Gnathia excavata, Ota, Yuzo, 2012

Ota, Yuzo, 2012, Gnathiidae from Kumejima Island in the Ryukyu Archipelago, southwestern Japan, with description of three new species (Crustacea: Isopoda) *, Zootaxa 3367, pp. 79-94 : 81-84

Gnathia excavata n. sp.

New Japanese name: Eguri-umikuwagata ( Figs. 2 , 3 )

Material examined. Holotype. Male, 2.45 mm in total length (RUMF-ZC-1428), from Stn. Dredge 23 (triangular dredge), 26°16.380ʹN, 126°51.502ʹE – 26°15.982ʹN, 126°51.304ʹE, 147– 125 m depth, off Kumejima Island, the Ryukyu Archipelago, southwestern Japan, 12 November 2009.

Description. Male ( Figs. 2 , 3 ). Body 2.45 mm (n = 1).

Cephalothorax ( Fig. 2 A–C). Cephalothorax almost square, dorsal surface sparsely covered with setae. Dorsal sulcus deep; superior frontolateral processes acute, with 1, 2 setae on left, right process, respectively. Posterior margin slightly but widely concave. Mediofrontal process broad, rounded with slightly bifid apex, dorso-ventrally thinner than region surrounded by dorsal sulcus. Anterior parts of marginal carina visible below mediofrontal process in dorsal view. Eyes with 49 ocelli in 8 horizontal rows. Paraocular ornamentation composed of 3 indistinct tubercles. Supraocular lobe not acute.

Pereon ( Fig. 2 A). Pereonite 1 slightly shorter than pereonite II, not fused, separated into 2 lateral and 1 central parts by posterior margin of cephalothorax and distal margin of pereonite II ( Fig 2 C). Pereonite II slightly shorter than pereonite III; few setae on lateral margins. Pereonite IV with anterior constriction; anterolateral lobe absent. Pereonite V with areae laterales but not distinct. Pereonite VI slightly longer than combined length of pereonite IV and V. Pereonite VII not extending posterolateral margin of pereonite VI, overlapping pleonite I.

Pleon ( Fig. 2 A). Pleonites II–V subequal in length and width; epimera prominent.

Pleotelson ( Fig. 2 D). Width 0.90 length. Two pairs of setae on lateral margin and apex; lateral margins slightly convex.

Mandible ( Fig. 2 B, C). Mandible approximately half length of cephalothorax; apex curved inward and dorsally. Mandibular seta present on mid-dorsal surface near incisor. Dentate blade occupying approximately onethird of mandible length. Erisma prominent.

Antennula ( Fig. 2 E). 3 basal podomeres and 5 flagellar articles. Distal margins of basal podomeres I, II, III, and flagellar article V bearing 2, 2, 1, and 1 penicillate setae, respectively. Flagellar articles III–V each with 1 aesthetasc; article V bearing 3 terminal setae.

Antenna ( Fig. 2 F). 4 basal podomeres and 7 flagellar articles. Distal margins of basal podomeres III and IV bearing 2 and 6 penicillate setae, respectively. Flagellar articles I–VII with few setae on distal margins; article VII bearing 5 terminal setae.

Maxilliped ( Fig. 3 A). Endite extending to distal margin of palp article I. Palp articles I–IV external margins bearing 3, 6, 4, and 7 plumose setae, respectively; article IV bearing 5 simple terminal setae.

Pylopod ( Fig. 3 B). 3 articles expressed; distal margins of article I and II with 3 and 2 setae, respectively. Article I elliptical with 3 areolae bearing 27 plumose setae on internal margin and 3 setae near external margin. Article II elliptical, fringed with fine setae. Article III reduced and semicircular.

Pereopod II ( Fig. 3 C). Inner margins of merus, carpus, and propodus with pectinate scales. Basis oblong, outer margin with 3 penicillate setae and 2 processes. Ischium approximately two-thirds length of basis, becoming wider distally; distal margin bearing 1 long seta and 3 shorter setae. Merus approximately half of ischium length, bearing 3 setae on distal margin. Carpus subequal in length to merus, bearing 1 spine on inner-middle margin. Propodus rectangular and 1.3 times as long as carpus; inner-middle and inner-distal margins with 2 spines.

