive system (type I with a transitional loop oriented at 6 o'clock, in the largest individual), and the size of their accessory gland spine (1 mm in the largest individual), those specimens belong to P. verruculata, but could potentially belong to more than one mitochondrial unit (Table 4). Mörch (1872a: 28; 1872b: 325) first mentioned them as Peronia mauritiana. Semper (1880: 255) identified them as O. verruculatum. Bergh (1884a) described one of them in detail (see below). Hoffmann (1928: 44, 73) also listed them in his material examined for O. verruculatum.
Schmeltz (1874: 96) listed Peronia verruculata from Samoa in a catalog of the Museum Godeffroy. This possibly is a record of P. peronii, although P. platei could also live there (Fig. 6). Ihering (1877: 230-237, pl. IV, fig. 3) described the nervous system of Peronia verruculata but did not provide any information on the specimens he examined. It is impossible to determine what Peronia species he actually studied.
Fischer and Crosse (1878: 689-690, pl. XXXI, figs 13-15) briefly described the radula of specimens they identified as Onchidium (Peronia) verruculatum from New Caledonia. There are three Peronia species in New Caledonia, and it is not possible to determine what species they examined. Semper (1880: 255-257, pl. 22, figs 3, 4; 1882: pl. 21, fig. 1) re-described O. verruculatum based on specimens from a variety of localities (Red Sea, East Coast of Africa, Nicobar, Ambon, eastern Australia, Philippines). His written description mostly focuses on traits that are not informative for species identification (e.g., number of dorsal papillae, number of dorsal eyes, radular teeth). Some of Semper's records of P. verruculata most likely are correct, given the geographic origin of the material (Fig. 6): Ambon, Philippines, and Cape York (Queensland, Australia). Some other material could be a mix of more than one species: P. madagascariensis and P. verruculata in the Red Sea and eastern Africa; P. verruculata and P. sydneyensis in MacKay, Queensland. Semper's material from Brisbane (27°S) most likely was part of P. sydneyensis (Fig. 6). Finally, Semper's specimen from Nicobar was part of some material collected during the Galathea Expedition and first reported by Mörch (1872a: 28;1872b: 325) as Peronia mauritiana (NHMD 635300). Those specimens, re-examined for the present study, belong to P. verruculata (see above).
Bergh (1884a: 148-151, pl. VII, figs 7-12, pl. VIII, fig. 14) described in detail the anatomy of an individual of O. verruculatum from Nicobar. The animal size (33/23 mm) and the size of the accessory penial gland spine (1.76 mm) match well the anatomy of P. verruculata (unit #1). This specimen was part of a group of specimens collected during the Galathea Expedition in Sambelong, Great Nicobar, which were examined for the present study (NHMD 635300). Their size (35/28 to 20/15 mm), their digestive system (type I with a transitional loop oriented at 6 o'clock, in the largest individual), and the length of their accessory gland spine (1 mm in the largest individual) are also compatible with P. verruculata. However, those specimens could potentially belong to more than one mitochondrial unit (Fig. 6).
Von Martens (1897: 126) mentioned Onchidium verruculatum from both Ambon and Timor with no description. Our molecular data indicate that Peronia verruculata does live in Ambon and Timor. However, Peronia peronii also lives in Timor and likely lives in Ambon too. Farran (1905: 358-359, pl. VI, figs 13-22) described a Peronia slug he identified as Onchidium verruculatum from the Gulf of Mannar based on one preserved specimen. Given the specimen size (31/34 mm) and the length of the spine of the penial accessory gland (2.8 mm), it is likely a record of P. verruculata, but it is unclear whether it is the unit #2 (known from the Andaman Islands) or unit #4 (known from Mumbai, western India). It could also be a record of a small, immature individual of P. peronii (which has not been recorded from southern India but could possibly be found there). Our present study does not include any specimen from Sri Lanka or the Gulf of Mannar.
Onchidium verruculatum is one of the eight onchidiid species mentioned by Hedley (1909: 369) from Queensland, Australia, without any reference to any material. It is impossible to know what species Hedley refers to. Our data show that there are two Peronia species in Queensland which overlap geographically (Fig. 6).
