Desmosomatidae (Isopoda: Asellota) from the Kuril Basin of the Sea of Okhotsk: First data on diversity with the description of the dominant species Mirabilicoxa biramosa sp. nov.

During the SokhoBio expedition in 2015, the macrobenthos was sampled by an epibenthic sledge at eleven sites at the abyssal and bathyal depths of the Kuril Basin of the Sea of Okhotsk, in the Bussol Strait, and on the western abyssal slope of the Kuril-Kamchatka Trench (KKT). Desmosomatidae constituted 30% of all isopod specimens (Crustacea: Malacostraca). Twenty-three species in eight genera (Desmosoma, Chelator, Eugerda, Eugerdella, Mirabilicoxa, Momedossa, Prochelator, and Oecidiobranchus) were found in the abyss of the Kuril Basin (at depths of 3210-3366 m) and the KKT slope (3371-4798 m). From these 17 species were found in the Sea of Okhotsk. This is the first record of this family at the abyssal depths of the Sea of Okhotsk. At the bathyal sites, Desmosomatidae were absent. This is the first report of the genus Oecidiobranchus in the North Pacific. Twenty species (86%) are new to science. The abundance and species richness of Desmosomatidae in the abyssal zone of the Kuril Basin varied from 0 to 282 individuals and 14 species per station. No relationships were found between the data on desmosomatid composition and abiotic parameters in the abyssal zone of the sea. In contrast, low oxygen content in the bathyal zone of the Kuril Basin and strong currents in the Bussol Strait can prevent colonization by the Desmosomatidae. The desmosomatid fauna in the abyssal zone of the Sea of Okhotsk is linked to the Pacific abyssal fauna of the KKT area. Among desmosomatids collected in the Sea of Okhotsk, seven genera and 10 species were found previously in the KKT abyssal zone during the KuramBio expedition (2012). The shelf desmosomatid fauna of the Sea of Okhotsk is more similar to the fauna of the bathyal and even abyssal zones of the Sea of Japan than the abyssal zone of the Sea of Okhotsk. These distributional patterns can be explained by taking into account the hydrology of the Sea of Okhotsk. The dominant species in the abyssal zone of the Kuril Basin, which comprises 27% of all specimens of the family, Mirabilicoxa biramosa sp. nov., is described. The new species can be distinguished from all other species of the genus by the presence of biramous uropods. A modified diagnosis of the genus Mirabilicoxa is presented.