Integrative taxonomy of the genus Onchidium Buchannan, 1800 (Mollusca, Gastropoda, Pulmonata, Onchidiidae)

Abstract In an effort to clarify the species diversity of onchidiid slugs, the taxonomy of the genus Onchidium Buchannan, 1800 is revised using an integrative approach. New, fresh specimens were collected in a large number of places, including type localities. The genus Onchidium is redefined here as a clade including only three species which are strongly supported by both morphological and molecular data. All three species were already named: the type species Onchidium typhae Buchannan, 1800, Onchidium stuxbergi (Westerlund, 1883), and Onchidium reevesii (J.E. Gray, 1850). With the exception of a re-description of Onchidium typhae published in 1869, all three species are re-described here for the first time. First-hand observations on the color variation of live animals in their natural habitat are provided. The anatomy of each species is described. Important nomenclatural issues are addressed. In particular, Labella Starobogatov, 1976 is regarded as a junior synonym of Onchidium and Labella ajuthiae (Labbé, 1935) and Onchidium nigrum (Plate, 1893) are regarded as junior synonyms of Onchidium stuxbergi. The nomenclatural status of several other species names is discussed as well. Many new records are provided across South-East Asia and precise ranges of geographic distributions are provided for the genus Onchidium and its three species. Distinctive features that help distinguish the genus Onchidium from other onchidiids are provided, as well as an identification key for the three species.


Introduction
The systematics of the Onchidiidae, one of the higher clades of pulmonate gastropods, has been problematic for decades.Many species names were created up to the 1930s, and then the study of their diversity has been more or less abandoned.Identifications have remained nearly impossible at both generic and specific levels.As a result, there are 143 species names and 19 genus names in the literature but the actual species diversity is largely unknown (Dayrat 2009).
The taxonomy of onchidiid slugs has remained problematic primarily because no malacologist has dared to study it in the past 80 years, which in turn is explained by the fact that many serious issues have made onchidiids a nightmare for taxonomists (Dayrat 2009).For instance, most species were described based on preserved specimens with no information on the color and shape of live animals, which turns out to be critical for taxonomic identification; most species were described based on few specimens (and, in fact, a single specimen in many cases), denying individual variation; many type specimens are likely lost; even when types are available, they often were destroyed, with few internal organs left; and, finally, most specimens in museum collections are old and poorly-preserved.
A few years ago, the Dayrat lab embarked on a worldwide systematic revision of the Onchidiidae.Our goal has been to integrate both traditional taxonomy and modern molecular tools (Dayrat 2005).Thanks to local collaborators, thousands of slugs have been collected from 263 stations (as of September 2016) across the tropical Indo-West Pacific.These stations, which include a large number of type localities, are mangrove sites for the most part, although rocky shores and coral rubble areas (where some genera of onchidiids are found) were also visited.Hundreds of individually-numbered slugs were photographed and hundreds of DNA sequences obtained from tissue cuts of those individually-numbered specimens which were also preserved for anatomical dissection.In addition, all the types available were borrowed from museums as well as additional materials, and the most important collections in the world were visited.
Traditionally, especially in museum collections, any onchidiid slug from the tropical Indo-West Pacific was by default referred to as "Onchidium," and any onchidiid slug from outside the tropical Indo-West Pacific was referred to as Onchidella.Still to this day, Onchidella has not been recorded in the tropical Indo-West Pacific but is found nearly everywhere else except for polar waters (Dayrat et al. 2011b).Also, most authors have agreed that Onchidella did not need to be divided into subgroups.Reciprocally, "Onchidium" slugs (in the broad sense of non-Onchidella) have not been recorded outside the tropical Indo-West Pacific.Because "Onchidium" was found to be highly diverse, several generic names were proposed in addition to Onchidium, such as Peronia, Paraperonia, Paraoncidium, Platevindex, and Semperoncis.The application of all these names has remained ambiguous, to say the least (Dayrat 2009).
Our understanding of the onchidiid diversity in the tropical Indo-West Pacific has grown as new data were being gathered.Species diversity, distributions, and higher relationships have become clearer as more and more DNA sequences were obtained and more and more specimens were dissected from new places.As of November2016, our data set for the onchidiid slugs in the tropical Indo-West Pacific includes approximately 70 species and ten clades of generic level.All species and genera are strongly supported by both DNA sequences and morphology.
After careful examination of all type materials, detailed comparisons between the original descriptions and our own observations, the nomenclatural status of nearly all existing species-group and generic-group names of the Onchidiidae is now known.In other words, it is clear which names are valid names, which names are junior synonyms, which names are nomina dubia, and which taxa require new names.So, results on the systematic revision of the Onchidiidae can now be shared.
In a first step, the nomenclature and the alpha-taxonomy of each of the clades of generic level in our data set must be clarified.The present contribution, which focuses on the genus Onchidium, marks the beginning of a series of taxonomic papers dedicated to each clade in our data set.Then, in a second step, a phylogenetic tree of the entire family will be provided (still an ongoing endeavor) and that tree used to address broader questions on onchidiid diversification, evolution, and biogeography.
The genus Onchidium, the type genus of the family, has been traditionally used by default for many onchidiid species from the tropical Indo-West Pacific.Therefore, it is important to give it a proper definition.The genus Onchidium is a clade including only three species, which were already named: the type species, O. typhae Buchannan, 1800, with a type locality in Bengal (Ganges delta); O. stuxbergi (Westerlund, 1883), with a type locality from Brunei Bay, north-western Borneo; and O. reevesii (J.E.Gray, 1850), with a type locality from China (exact locality unknown).With the exception of a re-description of O. typhae by Stoliczka (1869), all three species are re-described here for the first time.New synonymies are proposed based on the examination of all available type materials and the careful study of all original descriptions, for the entirety of the Onchidiidae.New geographical records are provided.
Special attention has been given to type localities.Indeed, without going back to type localities to collect fresh specimens, it can be extremely challenging, and often impossible, to address the nomenclatural status of taxon names.For that reason, new specimens have been collected from type localities as far as possible.For the present study, for instance, new specimens of O. typhae were collected from the Sundarbans, in West Bengal, India, which corresponds to the type locality, and new specimens of O. stuxbergi were collected from Brunei Darussalam, north-western Borneo, extremely close to the type locality.