Pleopod II ( Fig. 3 D, E). Protopod distomedial corner with 1 coupling hook and 1 seta. Both rami lengths subequal, shape elliptical; endopod and exopod with 7 and 9 plumose setae, respectively. Top of appendix masculina bearing 1 plumose seta; another appendix masculina of left pleopod 2 lacking plumose seta. Pleopods IV and V rami shorter than pleopods I–III; exopods shorter than endopods. Exopods each with 6–11 plumose setae, endopods each with 7–9 simple setae.

Uropod ( Fig. 2 D). Both rami subequal in length, slightly extending to pleotelson apex. Exopod bearing 6 setae and 3 plumose setae laterally. Endopod bearing 8 setae laterally. Dorsal surface of endopod bearing 5 penicillate setae.

Penes ( Fig. 3 F). Fused but not prominent; tip of penes composed with 3 papillae.

Etymology. The scientific name excavata is derived from the Latin meaning “hollow”, referring to be deeply concaved on dorsal sulcus.

Remarks. Although gnathiid morphology quite differs between male adult, female adult, and larva, the taxonomy has traditionally been based on the male morphology only. Congeners of the adult males are distinguished by the frontal border, pleotelson, pylopod, maxilliped, and their combinations. In addition to these, number of setae on the mouthparts, the presence of tubercles and setae on cephalothorax and appendix masculina on pleopod II are also frequently used.

Among the 190 species of the family Gnathiidae , Gnathia and Caecognathia species comprise the most number of species of this family (containing approximately 110 and 50 species, respectively). These two genera are closely related and have been even synonymised with each other until Cohen & Poore (1994) reviewed the classification of the Gnathiidae . Cohen & Poore (1994) recognised 10 genera, including Gnathia and Caecognathia , in the family. According to their key, Gnathia species can be distinguished from the congeners by the presence of a frontal process on the frontal border of the cephalothorax (not deeply excavated frontal border), the presence of paraocular ornamentation, and depth and/or width of dorsal sulcus. On the other hand, Caecognathia species can be distinguished from the congeners by the absence of a frontal process on the frontal border (frontal border often rounded) ( Cohen & Poore 1994). However, the characters of the frontal “border” and the frontal “process” seem to be ambiguously interpreted by the authors. Some Gnathia species have large, rounded frontal “process” (or anteriorly pronounced frontal borders); e.g. G. triobata Schultz, 1966 ; G. prolasius Cohen & Poore, 1994 ; G. ubatuba Pires, 1996 ; G. r i c a rd o i Pires, 1996. However, these characters has been termed differently by different workers, e.g. “large frontal process” ( Cohen & Poore 1994), “frontal projection” ( Schultz 1966) or not even noted ( Pires 1996). Furthermore, the frontal borders of Caecognathia species are indeed pronounced anteriorly; e.g. C. hirusta ( Schultz, 1966) ; C. bicolor ( Hansen, 1916) (redescribed by Svavarsson 1999); C. dolichoderus Cohen & Poore, 1994 . Future study may provide a better understanding on the systematic position of the genera.

Gnathia excavata n. sp. is most similar to G. notostigma Cohen & Poore, 1994 , because of the following characters; the mediofrontal process is broad with bifid apex and the whole body is not covered with tubercles ( Cohen & Poore 1994). However, G. n o t o s t i g m a has very pronounced paraocular ornamentation forming slight mesolateral ridge ( Cohen & Poore 1994) rather than three small indistinct tubercles of the present new species.

Figures

FIGURE 2. Gnathia excavata n. sp., holotype, adult male (RUMF-ZC- 1428, total length, 2.45 mm): A, body, dorsal view; B, eye and frontal border, dorsal view; C, cephalothorax and pereonite I, lateral view; D, pleotelson and uropods, dorsal view; E, left antennula, medial view; F, left antenna, medial view.

FIGURE 3. Gnathia excavata n. sp., holotype, adult male (RUMF-ZC- 1428, total length, 2.45 mm): A, left maxilliped, ventral view; B, left pylopod, ventral view; C, left pereopod II, medial view; D, left pleopod II, ventral view; E, left pleopod IV, ventral view, plumose setae were omitted; F, penes, ventral view.