See Ariyama (2009).

Holotype : male (6.1 mm) (MBM 286557), dissected, Okinawa Trough, 27°32'N, 126°58'E, RY0067, ROV-3, depth 1243 m, 16 Apr. 2014. Paratype: female (4.4 mm), dissected, same data as holotype.

1 female (4.5 mm), 3 males (4.2–5.5 mm) (MBM 286557), Okinawa Trough, 27°32'N, 126°58'E, RY0067, ROV-3, depth 1243 m, 16 Apr. 2014. 1 female (3.3 mm) 1 male (4.1 mm) (MBM 286560), Okinawa Trough, 27°33'N, 126°58'E, RY0051, ROV-3, depth 1243 m, 16 Apr. 2014. 15 females and males (<5.5 mm) (MBM 286565), Okinawa Trough, 27°33'N, 126°58'E, RY0069, ROV-3, depth 1243 m, 16 Apr. 2014.

Head. Eyes not visible in ethanol material. Antenna 1 subequal in length to antenna 2; peduncular article 1 shorter than article 2 (0.8×); article 2 elongate, length 4.1× width; article 3 less than half the length of article 1; primary flagellum 5-articulate, not setose; accessory flagellum hardly reaching to end of 1^st flagellar article, 2-articulate, distal article tiny. Antenna 2 with peduncular article 4 1.7× longer than article 5; peduncular article 5 much narrower than article 4; flagellum 3-articulate, not setose.

Mouthparts. Epistome separate, upper lip rounded. Right mandible incisor well developed, with blunt denticles; palp 3-articulate, article 2 1.2× longer than article 3, bearing one long seta, article 3 bearing apical long seta; left mandible with dentate lacinia mobilis. Lower lip with inner lobes absent; mandibular lobes weak. Maxilla 1 with inner plate rounded, bearing single robust apical seta; outer plate broadly truncate, bearing five robust apical setae; palp 1-articulate, with two apical setae. Maxilla 2 with inner plate wider and shorter than outer plate, with 3 apical setae; outer plate with four apical setae. Maxilliped with very short inner plates, reduced to small lobes; outer plates short, slightly beyond distal margin of 1^st palp article, rounded, bearing few marginal and apical setae; palp 4-articulate, inner margins of article 2 and 3 bearing setae, dactylus elongate, slender.

Pereon. Coxae 1–4 longer than broad, overlapping. Gnathopod 1 parachelate, stouter than gnathopod 2; coxa with a small seta at posteroventral corner; basis linear, with 1 long seta on posterior margin; ischium subquadrate; merus subequal in length to ischium, posterior margin bearing three long setae; carpus shorter than palm, distally expanded, posterior margin lobate, bearing patch of long setae distally; palm slightly longer than deep, ventral margin fringed with long setae, with five bumps and few setae; dactylus curved, tapering. Gnathopod 2 elongate, chelate; coxa oval; basis linear, naked; ischium longer than menus, naked; merus subequal in length to carpus; carpus distally expanded; propodus slender, narrowing distally, more than 4× longer than carpus. Lower finger of chela straight, bearing row of short setae on palmar edge; dactylus slender, straight, fitting palm, distally with few short setae. Pereopod 3 with coxa slightly longer than coxa 2; basis not expanded posteriorly, anterior margin bearing four shorter setae in distal half length; ischium subrectangular, anterior margin notched; merus anterodistally drawn out, pointed, both anterior and posterior margins bearing few robust setae; carpus shorter than merus; propodus slightly longer than carpus; dactylus curved, tapering. Pereopod 4 slightly larger than pereopod 3, but of similar appearance; coxa smaller than coxa 3, posterior margin excavated. Pereopod 5 coxa bilobed, posterior lobe longer and larger than anterior lobe; basis evenly expanded, posteroproximal margin overlaps posterior coxal lobe; merus expanded, produced posteroventrally, well overreaching distal margin of carpus; carpus shorter than propodus; dactylus curved. Pereopod 6 larger than pereopods 5 and 7, of similar appearance with pereopod 5, coxa unilobate. Pereopod 7 smaller than pereiopod 5, coxa unilobate, much smaller than coxa 6; merus expanded, but smaller than that of pereopods 5 and 6, not extending past distal margin of carpus; carpus shorter than propodus.

