Gnathia capitellum, Ota & Kohtsuka & Tanaka, 2021

Ota, Yuzo, Kohtsuka, Hisanori & Tanaka, Katsuhiko, 2021, Description of a Small-headed Gnathiid Isopod (Crustacea), Gnathia capitellum sp. nov., from Coastal Japan, Species Diversity 26, pp. 207-216 : 208-215

publication ID	https://doi.org/ 10.12782/specdiv.26.207
publication LSID	lsid:zoobank.org:pub:7B906E72-9860-4DE3-9DF9-EB15E2902376
persistent identifier	https://treatment.plazi.org/id/D33A87DB-BA11-846A-187A-3FE1A202FD51
treatment provided by	Felipe
scientific name	Gnathia capitellum
status	sp. nov.

Treatment

Genus Gnathia Leach, 1814 View in CoL Gnathia capitellum sp. nov. [New Japanese name: Kogashira-umi-kuwagata] ( Figs 1–7 View Fig View Fig View Fig View Fig View Fig View Fig View Fig )

Type material. Holotype: adult male (NSMT-Cr 28254), 2.17 mm, molted from praniza larva that infested Platycephalus sp. 2 of Nakabo (2013) caught by longline fishing, off Nabeta Bay , 4 November 2011, coll. Y. Ota. Paratypes: two third stage praniza larvae (NSMT-Cr 28255), 2.49, 3.14 mm, from same host and locality as holotype; two adult males (NSMT-Cr 28256), 2.06, 2.14 mm, molted from praniza larvae that infested Takifugu snyderi (Abe, 1988) caught by longline fishing, off Nabeta Bay , 4 November 2011, coll . K . Tanaka; two adult males (NSMT-Cr 28257), 2.03, 2.14 mm, dredge, Koajiro Bay , 11–12 m depth, 14 March 2013, coll . H . Kohtsuka; two adult males (NSMT-Cr 28258), 2.28, 2.54 mm, dredge, Koajiro Bay , muddy substratum, 3 m depth, 16 October 2014, coll . H . Kohtsuka.

Locality. Nabeta Bay , Izu Peninsula (type locality) and Koajiro Bay, Miura Peninsula, Japan .

Etymology. The specific name “ capitellum ” was derived from the Latin meaning “small head”.

Description of adult male. Head ( Figs 2B View Fig , 3A–C View Fig ) oval, length ca. 1/8 of body length, width about 1.7 times of length, with concave posterior margin; dorsal sulcus shallow, narrow in anterior part; dorsal surface covered with sensory pits ( Fig. 2A View Fig ). Eyes well developed, occupying about 0.6 times of head length; paraocular ornamentation not developed; supraocular lobe not prominent. Frontal border centrally concave with three processes: mediofrontal process dentate; two superior frontolateral processes slightly protruded.

Pereonites ( Fig. 3A View Fig ) widest in pereonite 5. Pereonites 1–4 covered with sensory pits and pereonites 5–7 not covered; cuticula of posterior parts not well developed ( Fig. 2A View Fig ). Pereonite 1 short, not fused with head, not reaching lateral margins of head. Pereonite 2 obviously concave in anterior margin. Pereonite 3 rectangular shape. Pereonite 4 oval shape with convex in posterior margin. Pereonite 5 rectangular with areae laterales. Pereonite 6 longer than pereonite 5; lobi laterals not visible. Pereonite 7 short, narrow, overlapping pleonite 1. Pleonites 1–5 ( Fig. 3A View Fig ) with small setae on lateral margins. Epimera prominent in pleonites 2–4. Pleotelson ( Fig. 3E View Fig ) triangular, with blunt apex, with pair of setae; broad ridge visible, bearing three pairs of setae on dorsal surface.