The references listed by Bretnall (1919: 310) for Onchidium verruculatum are all commented on above already. Let us say a few words about the specimens he examined himself. Bretnall's (1919: 310) records of O. verruculatum from Broken Bay, New South Wales (33°30'S) are likely records of Peronia sydneyensis, the only Peronia species known in New South Wales (Fig. 6). Bretnall's (1919: 310) records of O. verruculatum from Port Curtis, Queensland (ca. 23°30'S) could be records of P. sydneyensis but they could also include P. verruculata because the known southernmost locality of the mitochondrial unit #1 of P. verruculata is at ca. 21S (see remarks on P. sydneyensis).
The record of Onchidium verruculatum from Katsepy (Catsèpe), northwestern Madagascar, by Odhner (1919: 23) is within the geographical range of both P. verruculata (unit #5) and P. madagascariensis (Fig. 6). The voucher specimen, re-examined here (SMNH 180724), clearly belongs to P. verruculata because of its intestinal loops of type I (with a transitional loop at 6 o'clock). Hoffmann (1928: 72) listed many references for O. verruculatum, all of which (but one) are commented upon elsewhere already: comments on the references for Onchidium peronii, O. punctatum, and Peronia mauritiana can be found in our remarks on P. peronii; comments on the references for Onchidium ferrugineum and O. elberti can be found above, in our remarks on synonymies; Peronia alderi is regarded as a nomen dubium and is commented on in the general discussion. Mörch's (1872a: 28;1872b: 326) record of Peronia (Onchidiella) marmorata from Nicobar Islands, which Hoffmann (1928: 72) included in his list of correct references for O. verruculatum, is commented on here: it is not possible to know to what species Mörch refers; Godwin-Austen (1895: 443) listed Mörch's record as Onchidium (Onchidiella) marmorata in a faunistic inventory of Nicobar and Andaman, without clarifying to what species that name was referring. At any rate, Lesson's (1831b) Onchidium marmoratum belongs to Marmaronchis (Dayrat et al. 2018).
More importantly, Hoffmann (1928: 44) examined specimens from the collections in Stockholm and Copenhagen which he identified as O. verruculatum. Most of those specimens could be re-examined for the present study and are commented on here. Several specimens are confirmed here to belong to P. verruculata based on diagnostic anatomical traits (Table 4): the material from Karachi (SMNH 180721) belongs to the unit #4 of P. verruculata; the material from Hong Kong (SMNH 180707) and Queensland (SMNH 180712, 180713, 180714) belongs to the widespread unit #1; the material from Singapore (SMNH 180716) and the Java Sea (SMNH 180719, 180720, 180722) could belong to any of the three units (#1, #2, #3) present in the region. However, several specimens listed by Hoffmann (1928: 44, 72) clearly do not belong to P. verruculata (see remarks on each corresponding species): the specimen from Port Natal, South Africa (SMNH 180711) belongs to P. madagascariensis; the specimen from Sagami Bay, Japan (SMNH 180725) belongs to P. setoensis; and the specimen from Port Darwin, Northern Australia (SMNH 180715) belongs to P. willani.
The Red Sea specimens from the Copenhagen collections listed as "Savigny leg., Mus. Marsil" belong to P. verruculata because of their intestinal loops of type I (NHMD 90791). The label in the jar says that they were obtained by the Copenhagen Museum in 1860 (journal entry) from Savigny and the museum of Marseille (erroneously spelled "Marsielle"). Given that the type material of O. verruculatum was originally illustrated by Savigny (1817), it is worth making it clear here that those specimens are not the type material of O. verruculatum (Hoffmann did not say they were). The type material of O. verruculatum is in Paris (MNHN-IM-2000-22941).
The other specimens mentioned by Hoffmann (1928: 44, 73) could not be reexamined for the present study: the specimens from the Red Sea could potentially belong to P. verruculata or P. madagascariensis; the specimens from Tharangambadi (Tranquebar), southeastern India, most likely belong to P. verruculata; the specimens from New Caledonia could potentially belong to any of the three species present there; the specimens from Hawaii clearly belong to P. platei.
All the references mentioned by Labbé (1934a: 192-193) for Peronia verruculata are already commented on above. Labbé (1934a: 193) blindly accepted the distribution provided by Hoffmann (1928: 44, 73), which was not accurate because, for instance, P. verruculata is not present in Hawaii (see above). Labbé (1934a: 193) mentioned intestinal loops of type II in one individual from the Red Sea, even though he did not list any material examined from the Red Sea. At any rate, those intestinal loops were most likely of type I as aforementioned Labbé often made that kind of mistake. For instance, Labbé (1934a: 196) described as P. anomala a species with supposedly anomalous intestinal loops of type II, but the type material, re-examined here, clearly is characterized by loops of type I (Fig. 86B). The specimens examined by Labbé from the Philippines likely belong to P. verruculata, but the individuals from New Caledonia or New Guinea could belong to several Peronia species. Finally, so far, only P. peronii and P. griffithsi are positively known from Mauritius and his record of P. verruculata there (as Ile de France) must not be taken for granted.
