Description of Seba longimera sp. nov. from hydrothermal vents in the Okinawa Trough, Northwest Pacific (Amphipoda, Amphilochoidea, Sebidae)

Abstract Seba longimerasp. nov., of the family Sebidae Walker, 1908, is described from hydrothermal vents in Okinawa Trough. Other two congenic species, S. bathybia Larsen, 2007 and S. profundus Shaw, 1989, are also reported from these hydrothermal vents, but the new species can be readily distinguished from them in having the merus of pereopods 5 and 6 extending beyond distal margin of carpus, coxa 4 smaller than coxae 2 and 3, and coxa 5 with the posterior lobe larger than the anterior one, rather than equilobate.


Introduction
The genus Seba Spence Bate, 1862 currently contains 24 species (updated by Köppen and Coleman 2011), and occurs in the Mediterranean Sea, Eastern and Southern Atlantic Ocean, southern United States, Hawaiian Islands, Indian Ocean, Antarctica, Japan, Eastern Pacific, and Australia from shallow to deep waters (Larsen 2007;Ariyama 2009;Yerman and Coleman 2009). When the Chinese research vessel "KEXUE" surveyed the biodiversity of hydrothermal vents in Okinawa Trough in the western Pacific in 2014, some individuals referred to Seba were collected. After careful examination, those specimens exhibited some distinctive characters differentiating them from the other described Seba species. The new species is most similar to S. bathybia Larsen, 2007 and S. profundus Shaw, 1989, which are also reported from hydrothermal vents. However, Seba longimera sp. nov. differs from above two species in having pereopods 5 and 6 with the merus extending beyond the distal margin of carpus, coxa 4 smaller than coxae 2 and 3, and coxa 5 having the posterior lobe larger than the anterior one, rather than equilobate. The present work describes this new species and compares it with closely related species.

Material and methods
The present material was collected by ROV "FAXIAN" during expeditions to the Okinawa Trough hydrothermal vents by the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) in April 2014. All the specimens examined are deposited in the Marine Biological Museum, Chinese Academy of Sciences (MBMCAS), Qingdao, China. Specimens were examined and dissected under a dissecting microscope (ZEISS Discovery V20). Line drawings were prepared with a graphics tablet using Adobe Photoshop CS6 software. Length measurements are made along the outline of the animals, beginning from the anterior margin of head to the end of the telson.
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.
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.