Seba latisexta, Ariyama, Hiroyuki, 2009

Ariyama, Hiroyuki, 2009, Four species of the genus Seba from Japan, with descriptions of two new species (Crustacea: Amphipoda: Sebidae), Zootaxa 2159, pp. 44-68 : 61-63

Seba latisexta sp. nov.

( Figs 15–18 )

Material examined. Holotype: male, 2.7 mm (OMNH-Ar-7725), off Tsuiura in Saeki City, Oita Prefecture (33°03'00''N, 131°55'41''E), 5.5–7 m deep, surface and/or inside of wooden test piece, 4 March 2008 (2 years after fitting on concrete block), coll. Oita Prefectural Agriculture, Forestry and Fisheries Research Center, Fisheries Research Institute. Allotype: female, 2.7 mm (OMNH-Ar-7726), same data as holotype. Paratypes: 3 males, 2.9, 2.8, 2.5 mm (OMNH-Ar-7727–7729) and 1 female, 2.5 mm (OMNH-Ar-7730), same data as holotype.

Description. Male [based on holotype, 2.7 mm, and paratype, 2.9 mm (OMNH-Ar-7727) for labrum, maxilla 1 and epimeral plates]. Body ( Fig. 15 ) relatively slender, urosomites 2 and 3 coalesced, eyes absent. Antenna 1 ( Fig. 16 A), ratio of peduncular articles 1–3 1:1.2:0.4, ventrodistal corner of article 1 with 2 penicillate setae, lateral surface with 3 penicillate setae, laterodistal edge of article 2 with penicillate seta; accessory flagellum present, with 2 articles; primary flagellum with 5 articles, distal ends of articles 1–3 with 1, 2 and 1 aesthetascs, respectively. Antenna 2 ( Fig. 16 B), ratio of peduncular articles 3–5 1:0.6:0.2, article 3 with 2 lateral, 1 medial, 1 ventral and 2 distal penicillate setae; flagellum with 2 articles. Labrum (Fig, 16C) pentagonal, without setae. Mandible ( Fig. 16 D, E), molar reduced to several spines with fine setae; palp article length ratio in left 1:3.3:3, article 3 with 2 apical setae. Maxilla 1 ( Fig. 16 G, G1), inner plate with single apical seta, outer plate with various-typed apical setae, palp with 3 apical setae. Maxilla 2 ( Fig. 16 H), inner plate with 3 simple setae, outer plate with 4 plumose setae apically. Maxilliped ( Fig. 16 I), inner plate with 1 long and 1 short robust setae, outer plate with 2 medial and 1 apical robust setae; palp article 3 with several mediodistal setae, medial surface of article 4 pectinate.

Gnathopod 1 ( Fig. 17 A, A1) small, weakly chelate; coxa rounded posterodistally; basis slender, with long seta on posterodistal margin; merus and carpus each with a few posterior setae; propodus projected posterodistally, projection with a pair of small robust setae, palm with 1 triangular and 2 obtuse processes, middle part of posterior margin setose; dactylus narrow, weakly curved. Gnathopod 2 ( Fig. 17 B, B1) large, strongly chelate; coxa square posterodistally, gill absent; basis broadened distally, curved anteriorly; carpus short, 0.4 times as long as propodus; propodus stout, protruded posteroproximally, distal end of propodus with a pair of minute robust setae; tip of dactylus curved. Pereopods 3 and 4 ( Fig. 17 C, D) slender, posterodistal corners of coxae square. Pereopod 5 ( Fig. 17 E), coxa bilobed; basis oval; merus slightly expanded; carpus short, 0.6 times as long as propodus. Pereopod 6 ( Fig. 17 F, F1), coxa bilobed, anterior lobe small; basis wide, protruded posterodistally; merus extremely expanded, with many robust setae on posterior margin; carpus short, 0.6 times as long as propodus. Pereopod 7 ( Fig. 17 G), coxa triangular; basis ovoid, greatly expanded posterodistally, posterior margin serrate, proximal part of medial surface with several setae; merus relatively narrow; carpus short, about half length of propodus.

