New data on nudibranch fauna in the North-West Pacific revealed an astonishing pseudocryptic diversity

Ekimova I.A., Mikhlina A.L., Stanovova M.V., Antokhina T.I., Schepetov D.M.

irenekimova@gmail.com

The North Pacific region is one of three major marine biodiversity origin hotspots, with high numbers of endemic taxa. However, while the North-East Pacific fauna is studied quite well due to contemporary USA and Canadian research programs, the biodiversity studies of another Pacific coast are much more scattered, especially in case of Russian coastal waters. Although preceding comprehensive surveys and expeditions revealed extremely rich fauna in this region, not every taxonomic group was covered in these studies. In particular, data on nudibranch molluscs of the North-West Pacific are permanently updated, and many new taxa have been described during last 30 years, but for many regions like deep-sea waters only a handful of species descriptions exist, scattered across several taxonomically focused papers.

In the present study we analyzed samples collected in 2017-2021 in different regions of the North-West Pacific: The Sea of Japan (Vladivostok, Rudnaya Bay), off Sakhalin Island, the Okhotsk Sea and coastal waters of the Kuril Islands. Molluscs were examined using standard anatomical dissections with help of the light microscopy and scanning electron microscopy. The identifications were supported by sequencing of standard barcoding marker COI and nuclear gene histone H3.

A total of 69 nudibranch species was found. Among them 23 species represent newly discovered undescribed taxa. The highest undescribed diversity (18 species) was found in the Kuril Islands area, in both shallow and deep waters. This region is characterized by high productivity, and deep straits between islands in the Kuril archipelago and hydrological conditions promote mosaic pattern of habitats and their restriction to island coastal waters. Our results highlight the necessity of further biodiversity studies of this region.

Acknowledgments: This study was supported by Russian Science Foundation grant #20-74-10012|