The record of Onchidium (Peronia) verruculatum from Natal, South Africa (Connolly 1939: 454) likely is a record of P. madagascariensis, the only Peronia species so far known from South Africa. However, P. verruculata (unit #5) could also be found in northeastern South Africa because its southernmost known locality is in Maputo (ca. 26°S), very close to South Africa. Allan and Bell (1947: 152) and Allan (1950: 368) reported onchidiid slugs living in dead coral which they identified as Onchidium verruculatum from Moreton Bay, Brisbane, Queensland, Australia. Given its latitude (ca. 27°S), Brisbane is clearly in the range of P. sydneyensis and possibly of P. verruculata (unit #1) as well. Indeed, it is still unclear how far south P. verruculata is distributed in southeastern Australia, although we did not find it in Sydney, ca. 33S (see remarks on P. sydneyensis).
For the record of O. verruculatum from New South Wales by Dakin (1947: 144), see remarks on P. sydneyensis.  published a detailed anatomical study of a species they identified as Onchidium verruculatum based on material from the western coast of India. They mention four localities: Vengurla (ca. 15°50'S), Malvan (ca. 16°06'S), Mumbai (ca. 19°S), and Kathiawar (ca. 21°S). The illustration of the intestinal loops provided by   fig. 6) leaves no doubt about the fact that they examined individuals of P. madagascariensis, a species with intestinal loops of type V distributed from South Africa all the way to (at least) Mumbai. Whether a type V was observed by the authors in all the specimens, including those from the southernmost localities (Vengurla and Malvan), is unclear. The presence of intestinal loops of type V in all the specimens examined by  would mean that P. madagascariensis is found much farther south than Mumbai. If the authors did not notice that some intestinal loops were of type I, then they described two species under the name Onchidium verruculatum: P. madagascariensis and P. verruculata (Fig. 6). Baba (1958: 144) reported that some individuals of Onchidium verruculatum from Tokara Islands (ca. 30°N), just south of Kyushu, were very large (up to 120 mm long), suggesting that they were P. peronii instead (see remarks on P. peronii). The smaller specimens, however, could be P. verruculata (unit #1) and possibly P. setoensis (see remarks on P. setoensis). The two species which Baba (1958: 21) seems to distinguish (as Onchidium and Onchidium verruculatum) from Misaki (ca. 34°N), near Osaka, could be P. verruculata (unit #1) and P. setoensis, which, based on our DNA sequences, are sympatric near the Seto Marine Laboratory, which is close to Osaka (Fig. 6).
According to Quoy and Gaimard (1825: 428, our translation), Onchidium planatum is "related to Onchidium peronii, with which it differs by its smaller size, its color [dirty greenish], and the shape and arrangement of the dorsal warts." Also, the "extremely small eyes placed at the superior part of the tentacles" likely refer to the eyes at the tip of the ocular tentacles. The most striking trait of Peronia peronii, its dorsal gills, is not mentioned in the original description of O. planatum, and it is clearly indicated that the dorsal "warts" of O. planatum differ from those of P. peronii. So, based on the original description, one could say that O. planatum may or may not refer to an onchidiid species. Given that Quoy and Gaimard (1825: 429-430, pl. 66, fig. 9) were able to describe and illustrate as Onchidium secatum a slug that obviously is not an onchidiid, the name Onchidium planatum is regarded as a nomen dubium (which may or may not refer to an onchidiid). There is, at the MNHN, a specimen which is part of the type series of O. planatum (MNHN-IM-2000-33706). That specimen is accompanied by three labels: the oldest label says "Onchidium planum, Q. G. Freyc. p. 428., de Guam, MM Quoy et Gaimard, Expn Freycinet." The name "Peronia" was subsequently added on that oldest label in pencil. Because the oldest label clearly refers to Onchidium planatum described from Guam by Quoy and Gaimard (1825: 428), a recent label indicates that the specimen is a syntype of O. planatum, from Guam. The third label says "Oncidium Peronii, Guam, Quoy et Gaimard, A. Labbé, dét. 1933," suggesting that Labbé re-identified