Redescription of poorly known species of Ceratothoa Dana, 1852 (Crustacea, Isopoda, Cymothoidae), based on original type material

Abstract Due to the difficulty in accurately identifying cymothoids, these parasitic isopods are often incorrectly named or confused with other species. Within the genus Ceratothoa, a number of recent studies have aimed at clarifying some of the problematic species; however, several of the less studied species still require revision. This paper redescribes, from type material, several poorly known Ceratothoa species including Ceratothoa angulata, Ceratothoa capri, Ceratothoa carinata, Ceratothoa collaris, Ceratothoa gilberti, Ceratothoa gobii, Ceratothoa guttata, Ceratothoa italica, Ceratothoa oestroides, and Ceratothoa verrucosa, further resolving taxonomic uncertainties within the genus.


Introduction
Although being one of the physically larger parasitic isopods, cymothoids are still relatively understudied. Often easily observed, these isopods can be located inside the gills, mouths, body cavities and on external surfaces of their fish hosts (Hadfield et al. 2011). Originally, most studies of these parasites were limited to the more populated and accessible regions of the world, such as Europe and North America (Smit et al. 2014). Early taxonomists included the cymothoid isopods in their extensive monographs, but often these accounts were limited in descriptive information. Over the years, several scientists started focusing on this group in more detail and made significant contributions to knowledge on cymothoids. One such notable work is that of Joergen Christian Schioedte and Frederik Vilhelm August Meinert in their series of monographs from 1881 to 1884, where the different life stages, hosts and distributions were all observed (Schioedte and Meinert 1881, 1883.
However, several cymothoid species have not been studied in many years. A number of factors could be responsible for this lack of research, including a lack of cymothoid specialists, but it is highly probable that many of these species cannot be accurately identified from the original descriptions. This paper revises these poorly known species of Ceratothoa Dana, 1852 with redescriptions based on their type material.
Currently there are 30 Ceratothoa species known worldwide (according to the World List of Marine, Freshwater and Terrestrial Isopod Crustaceans database (Bruce and Schotte 2016). Ceratothoa is one of the more speciose genera within the family Cymothoidae, and is usually found residing in the buccal cavity of the fish host. Recent descriptions incorporate comprehensive descriptions and figures essential for accurately identifying specimens to species level, often absent in many of the original descriptions (Martin et al. 2013, 2015a, Hadfield et al. 2014a, 2014b. These papers have added new species and made several taxonomic changes (bringing species in and out of synonymy) within this genus, but more work is still required for the remaining species. Several of these species are considered questionable or no longer valid due to the lack of type material or inadequate descriptions. Here these lesser known Ceratothoa species are revised and updated, separating valid from invalid information and determining their correct taxonomic status where possible.

Methods
Type material for the Ceratothoa species was borrowed where available or drawn at their respective museums. Isopods were processed according to the techniques described in Hadfield et al. (2010Hadfield et al. ( , 2013. Species descriptions were prepared in DELTA (Descriptive Language for Taxonomy, see Coleman et al. 2010) using a general Cymothoidae character set. Classification follows that of Brandt and Poore (2003). Host authorities are not included in the text or references; host nomenclature and distribution being sourced from FishBase (Froese and Pauly 2015) and Catalog of Fishes (Eschmeyer 2016).
projected ridges, slight depression at base of each ridge. Posterior margins of pereonites smooth and straight, with posteroventral angles rounded. Coxae 4-7 rounded; not extending past pereonite margin. Pereonites 1-5 increasing in length and width, 6-7 decreasing in length and width, becoming more progressively rounded posteriorly.
Pereopod 1 basis 1.6 times as long as greatest width; ischium 0.8 times as long as basis; merus proximal margin with bulbous protrusion; carpus with straight proximal margin; propodus 1.2 times as long as wide; dactylus slender, 1.5 as long as propodus, 3 times as long as basal width. Pereopod 7 basis 0.8 times as long as greatest width; ischium as long as basis, with a large proximal bulbous protrusion; merus proximal margin with large bulbous protrusion, merus 0.5 times as long as wide, 0.3 times as long as ischium; carpus 0.8 times as long as wide, 1.1 times as long as ischium, without bulbous protrusion; propodus 1.3 times as long as wide, 0.5 times as long as ischium; dactylus slender, 1.7 times as long as propodus, 3.3 times as long as basal width.
Uropod same length or slightly longer than the pleotelson; peduncle 1.3 times longer than rami, peduncle lateral margin without setae; rami subequal, extending beyond pleotelson, marginal setae absent. Endopod 2.6 times as long as greatest width, straight medial margin, convex lateral margin, apically slightly pointed; exopod 2.3 times as long as greatest width, extending to end of endopod, apically rounded.