Mandible ( Fig. 3C, D View Fig ), length less than 1/2 of head length; apex curved inward; dentate blade occupies ca. 3/4 of mandible length; basal neck, erisma prominent; mandibular seta, incisor, armed carina, internal lobe lacking. Antennule ( Fig. 3F View Fig ) composed of three peduncular, four flagellar articles; one, one, two penicillate setae on distal margins of peduncles 1, 2, 3, respectively; articles 2–4 each with aesthetasc; article 4 terminating in simple seta. Antenna ( Fig. 3G View Fig ) with four peduncular, seven flagellar articles; large seta on peduncle 3, penicillate seta on peduncles 3, 4, respectively; 0–3 setae present on each distal margin of flagellar articles 1–7. Maxilla 2 reduced, fused with head ( Fig. 2C View Fig ). Maxilliped ( Fig. 4A View Fig ) composed of basis, four palp articles; endite absent; palp articles 2, 3 longer than articles 1, 4; palp articles 1–4 with two, four, four, one plumose setae on external margin, respectively; three simple setae on internal margin of article 4. Pylopod ( Fig. 4B View Fig ) with three articles; article 1 semicircular, without both areolae, plumose setae, bearing three simple setae subdistally; article 2 same length as article 3; article 3 with simple seta.

Pereopods ( Fig. 5A–E View Fig ), pereopods 2, 3 entirely shorter, thicker than pereopods 4–6; ischium, merus, carpus of pereopods 2–4 covered with tubercles. Penes ( Fig. 4C View Fig ) composed of two contiguous, non-prominent papillae. Pleopodal basis ( Fig. 5F, G View Fig ) with single seta, two coupling hooks on distolateral, distomesial margins, respectively; pleopod 1, 2 rami with 8 or 9 plumose setae on exopod, 7 or 8 plumose setae on endopod; pleopod 2 lacking appendix masculina. Uropodal rami ( Fig. 3E View Fig ) subequal in length, not extending beyond apex of pleotelson; exopod with nine simple setae on external margin; endopod with eight simple setae on external, internal margins, five penicillate setae on dorsoexternal surface.

Size of adult males. 2.06–2.54 mm (between frontal border and pleotelson, mean±SD, 2.19± 0.17 mm; n=7). Adult males molted after six or seven days after detaching from their host.

Pigmentation of adult males. ( Fig. 1A View Fig ). Body light brown; digestive organs green due to congealed host blood; brown spots often remaining in fixed specimens.

Adult female characters. Unknown.

Description of third larva. Head ( Fig. 6A, B View Fig ) semicircular, length ca. 3/5 of width. Clypeus pentagonal with slightly concave in anterior margin. Eyes occupying ca. 7/10 of maximum length of head. Pereonite 1 ( Fig. 6A View Fig ) short. Pereonites 2 concave on anterior margin, with one seta on lateral margin. pereonite 3 closely similar to, slightly larger than pereonite 2. Segments of pereonites 4–6 visible, slen- der in zuphea larva ( Fig. 1B View Fig ), fused, swollen by fish’s body fluid in praniza larva ( Fig. 6A View Fig ); lateral shields of pereonites 4, 5 visible in dorsal view, elliptical. Pereonite 7 reduced, overlapping pleopod 1. Pleonites 1–5 ( Fig. 6A View Fig ) almost same length, epimera prominent in pleonite 5. Pleotelson ( Fig. 6C View Fig ) with broad ridge and three pairs of setae on dorsal surface and apex. Apex not acute.

Both antennae ( Fig. 6D, E View Fig ) almost same shapes, setations as those of male adult; peduncle 3 of antenna lacking distomesial large seta ( Fig. 6E View Fig ). Gnathopod (=pereopod 1) ( Fig. 6F View Fig ) smaller than pereopods; pectinate scales covered with inner margins of basis, ischium, carpus; carpus reduced. Maxilliped ( Fig. 6G View Fig ) composed of two basal, two palp articles; basal article 2 with endite and single coupling hook on inner margin; apex of palp 1 with five teeth and seta; apex of palp 2 with five setae and two spines. Mandible ( Fig. 6H View Fig ) with seven teeth. Maxillule ( Fig. 6I View Fig ) with three teeth on apex. Paragnath ( Fig. 6J View Fig ) stylet-like with groove on inner margin.

