Deepest known gastropod fauna: Species composition and distribution in the Kuril-Kamchatka Trench

The joint German-Russian expedition KuramBio II in 2016 provided a significant opportunity to reinvestigate the species composition and distribution of the abyssal and hadal gastropod fauna in the Kuril-Kamchatka Trench, which was sampled previously only by the Russian research vessel (R/V) Vityaz in the 1940-1960s. During the cruise of the German R/V Sonne, benthic animals were collected using an Agassiz trawl, an epibentic sledge, a multiple corer and a giant box corer. A total catch of 592 live specimens of non-heterobranch gastropods were classified into 86 species and 22 families of the subclasses Neomphaliones, Vetigastropoda, and Caenogastropoda. The abundance, richness and phylogenetic diversity were lower in the hadal zone than in the abyssal zone. Seven hadal stations (6,442-9,584 m) yielded a total of 101 specimens belonging to 22 species in 11 families, whereas catches from four abyssal stations on the landward and seaward slopes (5,102-6,221 m) included 491 specimens and 70 species in 21 families. The abundance, richness and diversity for the hadal zone are, however, particularly high in comparison with other oceanic trenches, due, presumably, to high primary production in the surface layer of the subarctic Northwest Pacific. The uniqueness of species composition increased with depth; sixteen out of the 22 species from the hadal stations (72.7%) were found exclusively in this depth zone. Two unnamed species of Neomphalidae and Sahlingia from 9,577 to 9,584 m represent the world's deepest records for Neomphaliones and Vetigastropoda and suggest the as yet unidentified presence of the world-deepest cold-seep habitats in the Kuril-Kamchatka Trench.