Gnathia mutsuensis Nunomura, 2004

Ota, Yuzo, 2013, Redescription of five gnathiid species from Japan (Crustacea: Isopoda), Zootaxa 3737 (1), pp. 33-56 : 51-54

Gnathia mutsuensis Nunomura, 2004

(Japanese name: Mutsu-umi-kuwagata) ( Figs 10 , 11 )

Gnathia mutsuensis Nunomura, 2004: 351 –354, fig. 1.—Nunomura & Shimomura 2013a: 142, 144.—Nunomura & Shimomura 2013b: 294, 295, fig. 136.

Material examined. Male adult, holotype, TOYA Cr-13050, 2.1 mm, from intertidal shore of Asamushi, Aomori City, northern Japan (40°54′N, 140°51′E), 8 Jul. 1983, coll. Makoto Tsuchiya.

Redescription ( Figs. 10 , 11 ). Body 2.1 mm, without tubercles. Color of fixed specimens white.

Cephalosome ( Fig. 10 A–C) almost square with posterior margin slightly convex, sparsely covered with setae. Frontal border convex with many setae and 3 small and dentate processes. Front lateral processes slightly rise with several setae. Dorsal sulcus wide and shallow V-shape. Oval-shaped translucent region visible anteromedianly on dorsal sulcus. Eyes composed with 44 ocelli. Paraocular ornamentation and supraocular lobe not prominent.

Pereonite I ( Fig. 10 A) short, three separated parts by posterior margin of cephalosome. Central part of pereonite I fused with cephalosome. Pereonite II and III almost same width and length. Pereonite IV with constriction anteriorly. Pereonite VII short and narrow, overlapping pleonite I.

Pleonite I–V ( Fig. 10 A) narrower posteriorly, with 1 seta on lateral margin of each segment. Pleonite V longer than each segment of pleonite II–IV. Epimera not prominent.

Pleotelson ( Fig. 10 D) triangular. Lateral margin straight and fringed with fine setae. Apex slightly acute. Two pairs of seta on dorsal surface and 1 pair of seta on distal apex.

Mandible ( Fig. 10 A–C) nipper-like shape, seems as triangular in dorsal view, not reaching half-length of cephalosome. Dentate blade occupies over half length of mandible. One mandibular seta present near armed carina on mid-dorsal surface. Basal neck prominent. Erisma not prominent.

Antennule ( Fig. 10 E) composed with 3 peduncle and 5 flagellar articles. Two, 4, and 1 feather-like bristles on distal margins of peduncle article I, II, and III, respectively. Four pectinate scales visible on peduncle article II. One feather-like bristle on flagellar article I. One aesthetasc on flagellar article III, IV, and V, respectively. Flagellar article V terminating in 2 setae.

Antenna ( Fig. 10 F) composed with 4 peduncle and 7 flagellar articles. One feather-like bristle on peduncle IV. Pectinate scales visible on peduncle article I–III. 1–3 setae on distal margins of flagellar articles I–VI. Article VII terminates in 4 setae.

Maxilliped ( Fig. 11 A) with endite extending palp article II. Three, 6, 5, and 7 plumose setae on external margins of palp articles I–IV, respectively. Four setae on internal margin of palp article IV.

Pylopod ( Fig. 11 B) 3-articled. Article I semicircular with 3 areolae, 32 plumose setae on internal margin, 4 setae on external margin, 10 setae on distal margin. Article II elliptical with 4 setae. Article III minute.

Pereopod II ( Fig. 11 C) sparsely covered with setae on inner margin. Pectinate scales visible on inner margin of ischium, merus, carpus, and propodus. Basis oblong with 2 feather-like bristles. Ischium shorter than basis, becoming larger distally. Merus with 2 projections on inner margin. Carpus with 3 projections and 1 denticulate compound spine on inner margin. Propodus rectangular, bearing 2 denticulate compound spines on inner-mid and inner-distal margins. Dactylus terminating in unguis.

Pleopod II lost.

Uropod ( Fig. 11 D) fringed with fine setae. Exopod with 6 simple and 4 plumose setae and endopod with 3 simple and 6 plumose setae, and 5 feather-like bristles.

Penes ( Fig. 11 E) composed with 2 contiguous papillae.

Remarks. Gnathia mutsuensis was originally described by Nunomura (2004) based on one male specimen. This redescription based on the holotype determined the following differences from original description: (1) pereonite I was not drawn in the original description but was actually present in this study; (2) the dorsal sulcus was not drawn but a v-shaped sulcus was actually present; (3) the mandibular seta and basal neck were not described but were actually present; (4) the endite of the maxilliped was present on the basis but was actually on the distal part of the basis. The basal neck of the mandible was difficult to confirm in the dorsal view because it was closed and hidden behind the frontal border ( Fig. 10 B).

Gnathia mutsuensis was most similar to G. nasuta and G. sanrikuensis among the all Gnathia species because of its triangular nipper-shaped mandibles and three frontal processes on the frontal border. However, the pleotelson of G. mutsuensis was fringed with fine setae, pectinate scales were visible on the peduncle articles of the antennae, and no protrusion raised anteromedially on the dorsal sulcus.

Figures

FIGURE 10. Gnathia mutsuensis Nunomura, 2004 holotype (TOYA Cr- 13050): A, body, dorsal view; B, cephalosome, dorsal view; C, cephalosome, lateral view; D, pleotelson, dorsal view; E, right antennule; F, right antenna.

FIGURE 11. Gnathia mutsuensis Nunomura, 2004 holotype (TOYA Cr- 13050): A, left maxilliped, ventral view; B left pylopod, ventral view; C, right pereopod II, lateral view; D, right uropod; E, penes. Pleopod II lost.