Pereopods ( Fig. 7A–E View Fig ) stouter, shorter than those of male adult. Basis of pereopods 2–4 oval shape. Exopod of pleopods ( Fig. 7F View Fig ) fan-shaped with eight or nine plumose setae; endopod oval, with seven or eight plumose setae on distal margins. Uropod ( Fig. 6C View Fig ) not reaching apex of pleotelson; exopod with five simple setae externally, four plumose setae internally; endopod with three simple setae externally, four simple setae dorsally, four plumose setae internally.

Size of third larvae. 2.49, 3.14 mm.

Pigmentation of third larva. Body light brown with brown spots, often remaining in fixed specimens ( Fig. 1B View Fig ).

Host. Platycephalus sp. 2 of Nakabo (2013) [Japanese name: Magochi] (type host) and Takifugu snyderi (Abe, 1988) [Japanese name: Shousai-fugu]. Site of infection: skin.

Remarks. The morphologies of our specimens having a small and oval-shaped head resembles the “premale” stage reported only in the Antarctic species, Caecognathia calva (Vanhöffen, 1914) . The premale stage is an intermediate stage between praniza larva and adult male defined by a broader head than that of praniza larva ( Wägele 1987, 1988). This stage has not been reported in the other gnathiid species for which the life cycle has been studied ( Mouchet 1928; Klitgaard 1991; Tanaka and Aoki 1998; Smit et al. 2003; Hadfield et al. 2009; Ota et al. 2012; Chong et al. 2015). Praniza larvae we reared in the laboratory metamorphosed into male adults in six or seven days after collection, and the male did not molt into the next stage for about two months (Ota, personal observation). The period is enough to mature after hatching for male gnathiids in temperate regions. For example, Elaphognathia cornigera (Nunomura, 1992) , of which the study site was the same as the locality of our specimens needed 52 days to reach male adults after eclosion ( Tanaka 2003). Smit et al. (2003) concluded that 62 days were required to complete the cycle from egg to egg in Gnathia africana Barnard, 1914 . Furthermore, the mean duration from eclosion to the molt to adult was approximately 39 days in males and 51 days in females in E. discolor (Nunomura, 1988) ( Tanaka and Nishi 2011) . Wägele (1988) reported third stage larvae of C. calva metamorphosed into male adults via premale stage within six weeks. However, the life cycle of the Antarctic C. calva was assumed to take four to five years ( Wägele 1988), and the premale period may be not long compared to the lifespan. Therefore, our observation during two months may be sufficient to examine the presence/absence of a premale stage in our specimens. In the premale stage in C. calva , dorsal pereonite 5 (called as sixth thoracic segment in the original description) has a dilatable membrane and no chitinized tergite, and the segment is swollen ( Wägele 1987). In mature male in C. calva , the membrane on pereonite 5 shrunk with a chitinized tergite (cf. Wägele 1987: fig. 12B). Pereonite 5 of a male G. capitellum had a shrunk membrane as well as a well-chitinized tergite. Thus, our specimens were considered not to have a premale stage, and males obtained in this study were fully matured males.

The general appearance of our specimens closely resembles the monospecific genus Afrignathia Hadfield and Smit, 2008 than the genus Gnathia . Afrignathia multicavea Hadfield and Smit, 2008 has an anterior body surface covered with sensory spots, body width widest in pereonites 5 and 6 oval head with compound eyes and dorsal sulcus, no paraocular ornamentation, a centrally concave frontal border, short mandibles with dentate blade, and an inner margin of pylopod without plumose setae (Hadfield and Smit 2008); and these characteristics are shared with our specimens. However, A. multicavea has a maxilla 1 which is absent in all other known male gnathiids around the world including our specimens.

