Deep-sea gastropods of the family Ringiculidae (Gastropoda, Heterobranchia) from the Sea of Okhotsk, Kuril–Kamchatka Trench, and adjacent waters with the description of three new species

Based on material from deep-sea expeditions including SokhoBio, KuramBio I, and KuramBio II, five species of ringiculid gastropods were found to inhabit the upper bathyal zone of the Sea of Okhotsk, lower bathyal and abyssal depths of the Kuril–Kamchatka Trench and adjacent waters. Three species are described here as new to science: Ringicula minichevi sp. nov., Microglyphis hasegawai sp. nov., and Ringiculoides vityazi sp. nov. Data on the anatomy of Microglyphis are provided for the first time. Two species occur in the bathyal zone (Microglyphis furukawai and R. minichevi sp. nov.), while the other three are exclusively abyssal (Ringiculoides kurilensis, R. vityazi sp. nov.) or eurybathic (M. hasegawai sp. nov.). The two species of Ringiculoides have closely similar shells and overlapping geographic and bathymetric distributions, thus highlighting the importance of combined molecular and morphological analyses for accurate taxonomy of the group. Our literature review suggests that the Northwest Pacific has the highest taxonomic diversity of bathyal and abyssal ringiculids, whereas the Sea of Okhotsk is inhabited solely by M. furukawai (≤ 591m).