Materials and methods
Collecting.All specimens examined here were collected by our team, except for the types of existing species and a few specimens found in museum general collections.Local guides (local villagers or fishermen) also often accompanied us.Sites were accessed by car (if next to a road) or by boat (by hiring local fishermen).Local fishermen and villagers are a great source of information to find good collecting sites.They know where to find well-preserved mangroves with old trees and they also know about potential dangers (snakes, crocodiles, wasps, and even tigers in the case of the Sundarbans, West Bengal).Each site was explored for an average of two hours but the exact time spent at each site also depended on the time of the low tide, the weather, etc.At each site, many photographs were taken to keep track of the kind of mangrove being visited (e.g., thick forest of young Rhizophora trees, open forest of large Avicennia trees) as well as the diverse microhabitats where specimens were collected (e.g., surface of the mud, old and muddy log).
Specimens.All available types were examined.Some additional non-type material was collected by others and borrowed from museum collections.However, most specimens were collected by us and our new collections provided fresh material for DNA sequencing and invaluable natural history observations.All our new specimens were deposited in local institutions as vouchers.Acronyms of collections are:

BNHS
Bombay Natural History Society, Mumbai, India Animal preparation and anatomical description.All anatomical observations were made under a dissecting microscope and drawn with a camera lucida.In addition, organs were prepared for scanning electron microscopy (SEM).Radulae were cleaned in 10% NaOH for a week, rinsed in distilled water for at least a week, briefly cleaned in an ultrasonic water bath (less than a minute), sputter-coated with gold-palladium, and examined by SEM.Soft parts (penis and penial hooks) were dehydrated in ethanol and critical point dried before coating.When a lot included several specimens, all pieces of the dissected specimens were carefully numbered, both inside the jar and on the SEM stubs.A range of minimum to maximum animal size is provided for each lot of specimens.In addition, individualized numbers and measurements are provided for the specimens being illustrated here as well as for those comprising our molecular data set.The anatomical description of O. typhae, the type species, is fully detailed.The written description of the many anatomical features that are virtually the same between species (nervous system, heart, etc.) is not uselessly repeated three times.
Phylogenetic analyses.Alignments were obtained using Clustal W in MEGA 6 (Tamura et al. 2013).Chromatograms were consulted to resolve rare ambiguous base calls.DNA sequences were all deposited in Genbank and vouchers clearly identifiable in museum collections (Table 1).The ends of each alignment were trimmed and sequences were concatenated.The concatenated alignment included 993 nucleotide positions: 582 (COI) and 411 (16S).In addition to analyses with the two concatenated markers, another set of analyses was performed with only COI sequences.Pairwise genetic distances between COI sequences were calculated in MEGA 6. Prior to phylogenetic analyses, the best-fitting evolutionary model was selected using the Model Selection option from Topali v2.5 (Milne et al. 2004).A GTR + G + I model was selected.Other (unpublished) analyses were performed using different models, which all yielded identical results.Maximum Likelihood analyses were performed using PhyML (Guindon and Gascuel 2003) as implemented in Topali v2.5.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 v2.5, with four simultaneous runs of 10 6 generations each, sample frequency of 100, and burn in of 25% (and posterior probabilities were also calculated).Three other onchidiid species and their corresponding COI and 16S sequences were selected from previous studies from our lab as out-groups (Dayrat et al. 2011a: Onchidella celtica (Cuvier in Audouin and Milne-Edwards, 1832), Peronia sp.(Okinawa), and Peronia sp.(Hawaii)).Other (unpublished) analyses were performed using different combinations of outgroups, which all yielded identical results.(Sun et al. 2014) and the Peronia and Onchidella out-groups (Dayrat et al. 2011a).Sun et al. (2014) misidentified all specimens from China as Onchidium "struma" (nomen nudum).Information on individually-identified specimens can be found in the additional material examined (see Fig. 1).The individual numbers starting with "S" from China correspond to vouchers used by Sun et al. (2014).

Molecular phylogenetic analyses.
Here, the primary purpose of using DNA sequences is to test the species limits within Onchidium.The phylogenetic analyses yielded three species units that are all reciprocally monophyletic and strongly supported (Fig. 1).Each species is supported by a bootstrap support of 100 and posterior probabilities of 1 (except for O. stuxbergi with a posterior probability of 0.97).Within O. typhae and O. reevesii, there is virtually no phylogenetic structure.Within O. stuxbergi, the specimens from Bohol (Philippines), cluster together in a well-supported (100/1.0)subunit.This, however, does not warrant any species status to the Bohol specimens because they are nested within O. stuxbergi which, without them, would not be monophyletic.Pairwise genetic divergences.The pairwise genetic distances unambiguously support the existence of three species of Onchidium (Table 2).There is a wide gap between intra-and inter-specific distances.All intra-specific genetic distances are below 5.1%  Information on individually-identified specimens can be found in the additional material examined and in Table 1.
Remarks.The synonymy of Labella ajuthiae (Labbé, 1935) with Onchidium stuxbergi (Westerlund, 1883) is discussed in the remarks on O. stuxbergi.Labella is a junior synonym of Onchidium because the two generic names Labella and Onchidium apply to the same clade.Baker (1938)  Distinctive diagnostic features.Onchidium differs from all other onchidiids by the presence of unmistakably, large, conical, pointed papillae on the dorsum of live animals.Disturbed live animals and preserved animals are retracted and their dorsal papillae are significantly smaller.The identification to the genus level can then be more challenging.

Onchidium typhae
Habitat (Fig. 3).In West Bengal, O. typhae was collected at both sites on soft mud, just next to the river (brackish water) and a few sparse Avicennia trees.In the Andaman Islands, it was collected directly on the mud, on the muddy surface (most often at the base) of tree trunks and roots, and on muddy old logs, inside old and deep mangroves with tall trees as well as more open muddy areas; it was also found on the cemented walls of small bridges (under a road next to mangrove).In Singapore, it was collected on muddy old logs.In Malaysia, it was most often found on muddy old logs and on muddy tree trunks and roots, as well as on the surface of the mud (especially that of mud lobster mounds).Onchidium typhae is a very cryptic species, especially when it is on the surface of the mud, and one needs to patiently look for it to find it, especially because it is rarely abundant (see below).
Abundance.In the Sundarbans, O. typhae was found in two of the four mangrove sites that were visited, but for a total of only six specimens.In the Andaman Islands, it was found in four of the five mangrove sites visited.The mangrove site where O. typhae was not found was not at all muddy but rather, comprised a sandy patch of Avicennia by a coral rubble sandy beach.It was especially abundant at two sites (many specimens were observed but not collected), mostly on or near old logs.In Malaysia, O. typhae was found in only five of the 18 mangroves that were visited and it was abundant (17 specimens collected) only at one site, which happens to be one of the best, oldest, most pristine, and most diverse mangrove forests we have ever seen anywhere.In Singapore, O. typhae was found in three of the five mangroves that were visited (the two sites mentioned above, and one site by the Mandai River where one specimen was collected but ultimately escaped).However, it is rare there (only five specimens collected in total).