Distribution. Known from the western and central Indo-Pacific region: Philippines (Richardson 1910); Indonesia (Nierstrasz 1931, Bruce andBowman 1989); Guam, Micronesia (Williams et al. 2000); and India (Ravichandran et al. 2011, Rameshkumar et al. 2013. The record in Guam extends the range of this species by 2060 km and since this species has only ever been found on the one host species, the isopod range might extend even further as the host has a wider known geographic range in the Indo-Pacific. Ravichandran et al. (2011) recorded this species from India supporting suggestions by Bruce and Bowman (1989) and Williams et al. (2000) that C. angulata may have a similar distribution to its host.
Hosts. In the buccal cavity of Dussumier's halfbeak, Hyporhamphus dussumieri (previously H. laticeps) (Bruce and Bowman 1989, Williams et al. 2000, Ravichandran et al. 2011. Remarks. The distinguishing characters of C. angulata include the truncate anterolateral margins of pereonite 1 which form distinct ridges on both lateral sides and two small medial depressions, the slightly emarginate and truncate pleotelson, and the broadly rounded uropodal exopod. The unusually large, quadrate pereonite 1 formed from the lateral ridges is very characteristic for this species. Richardson's (1910) description was based on a single specimen, a female from an unidentified host in the Philippines, and consisted of a short description with a single figure. Bruce and Bowman (1989) provided a redescription based on the holotype (with only two figures) and additional material from Borneo (a non-ovigerous female and male), including a short description of the male and figures for both specimens.
Ceratothoa angulata resembles C. guttata with the narrow pleon and pleotelson but the unique pereonite 1 makes it readily distinguishable from other species.
Body oval, 1.7 times as long as greatest width, dorsal surfaces smooth and polished in appearance, widest at pereonite 5, most narrow at pereonite 1, lateral margins posteriorly ovate. Cephalon 0.5 times longer than wide, visible from dorsal view, triangular. Frontal margin rounded to form blunt rostrum. Eyes oval with distinct margins, one eye 0.3 times width and length of cephalon. Antennula more stout than antenna. Antenna with 8 articles.
Size. Female: 13-20 mm TL; male: 6-7 mm TL; second pullus: 2.5-3.5 mm TL (Trilles 1964a(Trilles , 1972a. Distribution. Throughout the Mediterranean with records from France (Trilles 1964a); Tunisia (Trilles and Raibaut 1973); Straits of Gibraltar and the Alborán Sea (southern Iberian Peninsula) (Rodríguez-Sánchez et al. 2001); Turkey (Kirkim et al. 2008, Innal andKirkim 2012); Cyprus (Kirkim et al. 2009);and Yemen (Al-Zubaidy and Mhaisen 2013). Rodríguez-Sánchez et al. (2001) stated that C. capri was found in the Atlantic and Mediterranean Sea including the Bay of Biscay (Bolívar 1892), Canary Islands (Koelbel 1892) and the Iberian Mediterranean (Barceló Combis 1875, Balcells 1953, Trilles 1977. None of these papers quoted by Rodríguez-Sánchez et al. (2001) mention C. capri, and it is possible that these were erroneous reference entries for this species. These references were probably intended for C. oestroides, which is mentioned in each of these articles and was also collected by Rodríguez-Sánchez et al. (2001). Junoy and Castello (2003) repeated this lapsus of the references in their checklist for the Iberian Peninsula and Balearic Islands. Rodríguez-Sánchez et al. (2001) also made reference to specific GPS co-ordinates in their paper which appear inaccurate as they correspond to localities on land instead of the expected aquatic points necessary for an oceanographic expedition.
Remarks. Ceratothoa capri can be distinguished by the acute anterolateral margins which extend past the prominent eyes; body widest at pereonite 5; a narrow pleotelson; and no appendix masculina on the second pleopod in males.
There are a number of species of Ceratothoa in the Mediterranean; however C. capri differs from them all. There are several differences between C. capri and C. gobii but the most obvious is the bilobed pleotelson in C. gobii which is absent in C. capri. The defining pereonite 1 characters of C. collaris are absent in C. capri and differences between C. capri and C. italica include less developed eyes, acute and produced anterior margin of the cephalon and the more truncate body of C. italica. Similar characters separate it from C. steindachneri as well as the number of articles of the antennae and C. oxyrrhynchaena differs from C. capri in the shape of the 7 th pereopod basis of the female. Lastly, C. oestroides is less globular or elliptical when compared to C. capri; is darker in the post-cephalic region due to more chromatophores; has shorter uropods; and a more stout body.
