Seba longimera, Wang, Yanrong, Zhu, Chaodong, Sha, Zhongli & Ren, Xianqiu, 2019

Wang, Yanrong, Zhu, Chaodong, Sha, Zhongli & Ren, Xianqiu, 2019, Description of Seba longimera sp. nov. from hydrothermal vents in the Okinawa Trough, Northwest Pacific (Amphipoda, Amphilochoidea, Sebidae), ZooKeys 899, pp. 141-149 : 141

Seba longimera sp. nov. Figures 1 , 2 , 3 , 4

Material examined.

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.

Additional materials.

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.

Description of male holotype.

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 1st 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 1st 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.

Sexually dimorphic characters.

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.

Variation.

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

Etymology.

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

Distribution.

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

Remarks.

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.

Figures

Figure 1. Seba longimera sp. nov., male holotype (6.1 mm) (MBM 286557), Okinawa Trough. G 1 R, right gnathopod 1; G 2 R, right gnathopod 2; A 1, antenna 1 (with accessory flagellum enlarged); A 2, antenna 2.

Figure 2. Seba longimera sp. nov., male holotype (6.1 mm) (MBM 286557), Okinawa Trough. P 3 R, right pereopod 3; P 4 R, right pereopod 4; P 5 R, right pereopod 5; P 6 R, right pereopod 6; P 7 R, right pereopod 7; U 1 R, right uropod 1; U 2 R, right uropod 2; U 3 R, right uropod 3; T, telson; E 1 - 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; Mx 1 R, right maxilla 1; Mx 2 R, right maxilla 2; Mxp, pair of maxillipeds.

Figure 4. Seba longimera sp. nov., female paratype (4.5 mm) (MBM 286557), Okinawa Trough. G 1 R, right gnathopod 1; G 2 R, right gnathopod 2; P 3 R, right pereopod 3; P 4 R, right pereopod 4; P 5 R, right pereopod 5; P 6 R, right pereopod 6; P 7 R, right pereopod 7.