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During two expeditions to the seamounts in the Yap-Caroline area of the Western Pacific, a new genus, Phoxirostus gen. nov., in the family Laphystiopsidae Stebbing, 1899, is erected for two new species, P. longicarpus sp. nov. (type species) and P. yapensis sp. nov. The new genus can be distinguished from the other three laphystiopsid genera by the acute rostrum not overreaching the distal end of the first peduncular article of antenna 1, the outer plate of maxilla 1 bearing 10–11 spines, and the elongated carpus of pereopods 3–7 being distinctly longer than half the length of the propodus. Phoxirostus longicarpus sp. nov. differs from P. yapensis sp. nov. by the shape of the eyes and coxa 4, the presence of posterodistal protrusions on pleonite 1, and the number of posterodistal protrusions on pleonite 2. Generic analysis of one mitochondrial (COI) and one nuclear (H3) gene using maximum likelihood and Bayesian inference clarified the phylogenetic position of the Laphystiopsidae within the superfamily Iphimedioidea Boeck, 1871.

The Formosa Ridge methane seep is one of three known active methane seepage areas discovered on the northern slope of the South China Sea. Cyphocaris is a genus of pelagic amphipods. Two new species of Cyphocaris collected from water columns above the Formosa Ridge methane seep are reported in the present study. Cyphocaris lubrica sp. nov. collected at a depth of 1,118 m is characterized by pereonite 1 dorsal projection not extending beyond head, and the pereopod 5 having a spur beyond pereonite 7. Cyphocaris formosa sp. nov. collected at a depth of 500–800 m is characterized by well-developed pereonite 1 dorsal projection extending beyond head and pereopod 5 without spur. The present work describes these two new species and compares them with closely related species, and provides a modified key of world Cyphocarididae species.

A new species of the subfamily Stegocephalinae, Stegocephalus carolus sp. nov., is described from a seamount in the Caroline Plate. Two related species, S. cascadiensis (Moore, 1992) and S. longicornis (Gurjanova, 1962), were previously reported in the North Pacific. Important morphological characters which differentiate S. carolus sp. nov. from S. cascadiensis are found in antenna 1, the mouthparts, pereopod 7 and the length of rami of uropods 2 and 3. The new species differs from S. longicornis by characters of antenna 1, the mouthparts and the shape of epimeral plate 3. Additionally, the morphological differences between the new species and the remaining seven species of Stegocephalus are also presented. [ABSTRACT FROM AUTHOR]

A new species of the family Tryphosidae, Orchomenella compressa sp. nov., is described from hydrothermal vents in the Okinawa Trough. This is the first known Orchomenella species found in vent fields. Important morphological characters that differentiate O. compressa sp. nov. from its congeners are the absence of eyes, the compressed distal three articles of gnathopod 2, the shape of the posterior margin of epimerons 2 and 3, and the number of dorsal spines on the telson. The genetic divergence of the analyzed COI gene clearly supports this new taxon. [ABSTRACT FROM AUTHOR]

Seba longimera sp. 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. [ABSTRACT FROM AUTHOR]