In the original description of this species, Trilles (1964a) did not designate a holotype; however, full descriptions of the female, male and second pullus were given along with figures of each. Several years later, Trilles (1972a) listed a male and female C. capri located in the buccal cavity of Capros aper from the Gulf of Lion, Mediterranean, which he stated were the types for the species. Examination of these specimens confirms that they are the syntypes of C. capri. The female specimen is here designated as lectotype and redescribed. This lectotype is necessary to fix and stabilise the identity of this species and use of the name. Figure 3 Cymothoa carinata Bianconi, 1869: 210-211, pl Description. Neotype female. Length 33 mm, width 15 mm. Body rectangular, 1.8 times as long as greatest width, dorsal surface with medial longitudinal ridge present, widest at pereonites 3-5, most narrow at pereonite 1, lateral margins slightly convex. Cephalon 0.6 times longer than wide, visible from dorsal view, subtriangular. Frontal margin rounded to form blunt rostrum. Eyes oval with distinct margins, one eye 0.1 times width of cephalon; 0.2 times length of cephalon. Antennula more stout and same length as antenna, with 7 articles; antennule peduncle articles 1 and 2 distinct and articulated. Antenna with 8 articles.
Pereopod 1 basis 1.5 times as long as greatest width; ischium 0.8 times as long as basis; merus proximal margin without bulbous protrusion; carpus with straight proximal margin; propodus 1.7 times as long as wide; dactylus moderately slender, 1.2 times as long as propodus, 2.9 times as long as basal width. Pereopod 7 basis as long as greatest width; ischium 1.2 times as long as basis, with a large proximal bulbous protrusion; merus proximal margin with large bulbous protrusion, merus 0.5 times as long as wide, 0.3 times as long as ischium; carpus 0.7 times as long as wide, 0.3 times as long as ischium, without bulbous protrusion; propodus 1.3 times as long as wide, 0.5 times as long as ischium; dactylus moderately slender, as long as propodus, 2.2 times as long as basal width.
Uropod same length as pleotelson, peduncle 0.8 times longer than rami, peduncle lateral margin without setae; rami not extending beyond pleotelson, marginal setae absent, apices narrowly rounded. Endopod apically rounded, 3 times as long as greatest width, lateral margin straight, mesial margin straight, terminating without setae. Exopod extending to end of endopod, 4 times as long as greatest width, apically rounded, lateral margin weakly convex, mesial margin straight, terminating without setae.
Remarks. Ceratothoa carinata can be identified by the characteristic medial ridge extending longitudinally along the dorsal pereon surface. Furthermore, it has a laterally depressed and wider than long pleotelson; pereonite 7 with an enlarged carinate ischium and large bulbous protrusion on the merus; uropods reaching the distal edge of the pleotelson; as well as a concave posterior margin on the pleotelson. Bianconi (1869) originally described this species from a single ovigerous female from Mozambique and stated that it was most similar to C. gaudichaudii and C. trigonocephala. Since then, another species, C. trillesi (Avdeev 1979) was also described from the Australia-New Zealand region, and shared many morphological characteristics. There has been some confusion surrounding the synonymy of C. trillesi with C. carinata (see Avdeev 1979, Trilles 1994, however, these species differ substantially with C. trillesi lacking the distinctive medial ridge, enlarged basis and ischium on pereopod 7, and a wide and depressed pleotelson seen in C. carinata. Species and names within Ceratothoa have been moved in and out of synonymy, an indication of both the difficulty of identifying and characterising species. Furthermore, many species are variable (Hadfield et al. 2014a) and species morphological boundaries are often unclear. In addition, the Cymothoidae also have groups of cryptic species such as has been seen in Mothocya (Bruce 1986) and Anilocra (Bunkley Williams and Williams 1981, Bruce 1987) and therefore the designation of a neotype is necessary in the long-term interests of nomenclatural stability.
Schioedte and Meinert (1883) mention a specimen from the type locality (Mozambique) that was originally deposited into Zoologisches Museum, Museum für Naturkunde, Homboldt-Universität, Berlin, Germany. Enquiries to that museum, as well as Muséum National d'Histoire Naturelle, Natural History Museum, London, the Naturalis Biodiversity Center and the Zoological Museum, University of Copenhagen, failed to locate any material that could be identified as the type for C. carinata. It is highly probable that this specimen was destroyed in World War II or lost during relocation of the material. As cymothoid isopods are among the most misunderstood and difficult isopods to identify (Brusca 1981), a complete description (or redescription) of the type material is essential for accurate identifications and research on the species. The current material of C. carinata was obtained from the type locality and corresponds with the original drawings of the species (Bianconi 1869). Both specimens have the noticeable medial ridge or hump running longitudinally down the length of the pereon. The pleotelson is medially concave and is wider than long. Furthermore, the uropods do not extend past the end of the pleotelson and the posterior margin of the pleotelson is indented medially; the eyes are small but clearly visible; and the antennae are stout and extend to the anterior margin of pereonite 1.