that specimen at some point as Peronia peronii even though he listed it as part of the material he examined for his re-description of Onchidium planatum. The specimen is now completely destroyed and poorly preserved: there are only two pieces of notum of which the length of 55 mm matches the original description; the oral area is totally destroyed; all internal organs are missing; no dorsal gills can be seen, but possibly because the notum is so poorly preserved; and it is unclear if a peripodial groove is present or not. Labbé (1934a: 225), who did not seem to realize that he was looking at one of the syntypes of O. planatum, mentioned several internal characters suggesting that O. planatum refers to an onchidiid species (intestinal loops of type I, accessory penial gland present with a muscular sac) although whether Labbé did see those structures or not remains an open question (Labbé often described and even drew structures that he could not have seen). The lack of dorsal eyes and gills could be due to the poor preservation, in which case a good guess would be that the (destroyed) syntype of O. planatum (MNHN-IM-2000-33706) belongs to a Peronia species. However, the fact that no dorsal gills and no dorsal eyes can be seen at all and that it is extremely unclear whether there is a peripodial groove or not seems to suggest that O. planatum may not even refer to an onchidiid species. It is very possible that the type series included more than one species. Semper (1882: 289) listed O. planatum as a problematic species name. Hoffmann (1928: 69, 84-85) thought that it was a valid Onchidium species name with Onchidella tabularis Tapparone Canefri, 1883 and Onchidium (Oncis) applanatum Simroth, 1920 as synonyms. Onchidella tabularis is a nomen dubium although it is clear that it does not refer to an Onchidella species (Dayrat et al. 2016: 37). Onchidium applanatum is a valid Platevindex species name. Platevindex applanatum was described only one jar at the MNHN with the following information (the label is not the original label): "Oncidiella, Tongatabou, Mrs. Quoy et Gaimard, 1829." The sizes provided by Quoy and Gaimard (13 to 15 mm long), likely for live animals, approximately match the preserved specimens considering preservation. The large syntype was dissected prior to the present study and was found empty with no internal organs. The small syntype, still intact, was opened for the present study. It is an immature individual with no reproductive parts. However, its intestinal loop of type I, the lack of rectal gland, and the lack of dorsal gills all indicate that it belongs to a Wallaconchis species. However, because the penial apparatus could not be checked, Onchidium cinereum is regarded as a nomen dubium, even though there is only one Wallaconchis species known so far in southwestern Pacific Ocean. Past authors transferred O. cinereum to Peronia (Oken 1834a: 287) or Onchidella (e.g., Gray 1850: 117; Adams and Adams 1855: 234), or just kept the original combination (Semper 1882: 286-287;Plate 1893: 142;Bretnall 1919: 319;Hoffmann 1928: 68, 81).
Peronia alderi JE Gray, 1850 was created by JE Gray (1850: 117) for a slug illustrated by his wife ME Gray (1850: pl. 226, fig. 3) and which Alder had apparently identified as P. punctata in a manuscript: the only information associated with that illustration says "P. Alderi. P. punctata, Alder, MSS, t. 226. f. 3." That slug clearly belongs to Peronia, based on the presence of dorsal gills. Alder and Hancock (1855: 34) briefly mentioned "Onchidium punctatum (Peronia Alderi, Gray)" in the context of a comparison between the dorsal gills in onchidiids and those in nudibranchs. However, given that no type locality is indicated and that no type specimen could be located (of which the label could have potentially indicated the type locality), Peronia alderi must be regarded as a nomen dubium (Table 1). Semper (1882: 268) transferred P. alderi to Onchidium but could not make any decision regarding its status because of insufficient data. Even though he does not seem to have examined any material, and for unclear reasons, Hoffmann (1928: 68, 72) mentioned New Guinea and the Torres Strait as records for Peronia alderi which he regarded as a synonym of Onchidium verruculatum.
