Unusually diverse, abundant and endemic deep–sea sponge fauna revealed in the Sea of Okhotsk (NW Pacific Ocean)

The deep–sea sponge fauna of the NW (North–West) Pacific has been re–examined with new and previously collected specimens. In total, 555 new sponge specimens were collected in the under–explored deep Kuril Basin (Sea of Okhotsk), Bussol Strait and adjacent NW Pacific Ocean sites during the Russian–German expedition SokhoBio (Sea of Okhotsk Biodiversity Studies) in 2015. Our knowledge of Sea of Okhotsk deep–sea sponge species has nearly tripled (additional 19 spp.) as a result of this expedition, taking the total of known deep–sea species to 32 for this region. Close to half of species collected were either new to science or new to this region, and two–thirds of species were only found at one site during this study. The Sea of Okhotsk is unlikely to be a sink population, and could be considered a source population for several species due to the large numbers of individuals found. The NE Kuril Basin (Sea of Okhotsk) harboured nearly half of all sponge individuals found during this expedition, and is more species diverse than other Kuril Basin sectors, highlighting the variability in abundance and diversity found in this basin; and is also far richer than the Bussol Strait and NW Pacific sites explored. These differences are potentially due to the presence of productive currents, variable substrates, and niche differences in throughout this region. The semi–enclosed abyssal sector of the Sea of Okhotsk is partially connected with the adjacent NW Pacific abyssal plains through the Bussol and Krusenstern straits, and these straits have enabled faunal connectivity. The long–term partial isolation of the Sea of Okhotsk, low population density of deep–sea sponges, high productivity, and its unusual marine environment, could be key to understanding the exceptional levels of regional deep–sea sponge endemism and diversity.