Abyssal–hadal cumaceans (Crustacea: Peracarida: Cumacea) exhibit depth-related patterns in species composition across the Japan and Kuril–Kamchatka Trenches

Although the abyssal and hadal zones are defined by faunal composition, the boundary depths vary widely among studies, likely reflecting regional and taxonomic differences. Therefore, it is essential to understand these boundaries within specific benthic groups. We focused on Cumacea, an order within Peracarida and a major component of deep-sea benthic communities. Cumaceans have low dispersal capabilities and are expected to be sensitive to geomorphological barriers. However, community structure analyses based on morphological and DNA data remain limited. Using specimens collected via high-density sampling (39 stations) in the Japan and Kuril–Kamchatka Trenches (3,500–8,000 m), we characterized the abyssal–hadal cumacean species composition and examined the effects of depth and geomorphology on its variation. Morphological and molecular analyses identified 60 morphospecies, 6 of which showed cryptic diversity. Community composition varied significantly with depth, whereas its differences between trenches and slopes were not significant. Our analysis revealed three depth-related clusters of sampling stations corresponding to the upper abyssal (<5,000 m), lower abyssal (5,000–6,500 m), and hadal (>6,500 m) zones, with the lower-abyssal cluster further subdivided into two subclusters (5,000–6,000 m and 6,100–6,500 m). Comparative analyses across additional taxa and regions are essential for determining the generality of these depth-related community boundaries during the abyssal–hadal transition.