Color and morphology of live animals (Fig. 4).Live animals are abundantly covered with mud and the color of their dorsum can hardly be seen at all.In fact, if it were not for their fecal pellets and their long ocular tentacles, they would be really difficult to find because they can be very cryptic.Once the mud is washed out, their dorsum is brownish, with no particular pattern.The color of the foot and of the hyponotum is important because it differs from the other species described here.The foot and the hyponotum vary from gray to yellow in color, and show sometimes even a greenish hue.The color of the foot and the hyponotum may or may not have the same color.It is remarkable that the color of the foot and of the hyponotum of a particular individual can change rapidly, especially when disturbed.The lateral sides of the foot remain dark grey.Distally, the long ocular tentacles are reddish brown.Proximally, they are darker brown and less reddish, speckled with many tiny white dots.The ocular tentacles are extremely long (easily 2 cm when the animal is undisturbed).The ocular tentacles of all Onchidium species described here are significantly longer than in other onchidiids, which is a useful character in the field to identify a slug as an Onchidium.The head is reddish brown to black, with many white dots evenly distributed.
When animals are not disturbed, the dorsum is typically covered by large pointed papillae that rest on larger, hemispherical bases.Those pointed papillae are unique to Onchidium and extremely useful for identification in the field.For a long time (before we realized those slugs were Onchidium in the strict sense), we called them the "spiky" slugs to refer to the unique pointed papillae.Those papillae may seem to be arranged in two to four longitudinal ridges (each with five to ten papillae), but this is not the rule.They bear from one to four black "dorsal eyes" at their tip but some papillae do not bear dorsal eyes (especially on the dorsum margin).As in many other onchidiids, there is a central peduncle entirely retractable within the notum.The central peduncle bears three or four "dorsal eyes" but its size is similar to the large pointed papillae (i.e., it is not significantly larger than the other large papillae).The large pointed papillae are surrounded by small papillae as well, which may be rounded or pointed.As soon as the animal is disturbed (by walking on the mud on which it crawls or by touching its dorsum), all dorsal papillae rapidly retract and the animal looks completely different.It then is evenly covered with minute pointed papillae.The body of disturbed animals also is more humped and their ocular tentacles are entirely retracted.One has to patiently wait for a long time for a disturbed slug to relax again (easily 10 or 15 minutes) so the rule is "take a picture first and then touch it!"Interestingly, the appearance of the dorsum of live, disturbed animals is very close to the appearance of preserved animals, which was quite useful when examining type materials of existing species and recognizing them as Onchidium species.Crawling individuals can easily measure 30 to 40 mm in length (largest individuals measured 65 mm).
External morphology (Figs.5A-C).Preserved specimens no longer display the distinct color traits seen in live animals.The color of preserved animals is meaningless and uninformative.The body is not flattened.The notum is elongated, occasionally oval.Dorsal gills are absent.The notum is evenly covered by papillae.Large papillae with hemispherical bases are typical for live animals but, in preserved animals, those papillae are pointier and smaller.These larger papillae are surrounded by even smaller papillae.As in live animals, papillae with so-called 'dorsal eyes' are present.There is a central, retractable peduncle in the center of the notum, but it can only be seen in live animals in the field.The hyponotum is horizontal.The width of the hyponotum relative to the width of the pedal sole varies among individuals.The width of the hyponotum ranges from about 1/3 to 1/2 its total width, occasionally extending to 4/5 its total width.The anus is posterior, medial, 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 pedal sole and the hyponotum, running longitudinally from the buccal area to the posterior end, a few millimeters from the anus and the pneumostome.The pneumostome is medial.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.The position of the female pore (at the posterior end of the peripodial groove) does not vary much among individuals.In the anterior region, the left and right ocular tentacles are superior to the mouth.They are outside if specimens were relaxed before preservation.Otherwise they are retracted.Eyes are located at the tips of the ocular tentacles.Inferior to the ocular tentacles, superior to the mouth, the head bears a pair of oral lobes.On each oral lobe, there is an elongated bump, likely with sensitive receptors.The male aperture (opening of the copulatory complex) is inferior to the right ocular tentacle, slightly to its left (internal) side.
Visceral cavity and pallial complex (Fig. 5D).Marginal glands (found in Onchidella) are absent.The anterior pedal gland is oval and flattened, lying free on the floor of the visceral cavity below the buccal mass.The visceral cavity is not pigmented internally and not divided (the heart is not separated from the visceral organs by a thick, muscular membrane).The heart, enclosed in the pericardium, is on the right side of the visceral cavity, slightly posterior to the middle.The ventricle, anterior, gives an anterior vessel supporting several anterior organs such as the buccal mass, the nervous system, and the copulatory complex.The auricle is posterior.The kidney is more or less symmetrical, the right and left parts being equally developed.The kidney is intricately attached to the respiratory complex.The lung is in two left and right, equally-developed, more or less symmetrical parts.
Digestive system (Figs 5E-G, 6).There are no jaws.The left and right salivary glands, heavily branched, join the buccal mass dorsally, on either side of the esophagus.The radula is in between two large postero-lateral muscular masses.Each radular row contains a rachidian tooth and two half rows of lateral teeth of similar size and shape.Examples of radular formulae are: 58 × 73-1-73 in USMMC 00005 #1 (42 mm long), 58 × 65-1-65 in USMMC 00005 #2 (28 mm long), 53 × 80-1-80 in USMMC 00002 #1 (60 mm long), and 65 × 70-1-70 in ZRC.MOL.6397#1 (38 mm long).The rachidian teeth are tricuspid: the medial cusp is always present; the two lateral  cusps, on the lateral sides of the base of the rachidian tooth, are small and inconspicuous.Rachidian teeth tend to be about half the size of the lateral teeth (with a length of the rachidian tooth usually not exceeding 40 µm).The lateral aspect of the base of the rachidian teeth is straight (not concave).The half rows of lateral teeth form an angle of 45° with the rachidian axis.With the exception of 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 seem to be unicuspid with a flattened and curved hook with a rounded tip (the length of the hook is between 50 and 60 µm), but there also is an outer pointed spine on the lateral expansion of the base.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 forming any protuberance).
