Munnopsidae (Crustacea, Isopoda, Asellota) from the Kuril–Kamchatka Trench with a regional and inter-ocean comparison of their biogeographic and richness patterns

•Munnopsidae was the most abundant Isopoda family in the KuramBio II samples.•The Northwest Pacific Munnopsidae inventory grew from 39 to 157 species.•87% of recently collected munnopsid species were new to science.•Kuril-Kamchatka Trench munnopsid diversity follows negative depth trend from abyssal to hadal.•Kuril-Kamchatka Trench does not constitute a dispersal barrier to 62 munnopsid species. The benthos of the Kuril–Kamchatka Trench (KKT) was sampled during the KuramBio II expedition on board the RV Sonne in 2016. Macrobenthos was, for the first time, collected in the KKT hadal zone using an epibenthic sledge. Isopod crustaceans were one of the dominant taxa, and Munnopsidae was the most abundant and diverse asellotan family, comprising 49% of all Isopoda. During the expedition, a total of 67 species of 24 genera from eight subfamilies of Munnopsidae were collected, including 41 species of 15 genera from 18 hadal stations. In general, the taxonomic diversity of the munnopsid fauna in the KKT hadal zone was lower than that of the Northwest Pacific (NWP) abyssal fauna adjacent to the KKT. Two subfamilies, Lipomerinae and Acanthocopinae, were not found in the hadal zone. Most of all collected munnopsid individuals (92%) belonged to the three subfamilies: Eurycopinae (42%; 15 species), Betamorphinae (33%; four species), and Storthyngurinae (17%; two species). The rest of all munnopsid individuals (8%) belonged to the subfamilies Ilyarachninae, Munnopsinae, Syneurycopinae, and to a group of incertae sedis genera. The eight most numerous species representing the three most widely distributed and speciose genera—Eurycope, Betamorpha and Rectisura—encompassed 78% of all munnopsid individuals. The study of the Munnopsidae collected during the four expeditions (SoJaBio in 2010, KuramBio in 2012, SokhoBio in 2015 and KuramBio II in 2016) extended the known diversity of the munnopsid species in the KKT area from 39 described species of 14 genera to 157 species of 36 genera. Most collected species (87%) were new to science. The fauna richness showed a tendency to decrease from the open abyssal towards the hadal zone of the KKT and towards the semi-isolated abyssal Kuril Basin of the Sea of Okhotsk, and especially towards the more isolated abyss of the Sea of Japan. A comparison of the munnopsids from NWP and those from other oceans showed their similar composition with the same dominant subfamilies. The main difference between the abyssal munnopsids from the NWP an