Four new deepwater Ophichthus (Anguilliformes: Ophichthidae) from Japan with a redescription of Ophichthus pallens (Richardson 1848).

In this paper, new and uncertain Japanese snake eels of the genus Ophichthus are treated. Ophichthus kusanagi, O. lupus, O. oligosteus and O. yamakawai are herein described as new species. Ophichthus kusanagi, collected from Suruga Bay of Shizuoka Prefecture, off Owase, Mie Prefecture, and Tosa Bay, Kochi Prefecture, is distinguishable from four similar congeners by having one + four supraorbital and two preopercular pores, 13 pectoral-fin rays, fewer total vertebrae (158–163), a longer tail [58–60% of total length (TL)], a small eye [7.5–9.0% of head length (HL)], an acute snout, a low knob on upper lip between nostrils, relatively slender teeth, and a pointed pectoral fin. Ophichthus lupus, collected from the East China Sea, is characterized by its numerous total vertebrae (180–182), shorter pectoral fins (20–25% HL), and the position of the dorsal-fin origin (slightly behind the pectoral-fin tip); it can be distinguished from similar species in its supraorbital and mandibular pores (one + four and six or seven), arrangements of teeth on its mandible and vomer (almost uniserial), its robust body (depth 2.6–3.1% TL), slightly larger head (7.8–8.1% TL), longer and acute snout (24% HL), longer upper jaw (45–48% HL), absence of protrusion along upper lip, and the narrow dark margin of its dorsal fin. Ophichthus oligosteus, collected from Tosa Bay of Kochi Prefecture, is characterized by its fewer preanal and total vertebrae (46 and 121). Ophichthus yamakawai, collected from Kochi Prefecture, is characterized by its large blotches along the lateral line and numerous preanal and total vertebrae (67 and 172). We refer an uncertain species, Ophichthus stenopterus Cope 1871, to the synonymy of Ophichthus pallens (Richardson 1848), a forgotten species described from China. Pisoodonophis zophistius Jordan and Snyder 1901 is recognized as valid in Ophichthus and a senior synonym of Ophichthys intermedius Regan 1905. Ophichthus habereri Franz 1910, originally described from Yokohama, central Japan, is a junior synonym of Ophichthus asakusae Jordan and Snyder 1901. Pisodonophis sangjuensis Ji and Kim 2011 is referred to Ophichthus. We currently recognize 20 species of Ophichthus from Japan. A key to the Japanese species of Ophichthus and its closely associated group Pisodonophis is provided. [ABSTRACT FROM AUTHOR]