The esophagus is narrow and straight, with thin internal folds.The esophagus enters the stomach anteriorly.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 in four chambers.The first chamber, which receives the esophagus, is delim-ited by a very thin layer of tissue, and receives the ducts of the dorsal and lateral lobes of the digestive gland.The second chamber, posterior, is delimited by a thick muscular tissue and receives the duct of the posterior lobe of the digestive gland.It appears divided externally but consists of only one internal chamber.The third, funnel-shaped chamber is delimited by a thin layer of tissue with high 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, narrow, and of type II.A rectal gland is present.It is a long, narrow, and coiled tube that opens in the left portion of the pulmonary complex.Its function is unknown.
Nervous system (Fig. 7A).The circum-esophageal nerve ring is post-pharyngeal and pre-esophageal.The two cerebral ganglia touch and the cerebral commissure is short.However, the length of the cerebral commissure does vary among individuals.Pleural and pedal ganglia are also all distinct.The visceral commissure is short but distinctly present and the visceral ganglion is more or less medial.Cerebro-pleural and pleuro-pedal connectives are short and pleural and cerebral ganglia touch each other.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.
Reproductive system (Fig. 7B).Sexual maturity is correlated with animal length.Mature individuals have large female organs (with a large female gland mass) and fully-developed, male, copulatory parts.Immature individuals may have inconspicuous female organs (or simply no female organs at all) and rudimentary anterior male parts.The hermaphroditic gland is a single mass, joining the spermoviduct through the hermaphroditic duct (which conveys the eggs and the autosperm).There is a large, bent, and approximately oval 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) which is not divided proximally, at least not externally.A prostate, not distinct externally, may be located within the walls of the spermoviduct.The spermoviduct is completely embedded within the female gland mass, at least proximally.Distally, the spermoviduct branches into the deferent duct (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, which should be near the proximal end of the spermoviduct.The ovate-spherical spermatheca (for the storage of exosperm) connects to the oviduct through a wide and strong duct.The oviduct is narrow, only slightly elongated and convoluted.The vaginal gland is absent.
Copulatory apparatus 8).The male anterior organs consist of the penial complex (penis, penial sheath, vestibule, deferent duct, retractor muscle) and penial accessory gland (flagellum, and hollow spine).The penial complex and the penial accessory gland share the same vestibule and the same anterior male opening.The 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 always heavily coiled.Distally, the flagellum ends in a hard, hollow spine protected by a sheath which is fused distally with the vestibule.The hollow spine is narrow and elongated, slightly curved.Its diameter is between 20 and 30 µm but it narrows down distally.The diameter of the opening at its tip is between 4 and 6 µm.Its length ranges from 0.7 mm (ZRC.MOL.6397#1) to 1.2 mm (USMMC 00005 #1).The hollow spine does not open directly into the vestibule.Instead, the end of the tube of the accessory gland is a disc which is more or less flat (between 0.4 and 0.6 mm in diameter) and bears approximately 20 conical papillae in its center.The hollow spine thus must go through that disc in order to be outside and shared with the partner.
The penial sheath is long and very strongly coiled in spirals.In less mature individuals, the coils may not be as strong and numerous but they are present.The penial sheath protects the penis for its entire length.The insertion of the retractor muscle marks the separation between the penial sheath (and the penis inside) and the deferent duct.The retractor muscle is shorter than the penial sheath and runs straight to the posterior half of the visceral cavity.The insertion of the retractor muscle varies among individuals: in the posterior half of the visceral cavity in all specimens but those from India; in the anterior half of the visceral cavity (just anterior to the heart) in the specimens from India.The deferent duct also is highly convoluted with many loops.In immature specimens, the deferent duct is significantly less coiled.The penis is elongated, round, narrow, and hollow; its diameter is less than 200 µm, and its distal part covered with hooks.When the penis is retracted inside the penial sheath, the hooks are inside the tube-like penis.During copulation, the penis is exerted like a glove and the hooks are then on the outside.Hooks are very densely packed inside the penis, with multiple, irregular rows of an average of 15 hooks around the circumference of the penis.Hooks are conical, slender, sharply pointed, and measure up to 300 µm in length.
Distinctive diagnostic features.Externally, O. typhae differs from other Onchidium species by the color of the hyponotum, which is not white but instead varies between grayish, yellowish, and even greenish (see below the dichotomous identification key, before the final conclusion).The color of the foot (yellowish, not bright orange) is not diagnostic.Internally, the spirally coiled penial sheath is not diagnostic.However, O. typhae is the only Onchidium species known so far with an intestine of type II.
Remarks.The original description of O. typhae by Buchannan was brief but it was based on first-hand observation of live animals, which is quite unusual for onchidiids since most onchidiid species were described based on preserved material with no information on shape and color of live animals.Even though the type material is likely lost, two features described and illustrated by Buchannan support the identification of the material described here as O. typhae.First, and most importantly, the long eye tentacles (only Onchidium species have such long eye tentacles) and the dorsal papillae of various sizes (Buchannan's "glandular tubercles").Buchannan's (1800: fig.2) illustration of an elongated body of a crawling slug is perfectly compatible with what we observed in the field.Buchannan originally described O. typhae as non-hermaphroditic; however, as pointed out by Cuvier (1804) shortly after Buchannan's article, onchidiid slugs are in fact hermaphroditic.
Stoliczka (1869) re-described O. typhae based on live animals that he collected himself near Calcutta.His identification is in agreement with Buchannan's original description.Stoliczka provided some information on the internal anatomy, which is compatible with our observations.Semper (1885) examined two specimens from Calcutta which he identified as O. typhae.According to Buchannan (1800), O. typhae is found on leaves of Typha elephantina (hence the specific name), a common reed in brackish waters of the Ganges delta.Wild areas with reeds have become very rare because Bengal was heavily developed in the last century and most coastal areas were converted to rice fields.According to Stoliczka (1869), however, O. typhae is also found "about old bricks" and "in ditches."Stoliczka also mentioned that it was the only species found near Calcutta, suggesting that in the past it could be found in brackish waters extending far inland.Finally, Stoliczka mentions O. typhae from the banks of the Hooghly River, which is very close to where our own specimens were collected in West Bengal.
Semper accepted Stoliczka's anatomical re-description of O. typhae and added some detail on the anterior male parts.In particular, Semper illustrated some penial hooks and the spine of the penial accessory gland.However, the sizes described by Semper (a maximum size of 170 µm for the penial hooks and a length of 4.5 mm for the spine of the penial accessory gland) are not really compatible with the sizes observed for the present study (penial hooks up to 300 µm and a spine less than 1.2 mm long).Therefore, Semper likely examined individuals of a different species, which cannot be identified at this stage.
