Depth-structured diversity: high number of undescribed species of Tanaidacea (Crustacea) across depth gradient in subarctic regions of the NE Pacific

• The study recorded 63 morphospecies of Tanaidacea in the Bering Sea and Aleutian Trench, with 62 likely new to science, highlighting the exceptional, yet largely undocumented, deep-sea diversity in this region. Richness and species composition varied with depth and geography, with the highest diversity on the trench slopes and in the Bering Sea, and the lowest in the hadal zone. • Many analyzed tanaid morphospecies were rare or geographically restricted, over a quarter occurred as singletons or doubletons, and over a third were found in only one area, indicating strong spatial heterogeneity and suggesting narrow distribution ranges. • Sampling method had a significant effect on observed diversity: epibenthic sledges consistently recovered more morphospecies than box corers, likely due to broader spatial coverage and ability to sample more mobile taxa. • Depth, silicate concentration, geographic region, and gear type emerged as key predictors of tanaid richness and composition, shaping diversity across the study area. • Community differences were mainly driven by nestedness rather than turnover, suggesting that species filtering along environmental gradients, rather than faunal replacement, plays a central role in structuring these deep-sea assemblages. This study investigated the diversity and ecological structuring of deep-sea tanaidacean assemblages across four environmentally distinct regions of the North Pacific: the Bering Sea, the Aleutian Trench, and its northern and southern slopes. Sampling was conducted during the AleutBio (SO293) expedition aboard RV Sonnein 2023, using standardized benthic gear, including the epibenthic sledge (25 valid deployments) and box corer (11 valid deployments), at depths ranging from 3,500 to 7,200 m. Each region encompassed a unique deep-sea setting, characterized by specific bathymetric profiles, geomorphological features, and presumed trophic regimes. A detailed morphological analysis identified 63 morphospecies, of which 62 are likely undescribed. Species richness declined with increasing depth and reached its highest levels in the Bering Sea. Beta diversity analyses revealed significant differences in community composition among regions, primarily driven by depth, and secondarily by food availability, inferred from dissolved silica concentrations.