Peronia acinosa was described by Gould (1852: 291-292;1856: pl. 21, fig. 384a), based on an unspecified number of type specimens from Fiji Islands. Peronia acinosa may or may not refer to an onchidiid species, mostly because its long ocular tentacles lack eyes at their tip (some onchidiids illustrated by Gould distinctly have ocular tentacles with eyes at their tip), its color would be very unusual for an onchidiid (deep berylgreen dorsum and slatey violet foot), and it is "everywhere closely covered with large rounded papillae" (which are not characteristic of onchidiids). Also, the type material could not be located. Johnson (1964: 36) could not find it either. Therefore, Peronia acinosa is regarded here as a nomen dubium (Table 1). Adams and Adams (1855: 234) transferred P. acinosa to Onchidella. Bretnall (1919: 326) thought that P. acinosa was a valid name, although he admitted that data were insufficient. Hoffmann (1928: 68, 102) rejected that P. acinosa could be an Onchidella and questioned that it could be a Peronia, and proposed (with a question mark) that it could be a synonym of Onchidina australis. Given that, for instance, Gould described large rounded papillae, P. acinosa clearly does not refer to Onchidina slugs. ent duct which remain in a small vial. The female (hermaphroditic) posterior parts are still in place inside. The digestive system was destroyed and the type of the intestinal loops could not be determined. Semper did not indicate the length of the spine of the accessory penial gland nor the length of the muscular sac, which is missing. Without any of those critical characters, it is impossible to determine the application of Onchidium nebulosum, especially given that both P. verruculata and P. peronii are known to be present in Palau and that both P. okinawensis and P. platei could potentially be there as well. As a result, O. nebulosum is regarded as a nomen dubium even though it is clear that it applies to a Peronia species (Table 1).
Plate (1893: 171-172) described a "medium-sized" specimen which he identified as Onchidium nebulosum, from Pohnpei, Micronesia, which is 2600 km east of Palau. Plate's description is problematic for at least two reasons. First, the presence of dorsal gills is not mentioned, which means that it is not certain that Plate did examine a Peronia individual. Second, Plate indicated a series of traits and measurements (intestinal loops of type I, muscular sac of the accessory penial gland 11 mm long, spine of the accessory penial gland 2.5 mm long, retractor muscle of the penis inserting near the heart) but those cannot be compared to the original description of Onchidium nebulosum because Semper did not mention them. Thus, there is no reason to admit that Plate did examine what Semper had originally described as Onchidium nebulosum. If Plate examined a specimen with dorsal gills, then it possibly was an individual of P. okinawensis, from Okinawa, which is 3800 km west of Pohnpei. Some of the characters seem to match (Table 4). It is unlikely, however, that Plate examined a specimen of P. platei, because most characters do not match (Table 4). Finally, it is not excluded that Plate examined a Paromoionchis or a Laspionchis individual instead (with no dorsal gills). Bretnall (1919: 310-311), Hoffmann (1928: 71), and Labbé (1934a: 224) assumed that Plate's identification was correct and accepted Palau and Pohnpei as two records of O. nebulosum, but did not add any new material.
Onchidium multiradiatum Semper, 1882 refers to an onchidiid species which belongs to Peronia or not, but is regarded as a nomen dubium because the type locality is unknown (Table 1). Semper (1882: 269) mentioned two individuals in the original description. One syntype, 30/22 mm, was located (ZMB/Moll 39026): the male and female parts are missing and the region of the male opening is partly destroyed; the radula is still present but the type of the intestinal loops cannot be determined; dorsal gills are not obvious but are present. Plate (1893: 141) merely mentioned Onchidium multiradiatum as an available species name. Both Hoffmann (1928: 79-80) and Labbé (1934a: 225) listed Onchidium multiradiatum as a valid species name, exclusively based on Semper's information. Hoffmann also briefly compared it to Onchidium griseum, which seems to refer to a species of Paromoionchis but also is a nomen dubium because its type locality is unknown (Dayrat et al. 2019: 70).