Hoffmann (1928) re-described a specimen that he identified as O. typhae.That identification as an Onchidium is possibly correct (because of the presence of a rectal gland and of a penial accessory gland).However, the specimen examined by Hoffmann came from an unknown locality and it remains unclear whether Hoffmann did actually examine O. typhae.Labbé (1934) simply mentions the name O. typhae with no additional description or records.Dey (2006) illustrated a preserved (and not relaxed) specimen identified as O. typhae from the Sundarbans.The identification is possibly correct (the photograph is fuzzy).However, it is unclear whether the brief comments on the natural history of O. typhae refer only to O. typhae or a mix of species because Dey claims that O. typhae is found climbing on the trees, which is uncertain.Based on our observations, O. typhae can be found on muddy old logs, but not actually climbing on trees.
Finally, a search for potential synonyms of O. typhae revealed no synonyms (the available type materials of all onchidiid species were personally examined) and it does seem that it was named only once.
Type locality (V.stuxbergi)."Borneo in silva, ad flum Kalias" means that the slugs were found in forests by a river now called the Klias River.The latter runs into the Bru-nei Bay, which is a small bay bordered by Brunei Darussalam in the South, by Sabah (Malaysia) in the north, and by the small island of the Labuan Territory (Malaysia)  Vega-Expedition, Westerlund det.Jena, Jan 1927, Hoffmann determ."Our specimens here were collected from Brunei Darussalam, which borders the Brunei Bay and faces the island of Labuan, i.e., from a locality that is extremely close to the type locality.
Type locality (O.nigrum)."Borneo" is the only geographic information provided by Plate (1893) in the original description as well as on the label of the type, which reads "Oncidium nigrum Plate.22749.Borneo.Gera S." The mention of "Gera S." does not refer to Sungai Geras, a river near the city of Bintulu, on the west coast of Borneo, in Sarawak, Malaysia, but most likely to the fact that the specimen was collected by Gerard, as indicated in Plate's (1893: 188) original description ("1 Exemplar von Borneo, durch Gerard gesammelt").Thus, the type locality of O. nigrum could be anywhere in Borneo.
Type locality (E.ajuthiae).The "Province d'Ajuthia (Siam)" is the province of Ayutthaya, in Thailand, approximately 80 kilometers north of Bangkok, which used to be the capital of the Kingdom of Siam.Because that province is inland, Labbé assumed that the slugs had been collected in fresh water.However, the Chao Phraya River of the basin than runs through the province of Ayutthaya actually is under the influence of the tide year round.The salt front (brackish water) goes up to 75 and 175 kilometers from the river mouth in the wet and dry seasons, respectively, and it was even more so the case in the past when the river side was still not developed (syntypes collected by M. Bocourt in 1862 according to the label of the type material).In the province of Ayutthaya, the river is approximately at its kilometer 140 (Singkran 2015: 28).The label of the type specimens of Elophilus ajuthiae says that it lived in the "eaux dormantes de la province d'Ajuthia."The French expression "eaux dormantes" means "swamps."Given what is known of the basin of Chao Praya River, those swamps were brackish water under the influence of daily tides.
Type material (V.stuxbergi).One lectotype hereby designated (43/25 mm; entire and never dissected; SMNH 1334).All eleven other syntypes become paralectotypes with no name-bearing status.Originally, the type material included a total of twelve specimens split in two different jars: five specimens (all paralectotypes) with catalog number SMNH 7523 (from 35/30 to 15/12 mm, all entire except one specimen opened by a previous investigator, with a vial including the male copulatory system); and seven specimens with catalog number SMNH 1334 (the lectotype 43/25 mm, entire, and six paralectotypes from 20/18 to 15/15 mm, all entire except for one specimen dissected by a previous investigator, with a vial including a male copulatory system).All paralectotypes are Platevindex and the lectotype clearly is an Onchidium.The only one Onchidium specimen was selected as the lectotype because it is the only specimen that Westerlund (1885) illustrated for his new species Vaginulus stuxbergi.In fact, Westerlund's figures are a perfect image of the lectotype and, most importantly, it is exactly how Onchidium slugs (in the strict sense) look like when they are preserved without relaxation or when they are alive but disturbed.
Type material (O.nigrum).Holotype, by monotypy (ZMB 22749).One specimen 40/30 mm, completely dissected (by Plate) and empty.There is a vial with destroyed pieces of the digestive system (mostly the digestive gland and the intestine).
Additional material examined.Sun et al. 2014, one individual misidentified as "Onchidium struma" nomen nudum).A specimen of O. stuxbergi was found in Singapore (in the mangrove by the Mandai River) but was unfortunately lost.The presence of O. stuxbergi was also documented (as Onchidium nigram, which is a spelling mistake) in a guide to the mangroves of Singapore (Ng and Sivasothi 2002: 115).The type locality of Onchidium nigrum simply was cited as "Borneo," which could be anywhere on the island in Indonesia (Kalimantan) or Malaysia (Sabah or Sarawak).Our record in Macau is the northernmost (22°10'N) confirmed locality on the coast of southern China.
Habitat (Fig. 9).The habitat of O. stuxbergi is very similar to that of O. typhae: directly on mud (not soft, i.e. mud that is not very watery), on muddy trunks, old logs, lobster mounds, and even under Nypa leaves (Philippines).However, O. stuxbergi was not observed on mud as soft as the mud on which O. typhae was found in West Bengal.
Abundance.Onchidium stuxbergi is a rare species.Only one individual was found in Malaysia (where 18 mangrove sites were explored), three individuals at one site in Brunei Darussalam (7 sites), nine individuals at four sites in Bohol (17 sites), and 12 individuals at four sites in Vietnam (19 sites).Even though it will need to be confirmed in the future, it seems that O. stuxbergi tends to be slightly more common (although still rare, overall) in more northern latitudes (Vietnam and Philippines).
Color and morphology of live animals (Fig. 10).Live animals are not always covered with mud and the color of their dorsum can normally be seen.The dorsum is brown, with no particular pattern.Exceptionally, it can be almost black.The hyponotum varies between grayish and yellowish, and sometimes even greenish.It occasionally bears conspicuous black dots.The foot is bright orange, which is different from the two other species described here.In a tiny specimen (12 mm in length, St. 195/17), the foot was pale yellow.The long ocular tentacles are cream to brown distally, and darker proximally.The head is brown to black, with many evenly distributed white markings.The morphology of live specimens is similar to that of O. typhae.The only difference is that the central papilla with a few dorsal eyes is prominent in O. stuxbergi.Crawling individuals normally measure between 30 and 40 mm in length.Preserved specimens no longer display the distinct color traits seen in live animals.The color of preserved animals is meaningless and uninformative.The background color of the notum is brown.Some individuals, including old ones (SMF 333491) bear a few irregular, darker markings.The hyponotum and the foot of preserved animals are homogenously white.
