Changes in species composition of Haploniscidae (Crustacea: Isopoda) across potential barriers to dispersal in the Northwest Pacific

Speciation processes as drivers of biodiversity in the deep sea are still not fully understood. One potential driver for species diversification might be allopatry caused by geographical barriers, such as ridges or trenches, or physiological barriers associated with depth. We analyzed biodiversity and biogeography of 21 morphospecies of the deep-sea isopod family Haploniscidae to investigate barrier effects to species dispersal in the Kuril-Kamchatka Trench (KKT) area in the Northwest (NW) Pacific. Our study is based on 2652 specimens from three genera, which were collected during the German-Russian KuramBio I (2012) and II (2016) expeditions as well as the Russian-German SokhoBio (2015) campaign. The sampling area covered two potential geographical barriers (the Kuril Island archipelago and the Kuril-Kamchatka Trench), as well as three depth zones (bathyal, abyssal, hadal). We found significant differences in relative species abundance between abyssal and hadal depths. Haploniscus belyaevi Birstein, 1963 was the dominant species at abyssal stations while H. hydroniscoides Birstein, 1963 was prevalent in the hadal. Species composition also differed significantly across geographical barriers. While H. hydroniscoides was the most abundant species in the open NW Pacific, not a single specimen of this species was found in the Sea of Okhotsk, which is separated from the Pacific basin by the Kuril Islands. Furthermore, H. belyaevi and two morphologically highly similar morphospecies were the only species found in samples from the Sea of Okhotsk, meaning that the other 18 species, which we identified from our samples, did not occur west of the Kuril Islands. In this study, haploniscid species show very diverse patterns in geographic distribution between geographic areas and with depth. Therefore, the KKT might have an isolating effect on both the bathymetric as well as geographic distribution of some haploniscid species from the NW Pacific into the Sea of Okhotsk and from the western to the eastern abyssal margin of the KKT.