Quoya indica , the type species, by monotypy, of the genus Quoya Labbé, 1934a, was originally described based on three specimens for which, according to Labbé, there was no information other than the locality, "Mer des Indes," i.e., Indian Ocean. Because the type locality is too vague, Quoya indica is regarded as a nomen dubium. Three specimens (16/8, 10/8, and 7/5 mm) were found (MNHN-IM-2000-33679), which seem to match the material used by Labbé to describe Quoya indica. The only information on the labels tells us that they are from the "Mer des Indes." No collector or collecting date are indicated. As often with Labbé, no species identification is indicated either. Those three specimens possibly are the syntypes of Quoya indica. They dried and are very hard and poorly preserved. However, dorsal gills are present on the largest specimen and possibly on the smallest specimen too. Labbé (1934a: 216, fig. 51) described a double male opening (with the openings of the penis and of the accessory penial gland being separated). This could not be confirmed and is by no means a trait of generic value. Indeed, the opening of the penis and the opening of the accessory penial gland occasionally appear to be separated due to preservation (when the vestibule is everted). Internal characters could not be checked. In particular, the presence of intestinal loops of type V (not illustrated by Labbé) and of an accessory penial gland   fig. 53) could not be confirmed. There is at the MNHN another jar (numbered "31" on an old label) with a single specimen from the "Mer des Indes." However, that specimen is identified as "Oncidium," suggesting that it is not part of the type series of Quoya indica. Instead, it possibly is the non-type specimen that Labbé (1934a: 204) identified as Scaphis punctata. It is confirmed here that Quoya indica refers to a Peronia species (dorsal gills are present on the largest possible syntype), and that, therefore, Quoya is a junior synonym of Peronia. However, because the type locality is too vague and because no internal characters could be confirmed, Peronia indica is regarded as a nomen dubium (Table 1).
The species-group name hombroni, created before 1961 as a variety name, is now of subspecific rank (ICZN 1999: Article 45.6.4). Labbé (1934a: 202, fig. 23) described Paraperonia gondwanae hombroni based on one specimen from the Torres Strait, Australia. Originally, no jar clearly labeled as the type material of P. gondwanae hombroni was found at the MNHN. However, only one old jar was found at the MNHN with material collected from the Torres Strait by M. Hombron aboard the Astrolabe, as in Labbé's original description of P. gondwanae hombroni (MNHN-IM-2000-33694). Therefore, it most likely contains the holotype, by monotypy, of P. gondwanae hombroni. Unfortunately, there is little doubt that whatever is in that jar is not an onchidiid (it seems to be an empty notum of a nudibranch). Three explanations are possible. First, this material (MNHN-IM-2000-33694) is not the holotype of P. gondwanae hombroni, even though all the collecting information matches. Second, it was originally the holotype of P. gondwanae hombroni, but that holotype was switched by mistake with something completely different. Third, the material inside the jar is the material examined by Labbé, which means that he would have completely made up the description. Given the description of dorsal gills and of an accessory penial gland by Labbé (1934a: 202), it is likely that P. gondwanae hombroni applies to a Peronia species. However, the original description is problematic because Labbé (1934a: 202) writes that the intestinal loops are of type V and sometimes of type I, which is just impossible given that he examined only one individual. Because the type of the intestinal loops is uncertain, P. gondwanae hombroni cannot be applied reliably to any Peronia species and is regarded as a nomen dubium (Table 1).
As discussed in detail in our revision of Paromoionchis, Onchidium straelenii Labbé, 1934b is a nomen dubium (Dayrat et al. 2019: 70-72). The examination of the two syntypes used by Labbé (RBINS I.G.9223/MT.3823) revealed that Labbé's original description is erroneous regarding several important characters. In particular, Labbé (1934a: 213) subsequently transferred Onchidium straelenii to his genus Scaphis based on dorsal gills being supposedly numerous and highly ramified. However, there are no gills at all on the dorsal notum so it is clear that Onchidium straelenii cannot be classified in Peronia. Onchidium straelenii was arbitrarily placed in the genus Onchidium but it clearly should not be classified in Onchidium because several traits, such as the lack of a rectal gland, are incompatible with Onchidium . The generic placement of Onchidium straelenii remains unclear, hence its status as a nomen dubium (Table 1).

Species delineation
Peronia species cannot be distinguished externally, except for the longest individuals of P. peronii (more than 100 mm). However, they all differ internally, apart from P. platei and P. setoensis which cannot be distinguished (Table 4). This situation is similar to what has been observed in several other onchidiid genera: in Wallaconchis, Laspionchis, Paromoionchis, and Peronina, species cannot be distinguished externally but they all differ with respect to their copulatory apparatus (Dayrat et al. 2019a, b;Goulding et al. 2018b, c). The special difficulty in Peronia is that species differ in minute details. In other genera, species differences tend to be obvious. For instance, an accessory penial gland is present in Peronina tenera and absent in P. zulfigari (Goulding et al. 2018c). In comparison, Peronia species may only differ with respect to the length of the spine of the accessory penial gland (Table 4). This has made it very difficult for past authors to interpret anatomical differences. Peronia species diversity has been interpreted in two opposite directions, both of which were unfortunately erroneous. At one end of the spectrum,  considered that every single difference justified the creation of a new taxon name. As a result, while  accepted only six species of slugs with dorsal gills, all being still classified in Onchidium along with 34 species without dorsal gills, Labbé (1934a: 187) thought that there were five genera and 21 species of slugs with dorsal gills. However, the present monographic revision shows that only one of all Labbé's new names is valid (Table 1): Peronia madagascariensis . At the other end of the spectrum, more recent authors accepted only two species, P. peronii and P. verruculata, which they could not even really distinguish (e.g., Solem 1959: 38-39;Marcus and Marcus 1970: 213-214;Britton 1984: 183).