The oviduct is narrow, short, and straight.The hollow spine of the penial accessory gland is slender and slightly curved.It measures between 0.5 (USMMC 00006) and 1.4 mm (PNM 041202 #2) in length, and between 20 (USMMC 00006) and 35 (PNM 041202 #2) µm in diameter.The diameter of the opening at its tip is nearly 10 µm.The hollow spine does not open directly into the vestibule.Instead, the end of the tube of the accessory gland is a disc which is more or less flat (approximately 0.3 mm in diameter) and does not seem to bear distinct conical papillae.The hollow spine must thus go through that disc in order to be outside and shared with the partner.The penial sheath is long (to the posterior third of the visceral cavity) and coiled in a few spirals.In less mature individuals, the coils may not be as marked.The retractor muscle is short and inserts into the posterior third of the visceral cavity.There is an additional retractor muscle attaching the anterior portion of the penial sheath to the left wall of the visceral cavity, near the buccal mass.In some individuals, that left additional retractor muscle is very thick and strong.The deferent duct is highly convoluted with many loops, but less so in immature specimens.The penis is elongated, round, narrow, and hollow.Its diameter is less than 200 µm.Its distal part is covered with hooks.When the penis is retracted inside the penial sheath, the hooks are inside the tube-like penis.During copulation, the penis is exerted like a glove and the hooks are then on the outside.Hooks are very densely packed inside the penis, with multiple, irregular rows of an average of 15 hooks around the circumference of the penis.Hooks are conical, slender, sharply pointed, and measure from 40 µm up to 300 µm in length.The longer they are the more slender they are.
Distinctive diagnostic features.Externally, O. stuxbergi differs from other Onchidium species by the color of the foot, which is bright orange (see below the dichotomous identification key, before the final conclusion).Internally, O. stuxbergi is the only Onchidium species known so far (and the only onchidiid species, for that matter) with a strong, additional retractor muscle attaching the anterior penial sheath to the left, anterior wall of the visceral mass, near the buccal mass.
Remarks.The status of O. stuxbergi has been problematic from the start because Westerlund unknowingly based his original description on specimens that are part of two distinct species (see Type materials, above): eleven former syntypes (now paralectotypes) are Platevindex and another former syntype (now the lectotype) is an Onchidium (in the strict sense, as defined here).Two years after the original description, Westerlund (1885) again published the description of Vaginulus stuxbergi, as a new species again.Although that contribution is not the original description, its figure 2 helps confirm that Vaginulus stuxbergi is an onchidiid and, most importantly, illustrates the one former syntype (here designated as the lectotype) that is part of Onchidium.Therefore, even though the brief and vague description may be confusing (because it is based on two different species), the illustration makes the identification absolutely clear, hence our decision to designate the illustrated specimen as the lectotype.Note that Westerlund did not describe any internal characters.
However, as a direct consequence of Westerlund's ambiguous original description and type material, many authors have proposed synonymies between O. stuxbergi and some species names that clearly belong to Platevindex.Those cases are briefly discussed here, but they will be discussed in more detail in our revision of the genus Platevindex.Labbé (1934: 235) and Hoffmann (1928: 88) Hoffmann (1928: 88) suggested that Onchidium ponsonbyi Collinge, 1901, could possibly be a synonym of O. stuxbergi but it is very unlikely because O. ponsonbyi is a terrestrial species known from 850 to 1,050 meters high at Mt Penrissen, Borneo.Onchidium ponsonbyi likely belongs to the genus Semperoncis Starobogatov, 1976.There is no doubt that O. nigrum, which is only known from the holotype, belongs to the genus Onchidium as re-defined here: the mantle of the preserved holotype bears the typical papillae of Onchidium.Also, Plate described both a rectal gland and an accessory gland, which are found in all three known Onchidium species.Plate did not mention the presence of an additional, left, retractor muscle for the penial sheath.He only mentioned that the insertion of the retractor muscle is of "type II" (i.e., near the pericardium).According to Plate, the penial hooks are from 14 to 87 µm long and the spine of the penial accessory gland is 1.2 mm long.The penial hooks observed here are from 40 to nearly 300 µm in length.It is possible that Plate, who observed only one specimen, could not fully evaluate the variation of penial hooks.Also, penial hooks are extremely challenging to extract and observe without SEM.However, Plate's description of the penial accessory gland spine is fully compatible with our observations (from 0.5 to 1.2 mm long).Finally, Plate described the intestine loops of O. nigrum as of a unique and exceptional pattern, which he referred to as "type III."The latter, as illustrated by Plate (1893: plate 8, fig.31a) is slightly more coiled than what was observed for the present study, but they are basically identical patterns.That intestinal pattern is absent from O. typhae but it has also been observed in some species from other genera.Given that Plate did not know the color of the live animal, it will never be known whether it matched the diagnostic color of the foot that was observed for our specimens (bright orange).According to our data, O. stuxbergi is distributed from Malaysia to Vietnam and the Philippines and therefore mostly encompasses Borneo.As a result, the synonymy of O. nigrum and O. stuxbergi is warranted, even though it cannot be completely excluded that O. nigrum could refer to an Onchidium species remotely endemic to the south east of Borneo.