In the family Gnathiidae , four genera, Caecognathia Dollfus, 1905 , Elaphognathia Monod, 1926 , Gnathia , and Tenerognathia Tanaka, 2005 , are known to have two- or three- articled pylopod as in the present species (Cohen and Poore 1994; Tanaka 2005). The genus Caecognathia is defined by having a rounded frontal border without frontal process and head lacking paraoccular ornamentation and dorsal sulcus (Cohen and Poore 1994). Our specimens have three frontal processes and dorsal sulcus. The genus Elaphognathia is defined by an excavated frontal border and long mandibles without a dentate blade (Cohen and Poore 1994). The present new species has a centrally concave frontal border and small mandibles. The genus Tenerognathia is defined by eyes occupying the whole of the side of head, narrow L-shaped pylopods and weakly chitinized pereonites ( Tanaka 2005). In the present new species, eyes are ordinary, pylopods are wide and pereonites are well-chitinized. Therefore, our specimens are separated from Caecognathia , Elaphognathia , and Tenerognathia .

The genus Gnathia is defined by 1) a frontal border has mediofrontal processes, 2) head may possess paraoccular ornamentation and/or a dorsal sulcus, 3) frontal border of cephalosome is not deeply excavated, and 4) mandibles are not elongated (Cohen and Poore 1994). Our specimens have a centrally concave frontal border of cephalosome with three frontal processes, a dorsal sulcus on the head, and short mandibles ( Fig. 3C View Fig ). These characteristics correspond to be a species in the genus Gnathia .

In the genus Gnathia , over 130 species have been record- ed from around (Boyko et al. 2008 onwards). our specimens have unique characteristics in this genus because its small oval head and the inner margin of pylopod which lacks plumose setae. Gnathia prolasius Cohen and Poore, 1994 , G. stigmacros Cohen and Poore, 1994 , and G. limicola Ota and Tanaka, 2007 also lack plumose setae on pylopods. However, the head shapes are rectangular in G. prolasius and G. stigmacros or pentagonal in G. limicola (Cohen and Poore 1994; Ota et al. 2007). Thus, our specimens were identified as a new species of Gnathia .

On the contrary of adult male morphology above mentioned, larval morphologies of G. capitellum sp. nov. were remarked here. Among Gnathia species around the world, G. capitellum sp. nov. is most similar to G. marleyi Farquharson, Smit, and Sikkel, 2012 . Gnathia marleyi is easily distinguished from G. capitellum sp. nov. by compound eyes occupying almost same length as head, triangular pleotelson with a slightly concave lateral margin, and uropodal endopod extending to approximately same length as apex of pleotelson (Farquharson et al. 2012). In contrast, eyes occupy ca. 7/10 of length of head, lateral margin of pleotelson is straight, and both uropodal rami do not extend to apex of pleotelson in G. capitellum sp. nov.

Of 24 Gnathia species known from Japanese coastal and adjacent waters, larval morphologies have been described in 12 species ( Nunomura and Honma 2004; Tanaka 2004; Ota et al. 2007; Coetzee et al. 2008, 2009; Ota and Hirose 2009a, b; Ota 2011, 2014, 2015). Most of the species (10) have been reported from elasmobranch fishes. Larvae of these species (4–13 mm; Nunomura and Honma 2004; Coetzee et al. 2008, 2009; Ota and Hirose 2009a, b; Ota 2011, 2014, 2015) are much larger than third praniza larvae (=finale stage larvae) of the present new species (2.5–3.1 mm). In the remaining two species, G. camuripenis Tanaka, 2004 is distinguished from G. capitellum sp. nov. by its rectangular head, subtriangular pleotelson, peduncle articles of antennula and antenna fringed with fine setae, narrower basis of pereopods 2–4 ( Tanaka 2004). Gnathia limicola Ota and Hirose, 2007 is distinguished by the triangular pleotelson with lateral margin sinuous and narrower basis of pereopods 2–4 ( Ota et al. 2007). The larva of G. capitellum sp. nov. is characterized by the remarkably oval-shaped basis in the pereopods 2–4.

Adult male of G. capitellum sp. nov. is quite unique with the small head and short mandibles. However, under the current classification of Gnathiidae , it is considered to belong to Gnathia based on the morphological features of the cephalosome and mouthparts. The genus Gnathia is the most diverse taxon in the family Gnathiidae and shows varied morphological features. Therefore, further discussions based on morphological as well as molecular data on the validity of the genus might be necessary to develop a clearer phylogenetic classification of gnathiid isopods.