Peronia is a taxon for which the use of DNA sequences as an independent test for species delineation has been indispensable. Without DNA sequences, it would have been impossible to determine which anatomical traits differ or not among species, which is perfectly illustrated by the species diversity of Peronia in Japan. Past authors have somehow sensed that there were more than one species in Japan but could not tell them apart (e.g., Baba 1958;Katagiri and Katagiri 2007;Ueshima 2007). Our data show that there are four species in Japan, two being endemic (Fig. 6). After species are delineated using DNA sequences, their anatomical differences become clear: for instance, P. setoensis is the only species in Japan with intestinal loops of type V (Table 4).
The present study also demonstrates that even though mitochondrial COI sequences are necessary, they must not be used blindly. Indeed, if one were to take into account only mitochondrial DNA sequences (Figs 2, 5), one might think that there are up to 16 Peronia species: P. verruculata could be split into five distinct species, and P. peronii, P. platei, and P. griffithsi could be split into two species each. However, our two other data sets (nuclear DNA sequences and comparative anatomy) strongly suggest that those merely are cases of species with high genetic structure: all individuals of P. platei, for instance, are completely indistinguishable anatomically, and nuclear ITS2 sequences do not support the existence of two distinct taxa in P. platei. The long geographic distances between sampling sites (e.g., Hawaii and Papua New Guinea for P. platei) may partly explain the high intra-specific genetic structure. In cases where mitochondrial units are sympatric (P. verruculata units #1 and #2 overlap in southeastern Sumatra, and P. verruculata units #1 and #3 overlap in Singapore), genetic distances could be explained by the fact that those mitochondrial lineages were isolated for some time before coming into contact again. The importance of investigating nuclear DNA sequences as well as comparative anatomy has been demonstrated in other onchidiid genera, especially in Paromoionchis, Wallaconchis, and Peronina (Dayrat et al. 2019a;Goulding et al. 2018b, c).
Species are not externally cryptic in all onchidiid genera: the six Melayonchis species and the four Onchidium species can all be distinguished in the field on external traits (Dayrat et al. , 2019c. When species can be distinguished externally, they are also unequivocally supported by both mitochondrial and nuclear sequences, i.e., DNA sequences do not support any cryptic diversity within those Melayonchis and Onchidium species. True cryptic diversity remains exceptional in onchidiids: Marmaronchis vaigiensis (Quoy & Gaimard, 1825) and M. marmoratus (Lesson, 1831) cannot be distinguished externally or internally (Dayrat et al. 2018).

Types of intestinal loops
It is not an exaggeration to say that identifying the types of intestinal loops, originally defined by Plate (1893) and , has remained challenging for authors. A thorough re-examination of the specimens examined by Plate and Labbé in the context of the complete revision of the Onchidiidae has shown that even they were confused about intestinal types. For instance, Dayrat et al. (2019c: fig. 2) demonstrated that Plate's original definition of the type III was based on an erroneous number of dorsal loops in a specimen of Onchidium stuxbergi (Westerlund, 1883).  also repeatedly made mistakes (see all the species remarks above), the most notorious being his original description of P. anomala: the specific name anomala was created to emphasize that the intestinal loops of that species were anomalous (i.e., of type II instead of type I as in most Peronia), but the intestinal loops of the type material of P. anomala are of type I (Figs 1, 86B). In that context, it is thus not too surprising that Maniei et al. (2020a) described the intestinal loops of P. persiae as of type II although they clearly are of type I, with a transitional loop at 5 o'clock (see above remarks on P. verruculata).