Ceratothoa collaris Schioedte & Meinert, 1883
Description. Holotype female. Length 40 mm, width 18 mm. Body oval, 1.8 times as long as greatest width, dorsal surfaces slightly bumpy, widest at pereonite 4 and pereonite 5, most narrow at pereonite 1, lateral margins posteriorly ovate. Cephalon 0.6 times longer than wide, visible from dorsal view, triangular. Frontal margin rounded to form blunt rostrum. Eyes oval with distinct margins. Antennula more stout than antenna, shorter than antenna, with 7 articles. Antenna with 8 articles.
Pereopod 1 basis 1.7 times as long as greatest width; ischium 0.6 times as long as basis; merus proximal margin with bulbous protrusion; carpus with rounded proximal margin; propodus 1.6 times as long as wide; dactylus slender, 0.9 times as long as propodus, 2.1 times as long as basal width. Pereopod 7 basis 1.4 times as long as greatest width; ischium 0.8 times as long as basis, without protrusions; merus proximal margin with large bulbous protrusion, merus 0.4 times as long as wide, 0.3 times as long as ischium; carpus 0.4 times as long as wide, 0.2 times as long as ischium, without bulbous protrusion; propodus 0.9 times as long as wide, 0.4 times as long as ischium; dactylus slender, 1.5 times as long as propodus, 2.3 times as long as basal width.
Lucas (1849) considered C. collaris to have a low host specificity (euryxenic) but Bariche and Trilles (2008) showed that there is a clear preference for Sparidae fish, particularly Pagellus erythrinus, which is commonly parasitised in Lebanon and Africa (Morroco and Algeria). Monod (1925) also stated how most of these isopods recorded from Dentex filosus were actually removed from Pagellus erythrinus, especially in the case of C. collaris forma africana.
Remarks. Ceratothoa collaris can be distinguished by the prominent anterolateral projections which do not extend past the eyes and form a collar-like structure from where it gets its name. It also has a wide pleotelson (same width or wider than pleon), uropods that do not extend past the pleotelson and a large bulbous protrusion on the pereopod 7 merus.
Ceratothoa collaris was described from Algeria, originally misidentified as C. oestroides by Lucas (1849). Later, Monod (1924a) described three different forms of this species, namely C. collaris forma globuligera, C. collaris forma africana and C. collaris forma typica based on morphological differences of their cephlon and antennae (Monod 1924a). Over the years, many researchers have identified other species where many forms are common, such as C. steindachneri (see Horton 2000), but naming the different forms are not necessary, thus Bariche and Trilles (2008) removed the three C. collaris forms.
Pereopod 1 basis 1.7 times as long as greatest width; ischium 0.6 times as long as basis; merus proximal margin with bulbous protrusion; carpus with rounded proximal margin; propodus 1.6 times as long as wide; dactylus moderately slender, 0.9 times as  (Richardson, 1904), female lectotype (22 mm) (USNM 1254761). A dorsal view B dorsal view of pereonite 1 and cephalon C ventral view of cephalon D pereopod 1 E pereopod 7 F dorsal view of pleotelson G lateral view. long as propodus, 2.1 times as long as basal width. Pereopod 7 basis 1.4 times as long as greatest width; ischium 0.8 times as long as basis, without protrusions; merus proximal margin with large bulbous protrusion, merus 0.4 times as long as wide, 0.3 times as long as ischium; carpus 0.4 times as long as wide, 0.2 times as long as ischium, without bulbous protrusion; propodus 0.9 times as long as wide, 0.4 times as long as ischium; dactylus slender, 1.5 times as long as propodus, 2.3 times as long as basal width.
Uropod more than half the length of pleotelson, peduncle 0.9 times longer than rami, peduncle lateral margin without setae; rami not extending beyond pleotelson, marginal setae absent, apices narrowly rounded. Endopod apically rounded, 3.6 times as long as greatest width. Exopod extending to end of endopod, 4 times as long as greatest width, apically rounded.
Remarks. Ceratothoa gilberti has an elongate body; pleon as wide as pereon; short uropods; a elongate, triangular cephalon; short anterolateral projections on pereonite 1; and a large pleotelson with a rounded posterior margin. Furthermore, Brusca (1981) previously noted that C. gilberti lacks an appendix masculina in the male. The female specimen is here designated as the lectotype and redescribed.
