Bathyopsurus sonnei sp. nov. – A new species of the rare deep-sea isopod genus (Crustacea, Isopoda, Asellota) from the Aleutian Trench, north-eastern Pacific with notes on the Bathyopsurinae

• Integrative description of one of a new munnopsid isopod belonging to a rare deep-sea genus of the subfamily Bathyopsurinae. • Description of the largest munnopsid isopod species ever recorded. • Northernmost record of the rare Bathyopsurinae and the genus Bathyopsurus. A new species of the genus Bathyopsurus Nordenstam, 1955 (Munnopsidae, Bathyopsurinae) is described from the Aleutian Trench in the North-east Pacific collected during the RV Sonne expedition AleutBio (Aleutian Trench Biodiversity Studies) (SO293) from depths between 4224–5170 m from both the northern sub-Arctic slope as well as the southern North Pacific slope of the Aleutian Trench. Bathyopsurus sonnei sp. nov. differs from the other species of the genus in several characters related to the mouthparts and pleopod 2 of the female. Bathyopsurus sonnei sp. nov. is 65 mm long, making it the largest munnopsid isopod ever sampled. The genus Bathyopsurus is considered rare, with only a few specimens sampled to date, and only limited genetic data available prior to this study. In this study, we present the first mitochondrial DNA data for the genus, specifically COI and 16S sequences. Our results indicate that, despite being collected from opposite sides of the trench, the specimens belong to the same species.