Deep-water Hydrozoa (Cnidaria: Medusozoa) in the Sea of Japan, collected during the 51st Cruise of R/V Akademik M.A. Lavrentyev, with description Opercularella angelikae, sp. nov.

A report is given about Hydrozoa collected at depths between 455 and 3666 m in the Sea of Japan during the Russian-German expedition on R/V Akademik M.A. Lavrentyev. Ten species were found, with four of them being typical bathyal–abyssal and abyssal zones. A new species, Opercularella angelikae, is described, and it was the dominant hydroid in samples from 970 to 3660 m. Four eurybathic species characteristics of the Sea of Japan were sampled between 455 and 582 m. Abyssal (pseudoabyssal after Andriashev, 1979) hydroid fauna in the Sea of Japan is reported. The hypothesis that an exclusively deep-water fauna is lacking in abyssal regions of the Sea of Japan is disputed. The author's personal opinion considered concerning the borders of 1000 m between shallow and deep hydrozoan species in the Sea of Japan.