The three syntypes described as Elophilus ajuthiae by Labbé (1935) were earlier identified by him as O. nigrum (Labbé, 1934).His first identification was supported by a pattern of intestinal loops (Plate's "type III") only known from O. nigrum.Labbé changed his mind after the observation of what he thought were tiny dorsal gills ("microbranchies") in those three syntypes from Thailand.Indeed, according to Labbé's (1934) onchidiid classification, dorsal gills are only found in the Dendrobranchiatae (which includes genera such as Peronia, Paraperonia, and Scaphis) while all other onchidiids (such as Onchidium, Onchidella, Platevindex and Onchidina), the Abranchiatae, lack dorsal gills.The three specimens from Thailand (with gills) could not belong to Onchidium (no gills) and, as a result, Labbé created a new species name and a new genus name for those specimens with an intestine of "type III" and dorsal gills.Labbé's new genus Elophilus, preoccupied, was replaced by Starobogatov (1976) by Labella.Those three syntypes from Thailand were re-examined for the present study; unfortunately, they are mostly empty.A few destroyed pieces of the intestine system remain but they are completely useless.However, the mantle clearly does not bear any "microbranchies" (i.e., microgills).It is very likely that Labbé's first intuition was correct and that he was just looking at large Onchidium papillae retracted within the mantle.Those three specimens from Thailand are part of O. nigrum, which means O. stuxbergi.Unfortunately, Labbé did not describe the male copulatory complex in detail and so the sizes and shapes of the penial accessory spine and of the penial hooks are unknown.However, our specimens from Vietnam suggest that there is only one species of Onchidium distributed in the region of the South China Sea, O. stuxbergi.Naturally, it cannot be excluded that the Gulf of Thailand actually hosts a distinct species; however, there is nothing in Labbé's description supporting that hypothesis.As a result, the synonymy of Labella ajuthiae with O. stuxbergi is warranted.Also, Labbé was confused about the type locality of O. nigrum because he claimed that "Plate's unique specimen came from Borneo (Guam)" (Labbé, 1934: 223, our translation) and that "Plate's unique specimen came from the Marianna Islands" (Labbé, 1935: 312, our translation).Borneo is with no doubt the type locality of O. nigrum.Finally, Labbé's claim that L. ajuthiae lived in fresh water was unfounded.Even though the specimens were collected far inland, it was still in brackish water and under the influence of the tides (see above, Type localities).
The name Onchidium struma, introduced by Qiu (1991) and used occasionally in the Chinese literature to refer to some onchidiids from the coast of China (e.g., Shen et al. 2006;Sun et al. 2014), is a nomen nudum (to our knowledge, O. struma has not been formally described as a new species).A survey of the diversity of onchidiids from China based on molecular data was recently published (Sun et al. 2014).The sequences of the specimens identified as Onchidium "struma" by Sun et al. were all included in our analyses here, and that name appears in two distinct species units (Fig. 1), which demonstrates that the name Onchidium "struma" used by Sun et al. referred to two distinct species.One of their species, identified here as O. reevesii, is mostly subtropical and is distributed from 22°30' to 34°36' latitude north along the coast of China.So far, it seems to be endemic to China.In the data set of Sun et al., O. reevesii is represented by eight individuals (under the name Onchidium "struma").Note that in that same contribution, Sun et al. (2014) apply the name Paraoncidium reevesii to a different species but that is also a misidentification.The species they refer to as Paraoncidium reevesii obviously cannot be O. reevesii, but it does not belong to Paraoncidium either.The other species referred to as Onchidium "struma", identified here as O. stuxbergi, is tropical and, in China, is only found in the southernmost coastline.In the data set of Sun et al., only one individual from Hainan Island (19°56'N) can be safely referred to as O. stuxbergi.Another individual in their data set (from Hong Kong, at 22°28'N) is problematic because its CO1 and 16S sequences give contradictory results, and so it is likely that one of those sequences is a mistake.The three specimens examined for the present study from Macau (SMF 333591/3) are the northernmost confirmed locality of O. stuxbergi in China at 22°10'N.
Type locality.China.Onchidella reevesii was not described by Gray.That name, which appears on page 117, simply referred to figures 5 and 5a of the plate 181).On page 117, there is no indication of the geographic origin, and there is no indication of geographic origin on the label of the holotype either.However, on page 36, those same figures are referred to as "Onchidium -.Mr. Reeve's drawings.China," clearly indicating that the animal illustrated on the fig. 5 (and 5a for the ventral side) of the plate 181 is from China.Hoffmann (1928) also accepted China as the type locality.
Type material.Holotype (43/25 mm), by monotypy (NHMUK 20160036).The label says "? Holotype Onchidella reevesii Gray 1850," but there is no reason to think it is not the holotype because the specimen fits perfectly the original illustration and the label also does refer to " Gray, Figs. Moll. Anim., IV, 1850, p. 117" where the name Onchidella reevesii was first published.The holotype is in excellent condition.It was opened dorsally prior to the present study so several key features could easily be checked in the digestive system as well as the male anterior parts, which are all drawn here (Fig. 14).The posterior reproductive parts (far less critical for species identification) were not removed from the visceral cavity (because it would have required our destroying the posterior region of the holotype) and so they are not illustrated here.Distribution (Fig. 2).China (type locality).Based on sequences misidentified as Onchidium "struma" (a nomen nudum) by Sun et al. (2014) and re-analyzed here, O. reevesii is found on most of the coastline of mainland China (except for southernmost and northernmost latitudes), from 22°30' to 34°36' of latitude north.
Internal anatomy (Fig. 14).The intestine of the holotype is between a type II and a type III, because a transverse line can intersect the intestine six times (four times for the type II as in O. typhae, and eight times for the type III as in O. stuxbergi).Although observed, the hollow spine was not extracted.At the end of the flagellum of the accessory gland, there by Sun et al. (2014), those two species are confused under a single name, Onchidium "struma", which is a nomen nudum.One of those two species, O. stuxbergi, is restricted to the extreme southernmost latitudes of the coast of China (up to 22°10'N) but is also distributed in Malaysia (Sabah, Peninsula), Vietnam, Brunei, and the Philippines (see above O. stuxbergi).The other species, O. reevesii, seems to be endemic to China and is distributed along nearly the entire coast of China (from 22°30' to 34°36'), except for the southernmost and northernmost latitudes.It makes sense that Onchidium reevesii applies to the species that is the most widely distributed in China because its type locality ("China") had a much higher chance to fall within the range of O. reevesii compared to that of O. stuxbergi.
Onchidium reevesii and O. stuxbergi, of which the distribution ranges do not seem to overlap, differ externally and internally.A few pictures of live animals of O. "struma" from Shangai, northern China (Shen et al. 2006: fig. 1), show that the foot of O. reevesii is whitish (the foot of O. stuxbergi is bright orange).Internally, the holotype of O. reevesii is lacking the additional, left, retractor muscle that is exclusively diagnostic of O. stuxbergi.The spiral coils of the penial sheath of the holotype of O. reevesii are compatible with the illustration (although fuzzy) of the anterior male apparatus of specimens misidentified as O. "struma" from Shangai (Wu et al. 2007: fig.1).However, spiral coils of the penial sheath are not diagnostic of O. reevesii because they are also found in O. typhae.Finally, there are fewer loops in the intestine of the holotype of O. reevesii than in O. stuxbergi (of type III) and, based on our data, the intestine of a species cannot be of different types.