Ceratothoa gilberti has been infrequently collected and seems to be confined to the region around the Gulf of California. It has only been found on mullet species and has often been compared to C. gaudichaudii (which has recently been placed into species inquirenda by Martin et al. [2015]). Description. Holotype female. Length 12 mm, width 5 mm. Body elongate, 1.9 times as long as greatest width, dorsal surfaces smooth and polished in appearance, widest at pereonite 5, most narrow at pereonite 1, lateral margins slightly convex. Cephalon 0.7 times longer than wide, visible from dorsal view, triangular. Frontal margin rounded to form blunt rostrum. Eyes oval with distinct margins, one eye 0.3 times width of cephalon; 0.6 times length of cephalon. Pereonite 1 smooth, anterior border straight, anterolateral angle with small distinct produced point and produced past frontal margin of cephalon, extend to middle of the eye. Posterior margins of pereonites smooth and straight. With posteroventral angles rounded; coxae 4-7 rounded; not extending past pereonite margin. Pereonites 1-5 increasing in length and width; 6-7 decreasing in length and width; becoming more progressively rounded posteriorly. Pleon with pleonite 1 most narrow, visible in dorsal view; pleonites posterior margin smooth, mostly concave. Pleonite 2 not overlapped by pereonite 7; posterolateral angles of pleonite 2 narrowly rounded. Pleonites 3-5 similar in form to pleonite 2; pleonite 5 free, not overlapped by lateral margins of pleonite 4, posterior margin produced medially. Pleotelson 0.4 times as long as anterior width, dorsal surface smooth, lateral margins weakly convex, posterior margin subtruncate and shallowly emarginate.

Ceratothoa gobii Schioedte & Meinert, 1883
Pereopod 1 basis 1.4 times as long as greatest width; ischium 0.7 times as long as basis; merus proximal margin with bulbous protrusion; carpus with rounded proximal margin; propodus 1.7 times as long as wide; dactylus slender, as long as propodus, 3 times as long as basal width. Pereopod 7 basis as long as greatest width; ischium 0.8 times as long as basis, without protrusions; merus proximal margin with large bulbous protrusion, merus 0.4 times as long as wide, 0.3 times as long as ischium; carpus 0.7 times as long as wide, 0.3 times as long as ischium, with slight bulbous protrusion; propodus 1.1 times as long as wide, 0.4 times as long as ischium; dactylus slender, 1.6 times as long as propodus, 2.7 times as long as basal width.
Uropod same length or slightly longer than the pleotelson, peduncle 0.6 times longer than rami, peduncle lateral margin without setae; rami extending beyond pleotelson, marginal setae absent, apices narrowly rounded. Exopod extending to end of endopod.
Remarks. Ceratothoa gobii has a triangular cephalon with a sub-truncate rostrum; large eyes which together take up more than half of the cephalon; uropods which extend past the posterior margin of the pleotelson; short anterolateral projections on pereonite 1; and pleonites 1-5 gradually becoming wider.
Pereopod 1 basis 1.5 times as long as greatest width; ischium 0.8 times as long as basis; merus proximal margin with large bulbous protrusion; carpus with straight proximal margin; propodus as long as wide; dactylus slender, 1.4 times as long as propodus, 2.4 times as long as basal width. Pereopod 7 basis 0.8 times as long as greatest width; ischium 1.3 times as long as basis, with a large proximal bulbous protrusion overlapping merus; merus proximal margin with large distal bulbous protrusion, merus 0.7 times as long as wide, 0.3 times as long as ischium; carpus 0.7 times as long as wide, 0.3 times as long as ischium, without bulbous protrusion; propodus 1.4 times as long as wide, 0.5 times as long as ischium; dactylus slender, 1.4 times as long as propodus, 2.6 times as long as basal width.
Uropod more than half the length of pleotelson, peduncle 0.8 times longer than rami. Other material. Holotype of Ceratothoa venusta. Russian Pacific Federal Fisheries Research Institute (AGK 75054) -on flying fish, Parexocoetus brachypterus, from the Red Sea (Avdeev 1978).
Remarks. Ceratothoa guttata is distinguished by the elongate body widest at pereonite 5; uropods which do not extend past the posterior margin of the pleotelson; a narrow pleon; an expanded merus on pereopod 1; and an expanded ischium and merus on pereopod 7.
Ceratothoa guttata is considered to be highly host specific as it has, to date, only been reported from a single host, Parexocoetus brachypterus. Avdeev (1978) briefly described Meinertia venusta from the Red Sea on Parexocoetus brachypterus, comparing it to C. guttata. Bruce and Bowman (1989) revised C. guttata and synonymised C. venusta with C. guttata after comparing drawings of the two species. Similar morphology of pereopods 1 and 7, coxae, the pleon and the pereon, as well as the host specificity all confirm this synonymy. The largest female is here redescribed and reillustrated and designated as lectotype.   Body rectangular and elongate, 1.7 times as long as greatest width, dorsal surfaces smooth and polished in appearance, widest at pereonite 5 and pereonite 6, most narrow at pereonite 1, lateral margins subparallel. Cephalon 0.5 times longer than wide, visible from dorsal view, triangular. Eyes irregular in outline, one eye 0.2 times width of cephalon; 0.2 times length of cephalon.