Hopefully, the method that Dayrat et al. (2019b: fig. 1; 2019c: fig. 2; 2019d: fig. 13) recently introduced to identify types of intestinal loops will help put an end to that confusion. This method is based on the coloration of different sections of the intestinal loops and, most importantly, takes individual variation into account (Fig. 1). It is very important to note that this method does not redefine the types of intestinal loops, it merely clarifies them, and the difference between the types I and II originally defined by Plate and Labbé is maintained. According to Plate and Labbé, type I is characterized by a transitional loop oriented to the right in dorsal view, at 3 o'clock (Dayrat et al. 2019b: Fig. 1A), and type II is characterized by a transitional loop oriented to the left in dorsal view, at 9 o'clock (Dayrat et al. 2019b: fig. 1C). The reality is that the orientation of the transitional loop varies between individuals, but a left or right orientation of the transitional loop remains true ( Fig. 1): in type I, the transitional loop is oriented between 12 and 6 o'clock (always to the right in dorsal view, as stipulated by Plate and Labbé); in type II, the transitional loop is oriented between 6 and 12 o'clock (always to the left in dorsal view, as stipulated by Plate and Labbé).
Only the types I and V are found in Peronia (Table 4, Fig. 1): the transitional loop of type I is always oriented to the right (from 12 to 3 o'clock, or from 3 to 6 o'clock); there is no transitional loop in type V. To this day, there is no positive record (proven with an illustration) of intestinal loops of type II in Peronia. In the future, a few individuals may be shown to exceptionally possess a transitional loop oriented at 7 o'clock (which strictly speaking would correspond to a type II), but this has never been observed among the hundreds of Peronia specimens dissected for the present study.

Geographic distribution
The genus Peronia includes the two most widespread onchidiid species, P. verruculata and P. peronii, as well as species that are endemic to comparatively small areas, at least according to the current data ( Fig. 6): P. setoensis and P. okinawensis are endemic to Japan, and P. willani is endemic to Northern Territory. One reason may be the development mode. In Japan, on the eastern coast of Honshu, near Sagami Bay (ca. 35°N), Katagiri and Katagiri (2007) documented two Peronia species, one characterized by a planktotrophic development (called "Isowamochi") and the other characterized by a direct development (called "Minneawamochi"). Most likely, those species correspond to P. verruculata (unit #1) and P. setoensis, which are the only two Peronia species found north of 30N in Japan (Fig. 6). Another reason may be that all species cannot compete ecologically with P. verruculata, one of the most abundant onchidiid species in the Indo-Malayan region (P. tumidus is also extremely abundant but it lives in mangroves, not in the rocky intertidal, although both P. verruculata and P. tumidus often are found together on muddy sand). A third reason may be related to diversification history. The fact that several species (P. okinawensis, P. platei, P. setoensis, P. sydneyensis, P. willani) are characterized by narrow distribution ranges at the periphery of broadly-distributed species (P. griffithsi, P. peronii, P. verruculata) raises the question of whether peripatric speciation events may have occurred. Phylogenetic relationships of sister species suggest that P. okinawensis could have emerged peripatrically from P. peronii. As for the other species, it remains uncertain because relationships among clades E, F, and G are still unclear (Figs 2-4). Finally, given that they are sister species, it is most likely that P. willani and P. sydneyensis are the result of a recent allopatric speciation (the Torres Strait serving as a biogeographic barrier).
In the future, it will be necessary to investigate the phylogenetic relationships of populations of P. verruculata from the regions from where no fresh material could be obtained, especially the northeastern Indian Ocean (the coasts of Somalia, Yemen, and Oman), the Persian Gulf, the Red Sea, and southern India. It will also be necessary to include fresh material from new localities for P. peronii (its distribution provided here is based on many specimens identified only based on anatomy). Dozens of new specimens of P. peronii may reveal some higher genetic structure within P. peronii, as observed in P. verruculata, given that both species are widely distributed. At the moment, the low level of genetic structure within P. peronii (compared to P. verruculata) may simply be due to the fact that our mitochondrial analyses include thirteen specimens of P. peronii while they include 102 specimens of P. verruculata. Populations of Peronia slugs also need to be investigated in southeastern Australia (southern Queensland and northern New South Wales) to determine more precisely the geographic range of P. verruculata. Also, it is possible that species that are endemic based on current data (P. okinawensis, P. setoensis, P. willani) will be found elsewhere and will thus be characterized by a wider range. Finally, it is not excluded that additional new species will be found.
Ngô Xuân (Vietnam). A research permit was awarded to Benoît Dayrat in Singapore