Identification key
A key is provided here to help identify the three known species of Onchidium.The key is based on external characters because they are the most readily available.However, internal diagnostic features also distinguish the species (see species descriptions).DNA sequences provided in the present contribution can also help clear any potential confusion, because, to date, DNA sequences of Onchidium have yielded unambiguous results.Because the new limits to the genus Onchidium are much more restricted than its traditional meaning, many specific names traditionally combined with Onchidium must be combined with different generic names (Dayrat 2009).Those names will be dealt with the systematic revision of each clade of Onchidiidae in our future contributions.Here, however, comments are being provided on existing Onchidium species names that are regarded as nomina dubia, i.e., names which have been validly published but that should simply be ignored because their application is doubtful.
Onchidium aberrans Semper, 1885 is a nomen dubium because its type locality (Singapore) was mentioned as uncertain in the original description.Also, no type material could be located.Onchidium griseo-fuscum (Tapparone-Canefri, 1874), originally described as Onchidella griseo-fusca from Singapore and for which no type material could be located, could not be associated with any of the species we collected in Singapore.It could belong to Peronia but it is uncertain.As a result, it is here regarded as a nomen dubium.Onchidium hardwickii (J.E. Gray, 1850) was originally described as Onchidella hardwickii from an unknown locality and is thus regarded here as a nomen dubium.The holotype (by monotypy, MNHN) of Onchidium harmandianum Rochebrune, 1882, originally described from the Côn Đảo archipelago, off southern Vietnam, is a piece of tissue that may not even be part of an onchidiid slug.Because its original description is too brief and uninformative, it is regarded as a nomen dubium.We explored this archipelago and no species could be associated with that name.Onchidium planatum Quoy and Gaimard, 1824, originally described from Guam, is a nomen dubium: the lack of type material (not located), a vague original description, and the lack of illustration make it nearly impossible for it to be re-identified.More importantly, it is not even sure, based on the original description, that it actually was an onchidiid (after all, Onchidium secatum, from the same publication and by the same authors, is clearly not an onchidiid).Onchidium tabularis (Tapparone-Canefri, 1883), originally described as an Onchidella tabularis from Wokam, Aru Islands, Indonesia, is a nomen dubium due to the lack of type material (not located), the lack of illustration, and a useless written description.Tapparone-Canefri suggested that O. tabularis might refer to the same species as Onchidium planatum, but the latter is also a nomen dubium.Additionally, none of the species that we collected from Kei Islands (which is very close to Aru Islands) could be linked to O. tabularis.Onchidium tricolor Simroth, 1918, also described from Aru Islands, is a nomen dubium because the type material could not be located, there is no internal description, and the drawings of the external morphology are not informative enough.Finally, Onchidium trapezoideum Semper, 1885 is a nomen dubium because the type locality is unknown.
The present contribution illustrates well some of the complicated and relatively common issues faced in taxonomy and the possible ways to address them within an integrative approach.Integrative taxonomy is more than simply comparing morphological data and molecular data: nomenclatural issues are at the core of integrative taxonomy (Dayrat 2005).Nomenclatural issues are not something one can think about after species units have been delineated.Nomenclatural issues need to be considered from the very beginning of a taxonomic study.For instance, as our field work was being planned, type localities were included as our top priorities and were actually visited as often as possible.That allowed us to include specimens from type localities in our mo-lecular and morphological data sets, and easily find available names for the species units being delineated.Specimens from type localities are not indispensable to obtain welldelineated species units, but they are critical to name them.For instance, in the present study, it would have been very challenging to determine the status of the names O. typhae and O. stuxbergi without newly-collected specimens from West Bengal and Brunei Darussalam, their respective type localities.The good news is that taxonomic work becomes possible and much easier if type materials are examined, original descriptions are carefully studied, and new specimens are collected from (as many) type localities (as possible).The not-so-good news is that there is no such thing as a quick taxonomic study because addressing all these issues can take a great deal of time and expertise.

Figure 1 .
Figure 1.Phylogenetic tree.Relationships within the genus Onchidium based on COI and 16S sequences.Numbers above branches are the bootstrap values (Maximum Likelihood analysis) and below are the posterior probabilities (Bayesian analysis); only significant numbers (> 80% and > 0.9) are indicated.Onchidella and Peronia sequences serve as outgroups.Numbers for each individual correspond to unique identifiers in our DNA collection.All sequences of Onchidium specimens are new, with the exception of the specimens from China which were all misidentified as Onchidium "struma" bySun et al. (2014).Information on individually-identified specimens can be found in the additional material examined and in Table1.

Figure 2 .
Figure 2. Geographic distribution of the genus Onchidium and its three valid species.The colored dots correspond to the known records for each species.Colored areas correspond to hypothetical ranges proposed based on those known records.Naturally, details about the distribution of each species remain uncertain.For instance, O. typhae may or may not be found on the western coasts of Thailand by the Andaman Sea.

Figure 3 .
Figure 3. Habitats for O. typhae.A India, West Bengal, very soft mud on the shore with recently-planted Avicennia trees (station 49) B India, Andaman Islands, by a large river, deep mangrove with tall trees, small creeks, and plenty of old muddy logs (station 57) C Malaysia, Matang, old and open Rhizophora forest with tall trees, hard mud, creeks, and many old logs (station 28) D Malaysia, Langkawi Island, open forest with high mud lobster (Thalassina) mounds (station 26).

Figure 9 .
Figure 9. Habitats for O. stuxbergi.A Malaysia, Kuala Sepatang, old forest with tall, old Rhizophora trees, high in the tidal zone (station 27) B Philippines, Bohol, mostly Nypa palms with Thalassina mounds (station 187) C Vietnam, Can Gio, open mangrove with large Avicennia trees, soft mud, some old logs (station 231) D Vietnam, Can Gio, hard mud with trees spread out by a small road and then a steep bank to the soft, deep mud (station 221).

Table 1 .
GenBank accession numbers for COI and 16S DNA sequences.All sequences are new, except for the specimens from China

Table 2 .
Intra-and inter-species pairwise genetic distances.Ranges of minimum to maximum distances are indicated (in percentage).For instance, within O. typhae, individual sequences are between 0% to 0.5% divergent, and individual sequences between O. stuxbergi and O. typhae are minimally 21.7% and maximally 26.2% divergent.
provided a list of misspellings and unjustified emendations of Onchidium: Onchidion, Onchydium, Orchidium, and Oncidium.Diagnosis.Body not flattened.No marginal glands in the notum.No dorsal gills.Dorsal eyes present on notum.Fully retractable, central papilla (with three dorsal eyes) present.Long eye tentacles.Male opening inferior to the right ocular tentacle, slightly to its left.Pneumostome medial.Intestine of types II and III.Rectal gland present.Accessory penial gland and hollow spine present.Penis with hooks.