Pereopod 1 basis 1.5 times as long as greatest width; ischium 0.9 times as long as basis; merus proximal margin with large bulbous protrusion; carpus with straight proximal margin; propodus 1.1 times as long as wide; dactylus slender, 1.2 times as long as propodus, 2.3 times as long as basal width. Pereopod 7 basis 1.2 times as long as greatest width; ischium 0.8 times as long as basis, without protrusions; merus proximal margin with slight bulbous protrusion, merus 0.5 times as long as wide, 0.4 times as long as ischium; carpus 0.5 times as long as wide, 0.3 times as long as ischium, without bulbous protrusion; propodus 1.1 times as long as wide, 0.5 times as long as ischium; dactylus slender, 1.4 times as long as propodus, 2.5 times as long as basal width.
Remarks. Ceratothoa italica can be distinguished by the arched body; large bulbous protrusion on the merus of pereopod 1; a pointed rostrum; and uropods that do not extend past the pleotelson. This species also has a prominent projection in the middle of pereonite 1 (hump-like) and a pleon which is usually as wide as the pereon. Ateş et al. (2006) stated that cymothoid isopods identified as C. italica were collected from the eggs of Nephrops norvegicus. As cymothoids are fish parasites, this interaction is an unusual association and is most likely accidental. Similarly, the record of C. italica (originally labelled as a Cymothoa sp. ) from Senckenberg Research Institute, Frankfurt, Germany (Trilles 2008) seems doubtful as the specimen was collected from Norway and this species has only previously been recorded from the Mediterranean Sea region.

Ceratothoa oestroides (Risso, 1826)
Description. Lectotype female. Length 22 mm, width 8 mm. Body oval and elongate, 1.9 times as long as greatest width, dorsal surfaces smooth and polished in appearance, widest at pereonite 4 and pereonite 5, most narrow at pereonite 1, lateral margins posteriorly ovate. Cephalon 0.6 times longer than wide, visible from dorsal view, triangular. Frontal margin rounded to form blunt rostrum. Eyes oval with distinct margins, one eye 0.3 times width of cephalon; 0.4 times length of cephalon. Antennula more stout than antenna, comprised of 7 articles. Antenna comprised of 8 articles.
Pereonite 1 smooth, anterior border straight, anterolateral angle with small distinct produced point, extend to middle of the eye. Posterior margins of pereonites smooth and slightly curved laterally. Coxae 2-3 narrow, with posteroventral angles rounded; 4-7 rounded, not extending past pereonite margin. Pereonites 1-4 increasing in length and width; 5-7 decreasing in length and width; 6 and 7 narrower. Pleon with pleonite 1 most narrow and same width as other pleonites, visible in dorsal view; pleonites posterior margin smooth, mostly concave. Pleonite 2 not overlapped by pereonite 7; posterolateral angles of pleonite 2 narrowly rounded. Pleonites 3-5 similar in form to pleonite 2; pleonite 5 free, not overlapped by lateral margins of pleonite 4, posterior margin with 2 indented points or produced medially. Pleotelson 0.5 times as long as anterior width, dorsal surface with lateral indent, lateral margins weakly convex, posterior margin rounded with medial indent.
Pereopod 1 basis 1.7 times as long as greatest width; ischium 0.6 times as long as basis; merus proximal margin with large bulbous protrusion; carpus with rounded proximal margin; propodus 1.5 times as long as wide; dactylus slender, 0.9 times as long as propodus, 2.3 times as long as basal width. Pereopod 7 basis 1.1 times as long as greatest width; ischium 0.9 times as long as basis, with slight bulbous protrusion; merus proximal margin with large bulbous protrusion, merus 0.6 times as long as wide, 0.3 times as long as ischium; carpus 0.7 times as long as wide, 0.3 times as long as ischium, without bulbous protrusion; propodus 1.4 times as long as wide, 0.6 times as long as ischium; dactylus slender, 0.9 times as long as propodus, 2.4 times as long as basal width.
Uropod same length or slightly longer than the pleotelson, peduncle 0.8 times longer than rami, peduncle lateral margin without setae; rami extending to pleotelson apex, marginal setae absent, apices narrowly rounded.
Remarks. Ceratothoa oestroides can be distinguished by having an acute rostrum; short antennae; prominent eyes; uropods which extend to or past the posterior pleotelson margin; large protrusion on the merus of pereonite 1; and a large carina on pereopod 7 in female specimens, as well as the appendix masculina absent in male specimens.
Ceratothoa sargorum Gourret, 1891, found on Sargus rondeletii, was described from a single female with large eggs, almost a millimetre in diameter (Gourret 1891). This species was later synonymised with C. oestroides as seen in Radujković et al. (1984). The original drawings by Gourret (1891) resemble the syntypic material of C. oestroides and confirm this synonymy.
There have been reported cases of C. oestroides involved in hyperparasitism. Dollfus (1922) recorded an unusual association with C. oestroides and a monogenean, Allodiclidophora charcoti (Dollfus, 1922) (Diclidophoridae) after being collected from C. oestroides in the mouth of Trachurus trachurus from Oviedo. Similarly, Monod (1923b) stated that the ectoparasite was found on one C. oestroides specimen from the mouth of Box vulgaris.

