We report a new commensal relationship between oerstediid nemerteans and deep-sea ascidians. A total of six nemertean individuals were collected from the peribranchial cavities of several Styelidae ascidians attached to small rocks at bathyal depths (1,400-1,409 m) off northeastern Japan in the northwestern Pacific Ocean. Phylogenetic analyses based on combined mitochondrial cytochrome c oxidase subunit I (COI), 16S rRNA, histone H3, 18S rRNA, and 28S rRNA gene sequences placed this species within the 'Clade Gononemertes' of the hoplonemertean family Oerstediidae. Previous records of commensalism between nemerteans and deep-sea ascidians are restricted to the genus Culeolus (Pyuridae). The present finding therefore suggests that ascidian-host diversity may be greater in Oerstediidae than previously recognised.
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