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

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.