Ceratothoa verrucosa (Schioedte & Meinert, 1883)
Description. Lectotype female. Length 40 mm, width 21 mm. Body oval, 1.9 times as long as greatest width, dorsal surfaces slightly bumpy, widest at pereonite 4, most narrow at pereonite 1, lateral margins slightly convex. Cephalon 0.7 times longer than wide, visible from dorsal view, subtriangular. Frontal margin rounded to form blunt rostrum. Eyes irregular in outline. Pereonite 1 with unique bulbous orientation, anterior border slightly indented, anterolateral angle with large wide projections, extend to anterior margin of eyes. Posterior margins of pereonites slightly damaged and bumpy. Coxae 2-3 wide; 4-7 large and produced on pereonite margins, not extending past pereonite margin. Pereonites subequal. Pleon with pleonite 1 most narrow, visible in dorsal view; pleonites posterior margin not smooth, mostly concave. Pleonite 2 not overlapped by pereonite 7; posterolateral angles of pleonite 2 narrowly rounded. Pleonites 3-5 similar in form to pleonite 2; pleonite 5 free, not overlapped by lateral margins of pleonite 4, posterior margin produced medially. Pleotelson 0.5 times as long as anterior width, dorsal surface with lateral indent, lateral margins weakly convex, posterior margin evenly rounded. Antennula more stout than antenna, same length as antenna, consisting of 7 articles. Antenna consisting of 9 articles.
Pereopod 1 basis 1.4 times as long as greatest width; ischium 0.7 times as long as basis; merus proximal margin with large bulbous protrusion; carpus with rounded proximal margin; propodus 1.2 times as long as wide; dactylus slender, 0.9 times as long as propodus, 1.9 times as long as basal width. Pereopod 7 basis 0.8 times as long as greatest width; ischium as long as basis, with slight bulbous protrusion; merus proximal margin with large bulbous protrusion, merus 0.4 times as long as wide, 0.3 times as long as ischium; carpus 0.6 times as long as wide, 0.9 times as long as ischium, without bulbous protrusion; propodus 0.7 times as long as wide, 0.4 times as long as ischium; dactylus slender, 1.8 times as long as propodus, twice as long as basal width.
Uropod half the length of pleotelson, peduncle as long as rami, peduncle lateral margin without setae; rami not extending beyond pleotelson, marginal setae absent, apices narrowly rounded.
Remarks. Ceratothoa verrucosa is distinguished by the large, oval body; wide anterolateral projections on pereonite 1; pleon as wide as pereon; and short uropods not extending to the posterior margin of the pleotelson.
This species was originally thought to infect a Greenland whale (Spengler 1775) but Spengler (1775) may have tried to relate this isopod to the whale lice Cyamus ceti. Cymothoid isopods had not been mentioned often before this time and researchers had some confusion with their identification. Schioedte and Meinert (1883), however, stated that this species (referred as "Oniscus Ceti") was undoubtedly the same as their "Rhexana" species. There is no detailed description and no type material for Spengler's (1775) species, so the original description is a nomen nudum; the correct authority for the species is Schioedte and Meinert (1883) who first made the name available. The name Rhexana was preoccupied, and the genus was then changed to Rhexanella Stebbing, 1911, this name being later synonymised with Ceratothoa (see Hadfield et al. 2014b). Nierstrasz (1931) listed this species from the East Indian Archipelago, specifically from Nangamessi Bay, Sumba (Indonesia) after being collected during the H.M. Siboga expeditions. If confirmed, this distribution range would increase the distribution of Ceratothoa verrucosa (previously only found from Japan and only from one host, Pagrus major). Remarks. This species was described from an immature specimen (4 mm in length) in only a few sentences and a single figure. It was found under the bell of a Rhizostoma in Port Jackson (Sydney, New South Wales) and noted as being similar to Aegathoa in many ways, but differed in the sudden narrowing of the body at the commencement of the pleon, and the uniramous character of the caudal appendages. Hale (1926) synonymised C. argus with C. imbricata as it appeared similar to the brood young of C. imbricata and according to the label it was also reported as coming from the jelly blubber, Catostylus mosaicus (recorded as Catostylus mosaicus).

Species excluded from
Due to the species being based on a single immature specimen (as well as a lack of a type specimen and an incomplete description), this species is hereby considered nomen dubium.

Hosts. Micromesistius poutassou (previously Gadus potassoa).