Gills present on coxae 3–6, small, not pleated.

Pleon. Epimeron 1–3 smooth, posteroventral margin rounded. Uropod 1 peduncle subequal in length to rami, with two distal, two lateral, and three medial robust setae; outer ramus slightly shorter than inner ramus, both outer and inner ramus with three robust setae. Uropod 2 extending as far as uropod 1, peduncle slightly longer than rami, with one distal and five marginal setae; outer ramus slightly shorter than inner ramus; outer and inner ramus each with three marginal robust setae. Uropod 3 uniramous, peduncle naked; ramus longer than peduncle, with two marginal setae and two setae at base of minute terminal article. Telson entire, laminar, tapering distally, smoothly rounded, distinctly overreaching end of uropod 3 peduncle, with one or two distolateral setae on each margin.

Figure 1. 

Seba longimera sp. nov., male holotype (6.1 mm) (MBM 286557), Okinawa Trough. G1 R, right gnathopod 1; G2 R, right gnathopod 2; A1, antenna 1 (with accessory flagellum enlarged); A2, antenna 2.

Figure 2. 

Seba longimera sp. nov., male holotype (6.1 mm) (MBM 286557), Okinawa Trough. P3 R, right pereopod 3; P4 R, right pereopod 4; P5 R, right pereopod 5; P6 R, right pereopod 6; P7 R, right pereopod 7; U1 R, right uropod 1; U2 R, right uropod 2; U3 R, right uropod 3; T, telson; E1–3, epimeron 1–3.

Figure 3. 

Seba longimera sp. nov., male holotype (6.1 mm) (MBM 286557), Okinawa Trough. UL, upper lip; LL, lower lip; Md R, right mandible (with incisor process enlarged); Md L, incisor process and lacinia mobilis of left mandible enlarged; Mx1 R, right maxilla 1; Mx2 R, right maxilla 2; Mxp, pair of maxillipeds.

Figure 4. 

Seba longimera sp. nov., female paratype (4.5 mm) (MBM 286557), Okinawa Trough. G1 R, right gnathopod 1; G2 R, right gnathopod 2; P3 R, right pereopod 3; P4 R, right pereopod 4; P5 R, right pereopod 5; P6 R, right pereopod 6; P7 R, right pereopod 7.

Based on female paratype, 4.5 mm.

Gnathopod 1 parachelate, but tending to chelate; propodus much narrower than that of male; palm nearly straight, only bearing few setae. Pereopods 5 and 6 with basis not as expanded as in male, narrower than that of pereopod 7; merus not as expanded as in male, and not extending to distal margin of carpus.

In one small male specimen (4.2 mm), the merus of pereopods 5 and 6 does not overreaching distal margin of carpus.

From the Latin longus (= long), referring to the merus of pereopods 5 and 6 overreaching the distal margin of carpus.

Northwest Pacific, Okinawa Trough, the hydrothermal vents at a depth of 1243 m.

The new species, reaching a length of 6 mm, is larger than all described Seba species that are usually less than 4 mm (Shaw 1989). Seba longimera sp. nov. is most similar to S. bathybia and S. profundus, which also are associated with hydrothermal vents, but it differs from these two species in the following characters. It differs from S. bathybia by having: the denticulate palm of gnathopod 1; coxa 2 lacking a notch, and coxa 4 smaller than coxae 2 and 3; the posterior lobe of the bilobed coxa 5 larger than the anterior lobe, and the expanded merus of pereopods 5 and 6 distinctly overreaching the distal margin of the carpus in male. Similarly, the new species differs from S. profundus by having coxa 4 smaller and the posterior lobe of coxa 5 larger than the anterior one; pereopod 6 larger than pereopods 5 and 7, rather than smaller than them as shown in Shaw (1989: fig. 3A); and the merus of pereopods 5 and 6 expanded beyond the distal margin of the carpus in male.