Symphurus thermophilus n. sp., described from 16 specimens collected by submersibles, ROV, epibenthic sled and dredge, occurs on a variety of substrata at several active hydrothermal sites located at 239-733 m between 21 degrees N and 35 S in the western Pacific Ocean. Symphurus thermophilus, the only pleuronectiform fish known to inhabit hydrothermal vent areas, is characterized by the combination of a 1-2-2-2-2 pattern of interdigitation of dorsal proximal pterygiophores and neural spines, 14 caudal-fin rays, 5 hypurals, 9 abdominal vertebrae, 47-51 total vertebrae, 88-94 dorsal-fin rays, 74-80 anal-fin rays, 100-112 scales in longitudinal series, ocular-side pigmentation pattern featuring 5-8, black, mostly incomplete crossbands, uniformly white blind side, and black peritoneum. Of specimens examined, seven including the holotype, were collected on Kaikata Seamount off southern Japan; one specimen was collected at the Kasuga-2 hydrothermal vent, Marianas Islands; and six were collected at sites on the Kermadec Ridge. In addition to specimens captured, many other S. thermophilus were observed from submersibles and ROVs at hydrothermal sites in the western Pacific including those in the Marianas Islands, at Nikko Seamount near Minami-Iohjima Island, and at Minami-Ensei Knoll, Mid-Okinawa Trough. Many of the specimens examined have skeletal anomalies including fused bones in the caudal skeleton, and missing or partially developed and/or misshapen fin rays.
