The Sea of Japan is a semi‐enclosed marginal sea in the northwestern Pacific whose bathyal communities exhibit a depauperate species diversity due to anoxic events during the last glacial maximum. Cold seeps associated with chemosynthetic ecosystems were discovered in this region over three decades ago, yet their fauna have not been characterised. Here, we surveyed three bathyal seeps in the northeastern Sea of Japan off Okushiri Island, Hokkaido, to reveal their faunal composition and environmental conditions. We documented only eight macrofaunal species across all sites, of which six are well‐known ambient fauna, with just two gastropod species in the genera Provanna and Hyalogyrina being seep endemics. Dissolved oxygen (DO) concentrations were consistently reduced on the seep bacterial mats, while the temperature remained similar to the ambient seawater. The Provanna species was new to science and is named P. cocytus sp. nov., characterised by a smooth shell with very convex whorls and a nearly holostomous aperture and a radula with a truncated central tooth cusp and the lateral teeth lacking a buttress. Phylogenetic reconstruction using the mitochondrial cytochrome c subunit I (COI) gene recovered P. cocytus sp. nov. as sister to all other congeners, indicating this genus did not recolonise the Sea of Japan after the last glacial maximum from seep or vent sources. The low species richness at seeps likely represents a combined effect of shallow connections limiting dispersal and impacts of the very cold Japan Sea Proper Water to larval settlement. Our results contribute to the understanding of the biodiversity in the Sea of Japan and close a major knowledge gap in the biogeography of chemosynthesis‐based ecosystems across the western Pacific.
Chen et al. (Thu,) studied this question.