Diversity and connectivity of deep-sea Amphipoda of the Bering Sea and the Aleutian Trench revealed with DNA barcoding

• The amphipod fauna of the abyss of the Bering Sea and the abyss as well as hadal of Aleutian Trench appears to be very diverse. • Species richness and faunistic composition of Bering Sea, both slopes and the Aleutian Trench itself highly differ. • There is a weak connectivity between the studied regions, particularly between Bering Sea and the open North East Pacific. • The abyssal and hadal of North East Pacific share species with the Kuril-Kamchatka Trench and adjacent abyssal plain. Amphipoda are speciose and abundant in all marine habitats and are proven to be important food source for higher trophic levels. These crustaceans are brooders with no planktonic larval stage, so migration of species depend mainly on swimming capacities of adult animals. The aims of the present work were: 1) characterizing the diversity of abyssal and hadal amphipods of the Bering Sea (BS) and the Aleutian Trench (AT), 2) studying the amphipod species distribution as well as 3) inferring the connectivity between North East Pacific amphipod fauna and the other deep-sea regions of the world ocean. The material was collected with and epibenthic sledge from two stations in the South East BS, three stations on the northern slope of the AT, three stations on the southern slope of the AT and six stations in the hadal of the AT. A representation of all collected amphipods was initially sorted into families and preferably morphospecies and subjected to cytochrome c oxidase subunit I gene (COI) barcoding. The barcoding of 575 individuals resulted in 460 sequences that were ascribed to 182 Molecular Operational Taxonomic Units (MOTUs). At least 25 families, 31 genera and six known species were identified. The families (or groups of families) that dominated barcoded material in terms of abundance were: Oedicerotidae (76 ind.), Eusiridae (75 ind.) and Lysianassoidea indet. (61 ind.), while the most speciose groups were: Lysianassoidea indet. (31 MOTUs), followed by Pardaliscidae (29 MOTUs) and Oedicerotidae (28 MOTUs). Almost three quarters of all MOTUs were singletons or doubletons, however as many as 44 MOTUs appeared to be shared with other regions of the World Ocean. The Bering Sea and both slopes of the trench appeared to harbour significantly higher number of amphipod MOTUs (51–61 MOTUs) than the hadal zone of the AT (33 MOTUs). The highest faunal exchange appeared to be between abyssal and hadal zones of the AT and similar depth zones of the Kuril-Kamchatka Trench (39 MOTUs), the second most connected region was the abyss of Clarion-Clipperton Zone (11 MOTUs). The BS fauna demonstrated high uniqueness with only four MOTUs shared with AT abyss and hadal as well as with four MOTUs shared with NW Pacific. The study confirmed high species richness of another deep-sea region of the World, formerly almost completely unknown.