A house in the abyss: description of the deepest known leucothoid amphipod (Crustacea: Amphipoda) associated with an ascidian from the Kuril-Kamchatka Trench

Leucothoidae Dana, 1852 is a diverse family of marine amphipods, members of which show commensal relationships with various immobile benthic invertebrates such as ascidians and sponges. Despite relatively abundant occurrence and host records from shallow-water environments, deep-sea records, especially at abyssal depths (> 3500 m), remain extremely limited. In this study, we describe the deepest leucothoid species collected during the Hakuho-maru KH-23-5 expedition at 6200 m. The new species, Leucothoe hakuhoae sp. nov., is mainly characterized by a large body size, lack of eyes, mandibular palp article 2 bearing 14–16 setae, dentate distal margin of the carpus in gnathopod 2, and weakly pointed posterodistal corner in epimeral plate 2 and 3. A phylogenetic tree based on nuclear 18S rDNA sequences revealed that L. hakuhoae sp. nov. is encompassed within the clade consisting of L. togatta White & Reimer, 2012b and L. ouraensis White & Reimer, 2012b. We also provided detailed information on the host, which represents the first record of a leucothoid-ascidian association at abyssal depths.