Epimeral plates 2 and 3 ( Fig. 17 H) slightly drawn out posteroventrally; ventral margins of plates 1–3 with 1, 1 and 3 lateral spines, respectively. Pleopods ( Fig. 18 D–F) long, pleopod 3 shortest; peduncles each with 2 coupling hooks and a few plumose setae; outer and inner rami each with 5 and 4 articles, respectively. Uropod 1 ( Fig. 18 A), peduncle longer than both rami, with 1 dorsal and 2 distal robust setae; outer ramus shorter than inner, both rami bare. Uropod 2 ( Fig. 18 B), peduncle shorter than that of uropod 1, with 2 distal robust setae; outer ramus shorter than inner, both rami each with dorsal robust seta. Uropod 3 ( Fig. 18 C, C1), 1st article of ramus with 1–2 fine distal setae, 2nd article minute. Telson ( Fig. 18 C) with a pair of distal setae.

Female (based on allotype, 2.7 mm). Gnathopod 1 ( Fig. 18 G, G1) small, weakly chelate; coxa rounded posterodistally; basis slender, with 2 long setae on posterior margin; merus and carpus each with several posterior setae; propodus projected posterodistally, projection with a pair of small robust setae, palm with indistinct process posteriorly, posterior margin setose in the middle; dactylus narrow, weakly curved. Gnathopod 2 ( Fig. 18 H, H1) strongly chelate; coxa roundish square posterodistally, gill absent; basis slightly curved anteriorly; carpus short, 0.3 times as long as propodus; distal end of propodus with a pair of minute robust setae; tip of dactylus curved. Pereopods 5–7 ( Fig. 18 I–K) almost same as those of holotype, but bases of pereopods 6 and 7 narrower and meri of pereopods 5–7 more slender.

Etymology. From the Latin latus (= broad) and sextus (= the sixth), referring to the expanded merus of male pereopod 6.

Remarks. This new species is characterized by the broadly expanded merus of male pereopod 6, and there is no other species having such a merus only in pereopod 6.

Habitat. Submerged wood, 5.5–7 m deep.

Distribution ( Fig. 5 ). Japan: off Tsuira in Oita Prefecture.

Figures

FIGURE 5. Distributions of Seba species dealt in the present study.

FIGURE 15. Seba latisexta sp. nov. Holotype, male, 2.7 mm (OMNH-Ar- 7725), habitus, left lateral view. Scale: 0.5 mm.

FIGURE 16. Seba latisexta sp. nov. A, B, D – F, H, I, holotype, male, 2.7 mm (OMNH-Ar- 7725); C, G, G 1, paratype, male, 2.9 mm (OMNH-Ar- 7727). A, B, left antennae 1, 2, medial views; C, labrum, anterior view; D, left mandible, medial view; E, right mandible, lateral view; F, labium, ventral view; G, right maxilla 1, ventral view; G 1, apical part of outer lobe of right maxilla 1, ventral view; H, right maxilla 2, dorsal view; I, right maxilliped, ventral view. Scales: 0.05 mm.

FIGURE 17. Seba latisexta sp. nov. A – G, A 1, B 1, holotype, male, 2.7 mm (OMNH-Ar- 7725); H, paratype, male, 2.9 mm (OMNH-Ar- 7727). A, left gnathopod 1, lateral view; A 1, palm of left gnathopod 1, lateral view; B, left gnathopod 2, lateral view; B 1, apical part of propodus of left gnathopod 2, lateral view, setae omitted; C – G, left pereopods 3 – 7, lateral views (gill on pereopod 3 and carpus – dactylus of pereopod 6 lost); F 1, carpus – dactylus of right pereopod 6, medial view; H, right epimeral plates 1 – 3, lateral view. Scales: 0.05 mm.

FIGURE 18. Seba latisexta sp. nov. A – F, C 1, holotype, male, 2.7 mm (OMNH-Ar- 7725); G – K, G 1, H 1, allotype, female, 2.7 mm (OMNH-Ar- 7726). A, B, left uropods 1, 2, lateral views; C, left uropod 3 and telson, dorsal view; C 1, tip of right uropod 3, lateral view; D, left pleopod 1, posterior view; E, F, left pleopods 2, 3 medial views; G, left gnathopod 1, lateral view; G 1, palm of left gnathopod 1, lateral view; H, left gnathopod 2, lateral view; H 1, apical part of propodus of left gnathopod 2, lateral view, setae omitted; I – K, left pereopods 5 – 7, lateral views. Scales: 0.05 mm.