Faunal turnover and spatial structuring of Pseudotanaidae (Crustacea; Tanaidacea) in the North Pacific deep sea

• First COI-based diversity assessment of Pseudotanaidae in the Bering Sea, Aleutian Trench and its vicinity, and Sea of Okhotsk. • Twenty-seven putative species were detected from the studied North Pacific regions. • Most species showed narrow regional and bathymetric distributions. • Beta diversity driven by high species turnover between regions • Molecular data reveal high endemism and faunal isolation in trench systems. We investigated the molecular diversity and biogeographic structure of the deep-sea crustacean family Pseudotanaidae across six regions of the North Pacific Ocean, spanning bathyal to hadal depths (3200–8200 m). Using cytochrome c oxidase subunit I (COI) barcoding and the Assemble Species by Automatic Partitioning (ASAP) species delimitation method, we identified 36 molecular operational taxonomic units (MOTUs), providing the first broad molecular assessment of this family in the region. Most MOTUs (n = 27) were confined to a single region, while only a few were shared between regions. Regional MOTU richness was highest in the Pacific Eastern Pacific Abyssal (EA), Aleutian Trench (AT), and Bering Sea (BS), whereas the Sea of Okhotsk (SO) and Kuril-Kamchatka Trench (KKT) exhibited the lowest values. β-diversity analyses based on the Sørensen and Jaccard dissimilarity indices revealed pronounced spatial differentiation among regions, driven mainly by species turnover with minimal nestedness. Dissimilarity between the western and eastern North Pacific basins was high (βSOR = 0.756), indicating limited faunal connectivity across the basin. These findings suggest a predominantly regionally restricted distribution and limited dispersal potential in Pseudotanaidae, likely influenced by geomorphological isolation and the patchy structure of deep-sea habitats.