Remarks. These two species names, published in the same year, refer to the same species. Brian (1939) stated that "… this isopod circa 2 cm in length … a species of Meinertia deserves to be described as it seems to be a new species, I hope to be able to publish the description of this species which I call Meinertia poutassouiensis". Horton (2000) added that both authors cited the species as found on Micromesistius poutassou (as Gadus potassoa), but it was inadequately described with two uninformative figures in Penso (1939). This lack of sufficient information, lack of types to redescribe the species and lack of the location of the type material lead Horton (2000) to place C. poutassouiensis (Brian, 1939) into nomen nudum. The mention of size alone by Brian (1939) does not meet the criteria of availability, specifically ICZN Article 13.1.1, as it does not differentiate or define the species; Brian's name is therefore not available so the authority has to be Penso (1939) as Penso provided two figures of the species thereby validating the name. The correct spelling of the epithet remains that proposed by Brian (1939). Given the lack of a descriptive data, lack of types and lack of a specific type locality, the species is here regarded as nomen dubium.
Hosts. Unknown. Remarks. Ceratothoa transversa was originally noted as having a cephalon only slightly immersed in pereonite 1; long antennae extending past pereonite 1; uropods slightly longer than the pleotelson; and a sub-triangular pleotelson. Richardson (1900) mentioned the name Meinertia transversa without differentiating characters or figures, without type locality, type deposition or type host, referring only to an "in press" paper and as such that name is a nomen nudum. Richardson (1901) later gave a short description of the species with figures of the cephalon and pleon were given. Schultz (1969) commented that this species is probably based on a young individual and Menzies and Kruczynski (1983) further stated that this species has never been adequately illustrated and probably also represents an Aegathoa-stage specimen as uropodal rami and pleotelson are shown with setae by Richardson (1900Richardson ( , 1901Richardson ( , 1905. Examination of the holotype confirmed that it is an immature specimen. The antennae extend into the middle of pereonite 1, the pleon is almost as wide as the pereon, there are a few setae on the uropods and pleotelson, the pleopods overlap and the appendix masculina is absent. Without an adult female to characterise the species, and no known hosts to assist in directing the collection of a new specimen, the identity of this species is uncertain and therefore C. transversa is hereby placed into species inquirenda.

Ceratothoa triglae Gourret, 1891, species inquirenda
Ceratothoa triglae Gourret, 1891: 19-20, pl. 11, figs 14-19. Remarks. Ceratothoa triglae (Gourret, 1891) was described from a male specimen measuring 7 mm TL. Gourret (1891) reported that it measured at least four times longer than wide and that it was found on the cheeks and belly of Chelidonichthys lucerna (previously Trigla corax). Second stage pulli were also found on the cheeks, probably newly released, along with the female; however, no other mention is made of the female after this statement. This species was subsequently placed into synonymy with C. parallela (see Radujković et al. 1984) and maintained there by Trilles (1994) and Horton (2000). As there is no known type material, and the description is based on a male specimen, this species can only be regarded as species inquirenda.
Remarks. Ceratothoa parva was originally described as having distinct eyes; rounded anterolateral processes on pereonite 1 which extend half the length of the cephalon; and short uropods which do not reach the end of the pleotelson.
Examination of the holotype revealed many characters not usually present in Ceratothoa. Pleonite 1 is as wide as the other pleonites (usually narrower), pleonite 4 is slightly wider than pleonite 5, slender and short antennae, and, most significantly, the bases of the antennae do not touch (a defining characteristic of Ceratothoa). These characters, together with the shape of the head and pereopod morphology all agree well with the generic characters for Elthusa Schioedte & Meinert, 1884(see Bruce 1990, and the species is here placed in that genus.

Ceratothoa species list
A number of recent papers revising Ceratothoa (Martin et al. 2013, 2015a, Hadfield et al. 2014a, 2014b, has resulted in a number of changes to the accepted species in the genus. Several species are no longer valid and some have been placed into synonymy  (Richardson, 1910), comb. n., female holotype (19 mm) (USNM 40938). A dorsal view B dorsal view of pereonite 1 and cephalon C ventral view of cephalon D pereopod 1 E pereopod 7 F dorsal view of pleotelson G lateral view. with other species. Currently accepted species of Ceratothoa are listed in Table 1; previous Ceratothoa combinations, showing the current status of the name as well as the earliest reference, are listed in Table 2.

Conclusion
A total of 50 Ceratothoa species names were found in the literature. Of these, 30 are regarded as valid (Bruce and Schotte 2016). Following recent work (Martin et al. 2013, 2015a, Hadfield et al. 2014b) and this present study, we accept 24 species of Ceratothoa as valid. This review of the poorly studied Ceratothoa resolves some of the uncertainties surrounding certain species and provides an updated list of valid Ceratothoa species. Table 1. Currently accepted species of Ceratothoa, their respective authorities and the most recent reference for each.

No. Accepted name
Authority Reference 1 Table 2. Species previously placed in combination with Ceratothoa, together with the current status and most recent reference. Full synonymies and nomenclatural details may be found in Trilles (1994).