(Figs 1–4)
Seba chiltoni Moore, 1987: 258, figs 14–15 (type locality: Tinderbox, d’Entrecasteaux Channel, Tasmania). Seba typica.— Chilton, 1921: 56, fig. 6 a–d.— Chilton, 1924: 269.— Ledoyer, 1978: 314, fig. 42.— Ledoyer, 1986: 948, fig. 374.
Material examined. Two males, 3.1, 2.3 mm (OMNH-Ar-7701, 7702) and 2 females, 3.7, 2.3 mm (OMNH- Ar-7703, 7704), Rinkai in Shirahama, Wakayama Prefecture, from a sponge Callyspongia confoederata (Ridley, 1884) cast ashore, 18 August 1992, coll. H. Ariyama.
Description. Male [based on mall '' 1 '', 3.1 mm (OMNH-Ar- 7701)]. Body (Fig. 1) roundish, urosomites 2 and 3 coalesced, eyes absent. Antenna 1 (Fig. 2 A), ratio of peduncular articles 1–3 1: 1: 0.3, surface of peduncle almost bare; accessory flagellum present, with 2 articles; primary flagellum with 4 articles, posteroproximal part of article 1 with bundle of aesthetascs, distal ends of articles 1 and 2 each with 2 aesthetascs. Antenna 2 (Fig. 2 B), ratio of peduncular articles 3–5 1: 0.6: 0.2, ventrodistal corner of article 3 with 2 penicillate setae; flagellum with 3 articles. Labrum (Fig, 2 C) longish triangular, without setae. Mandible (Fig. 2 C, D), molar reduced; palp article length ratio 1: 2.4: 2, articles 2 and 3 with many fine setae, article 3 with 2 apical setae. Labium (Fig. 2 E), outer lobes bearing thin setae on distal margin. Maxilla 1 (Fig. 2 F, F 1–3), inner plate bare, outer plate with bifid apical setae; palp with 3 apical setae: 1 large bifid, 1 triangular and 1 spear-like setae. Maxilla 2 (Fig. 2 G) with inner and outer plates bearing 3 and 4 apical thick setae, respectively. Maxilliped (Fig. 2 H), inner plate bearing 1 large bifid, 1 spear-like and 1 normal setae distally; outer plate with 3 medial robust setae; palp article 3 setose medially.
Gnathopod 1 (Fig. 3 A, A 1) large, subchelate; coxa rounded posterodistally; basis broadened distally, with 2 setae on posterior margin; merus and carpus setose posterodistally; propodus stout, projected posterodistally, projection with a pair of small robust setae, palm with 1 large triangular, 1 small trapezoidal and 2 minute processes, middle part of posterior margin setose; dactylus wide, gradually curved, posterior margin with large process in the middle. Gnathopod 2 (Fig. 3 B, B 1) slender, strongly chelate; coxa rounded posterodistally, gill absent; basis gradually curved anteriorly; carpus 0.8 times as long as propodus; propodus short, distal end with a pair of minute robust setae; tip of dactylus curved. Pereopods 3 and 4 (Fig. 3 C, D), posterodistal corners of coxae rounded; meri relatively wide, anterodistal corners acutely pointed. Pereopod 5 (Fig. 3 E), coxa bilobed; basis roundish rectangular; merus expanded, posterior margin with 4 robust setae; carpus subequal length to propodus. Pereopod 6 (Fig. 3 F), coxa bilobed, anterior lobe small; basis oval; merus broadened, posterior margin with 4 robust setae; carpus subequal length to propodus. Pereopod 7 (Fig. 3 G), coxa triangular; basis greatly expanded posterodistally, posterior margin serrate, proximal part of medial surface with a few setae; merus widely expanded posterodistally, distal margin of lobe rounded, posterior margin weakly serrate, anterior half of lateral surface bearing sparse short setae; carpus 0.9 times as long as propodus.
Epimeral plates 2 and 3 (Fig. 3 H) slightly drawn out posteroventrally; ventral margins of plates 1–3 with 1, 2 and 2 lateral spines, respectively. Uropod 1 (Fig. 4 A), peduncle shorter than both rami, with 1 dorsal, 1 lateral and 2 distal robust setae; both rami subequal in length, without setae. Uropod 2 (Fig. 4 B), peduncle subequal length to that of uropod 1, with 1 dorsal and 2 distal robust setae; both rami subequal in length, bare. Uropod 3 (Fig. 4 C) stout; peduncle with 2 distal fine setae; 1 st article of ramus with 2 fine setae dorsally, medial surface with minute distal process, 2 nd article small. Telson (Fig. 4 D) with a pair of dorsal setae.
Female [based on female '' 1 '', 3.7 mm (OMNH-Ar- 7703)]. Gnathopod 1 (Fig. 4 E, E 1) small, weakly chelate; coxa rounded posterodistally; basis slender, posterodistal corner with short seta; merus and carpus setose posteriorly; propodus projected posterodistally, projection with a pair of small robust setae, palm smooth, posterior margin setose in the middle; dactylus narrow, almost straight. Gnathopod 2 (Fig. 4 F, F 1) slender, strongly chelate; coxa rounded posterodistally, gill absent; basis slightly curved anteriorly; carpus 0.7 times as long as propodus; distal end of propodus with a pair of minute robust setae. Pereopods 5 and 6 almost same as those of male. Pereopod 7 (Fig. 4 G) almost same as that of male, but merus narrower and posterior margin with 5 robust setae. Pleopods (Fig. 4 H–J) long, pleopod 3 shortest; peduncles each with 2 coupling hooks and 1–4 plumose setae; outer and inner rami each with 4 and 3 articles, respectively.
Variation of male with growth. Male '' 2 '', 2.3 mm (OMNH-Ar- 7702): gnathopod 1 (Fig. 4 K), palm with 2 triangular and 1 small processes, posterior margin of dactylus without process; pereopod 7, merus narrower than that of male '' 1 '', but apparently different from female.
Remarks. The morphological characters of this species almost agree with the descriptions and figures of Moore (1987) and Ledoyer (1978, 1986). This species is distinctive in the robust subchelate gnathopod 1 and the spread merus of pereopod 7 in male. However, the width of the merus varied with the previous reports: the merus in the present study resemble that in Chilton (1921) and Ledoyer (1978), but is wider and posterodistally rounder than that in Moore (1987) and Ledoyer (1986).
Habitat. A sponge Callyspongia confoederata (present study); a brown alga Ecklonia holdfast and sponge, 3–4 m deep (Moore, 1987).
Distribution (Fig. 5). Japan: Shirahama in Wakayama Prefecture (present study); Australia: Flinders Island (Chilton, 1921), Tasmania (Moore, 1987); Mauritius (Ledoyer, 1978); Madagascar (Ledoyer, 1986).
In addition, S. typica recorded at Bunaken Island in Indonesia (Ortiz & Lalana, 1997) may be S. chilton, because Seba chiltoni is close to S. typica (Moore, 1987) which is distributed only in New Zealand (Chilton, 1884, 1906), off Tierra del Fuego (Holman & Watling, 1983) and the Beagle Channel (Chiesa & Alonso, 2007).