Gnathia sanrikuensis Nunomura, 1998

Ota, Yuzo, 2013, Redescription of five gnathiid species from Japan (Crustacea: Isopoda), Zootaxa 3737 (1), pp. 33-56 : 47-48

Gnathia sanrikuensis Nunomura, 1998

(Japanese name: Sanriku-umi-kuwagata) ( Figs 8–9 )

Gnathia sanrikuensis Nunomura, 1998: 55 –69, figs. 1–3.—Saito et al. 2000: 18.—Tanaka 2004: 59.—Nunomura, 2004: 354. Smit & Davies 2004: 300, table 1.—Nunomura & Shimomura 2013a: 142, 144.—Nunomura & Shimomura 2013b: 295, fig. 137.

Material examined of type series. Male adult, holotype, TOYA Cr-12473, 2.8 mm, from 42 m depth of Otsuchi Bay, Iwate Pref., northern Japan (39°20′–21′N, 141°53′–58′E), 2 Aug. 1993, coll Shin’ichi Ishimaru. Two male adults, paratypes, TOYA Cr-12474, 3.0 mm, TOYA Cr-12475, 3.0 mm, from the same locality and date of holotype.

New material. One male adult, KMNH IvR-500,719, 2.8 mm, from 46.5 m depth, sandy sediment, Otsuchi Bay, Smith-McIntyre grab, 25 May 1998, coll. Michitaka Shimomura.

Distribution ( Fig. 1 , Table 1). Otsuchi Bay, northern Japan (42–46.5 m depth).

Redescription ( Figs 8 , 9 ). Body 2.8–3.4 mm (mean ± SD; 3.0 ± 0.2 mm, n = 4) without tubercles. Color of fixed specimens white or light brown; digestive organs brown due to congealed host blood.

Cephalosome ( Fig. 8 A–D) almost square with posterior margin slightly concave, sparsely covered with setae. Frontal border of dorsal part convex with many setae and 2 pairs of small processes. Two projection with several setae slightly visible near frontal border. Frontal border of ventral part slightly extend beyond that of dorsal part. Dorsal sulcus wide and shallow V-shape. Oval-shaped translucent region visible anteromedianly on dorsal sulcus. Eyes composed with 39 ocelli. Paraocular ornamentation and supraocular lobe not prominent.

Pereonite I ( Fig. 8 A) short, not fused with cephalosome but anterior margin slightly visible. Widths and lengths of pereonites II, III, and IV almost same. Pereonite II anteriorly concave. Pereonite III and IV with anterolateral lobes. Areae laterales slightly visible on pereonite V. Pereonite VII short and narrow, overlapping pleonite I.

Pleonite I–V ( Fig. 8 A) almost equal length and narrower posteriorly, with 1 seta on lateral margin of each segment. Epimera not prominent.

Pleotelson ( Fig. 8 E) almost triangular with acute apex and covered with pectinate scales. One pair of seta on dorsal surface and 1 pair of seta on distal apex.

Mandible ( Fig. 8 A, B, D) nipper-like shape, seems as triangular in dorsal view, 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. 8 F) composed with 3 peduncle and 5 flagellar articles. One feather-like bristle on distal margin of peduncle article III. One feather-like bristle on flagellar article I. One aesthetasc on flagellar articles III and IV, and V, respectively. Flagellar article V terminating in 3 setae.

Antenna ( Fig. 8 G) composed with 4 peduncle and 6 or 7 flagellar articles. Two and 3 feather-like bristles on peduncle III and IV, respectively. 1–3 setae on distal margins of flagellar article I, III–VI. Article VII terminates in 4 setae.

Maxilliped ( Fig. 9 A) composed with basis and 4 palps. Endite of basis reaching distal part of palp article I. Three, 7, 5, and 7 plumose setae on external margins of palp articles I–IV, respectively. Five setae on internal margin of palp article IV.

Pylopod ( Fig. 9 B) 3-articled. Article I semicircular with posterior corner; 3 areolae visible, 27 plumose setae on internal margin, 7 setae on external margin, 6 setae on distal margin, and fine setae cover ventral surface. Article II elliptical with 4 setae. Article III minute.

Pereopod II ( Fig. 9 C) sparsely covered with setae. Basis oblong. Ischium shorter than basis, becoming larger distally. Merus slightly shorter than ischium. Carpus slightly larger than merus with 1 projection on inner margin. Propodus rectangular, bearing 2 denticulate compound spines on inner-mid and inner-distal margins. Dactylus terminating in unguis.

Pleopod II ( Fig. 9 D) with one seta on outer distal corner and coupling hook on inner margin. Both rami oval and equal in length. Appendix masculina of pleopod II extending half-length of endopod. 7–9 plumose setae on exopods. Six or 7 plumose setae on endopods.

Uropod ( Fig. 8 E) not extend beyond apex of pleotelson. Exopod with 5 or 6 simple and 3 or 4 plumose setae and endopod with 3 simple and 6 plumose setae, and several feather-like bristles.

Penes ( Fig. 9 E) composed with 2 contiguous papillae.

Remarks. Gnathia sanrikuensis was originally described by Nunomura (1998). This redescription based on a type series determined the following differences from the original description: (1) pereonite I was not described in the original description but was present in this study; (2) a smooth frontal border without setae was drawn but frontal processes with several setae were actually present; (3) the endite of the maxilliped and article III of the pylopod were not described; (4) penes were described as “relatively stout” but they were actually two contiguous papillae.

Among all the Gnathia species, the morphological characteristics of G. sanrikuensis were most similar to G. nasuta because of the nipper-shaped mandibles, three frontal processes on the central frontal border, acute apex of the pleotelson, and the pylopod was sparsely covered with fine setae. These species were separated because the paraocular ornamentation of G. sanrikuensis was not as remarkable as that of G. nasuta and the pleotelson was covered with distinct pectinate scales.

Figures

FIGURE 1. Sampling sites of holotype materials and new materials: k, Elaphognathia kikuchii; n, Gnathia nasuta; s, Gnathia sanrikuensis; m, Gnathia mutsuensis; b, Gnathia bungoensis. The numbers correspond to the specimen numbers in Table 1.

FIGURE 8. Gnathia sanrikuensis Nunomura, 1998, (A, C, E; TOYA Cr- 12474), holotype (B, D, F; TOYA Cr- 12473): A, body, dorsal view; B, cephalosome, dorsal view; C, frontal process; D, cephalosome, lateral view; E, pleotelson, dorsal view; F, right antennule; G, right antenna.

FIGURE 9. Gnathia sanrikuensis Nunomura, 1998 holotype (TOYA Cr- 12473): A, right or left maxilliped; B left pylopod, ventral view; C, right or left pereopod II, lateral view; D, left pleopod II; E, penes.