Deep-sea nemerteans collected along the Japan Trench and the southern Kuril-Kamchatka Trench onboard the R/V Hakuho-maru with descriptions of two new species in Hoplonemertea

A total of 80 nemertean specimens, representing 30 putative species, were obtained from the trawled material collected along the Japan Trench (JT) and southern Kuril-Kamchatka Trench (KKT) during the KH-22-8 and KH-23-5 cruises of the R/V Hakuho-maru in October 2022 and September 2023, respectively. Samples were collected from the 24 stations along these trenches at depths ranging from 2007 to 7654 m. Molecular and morphological analyses revealed nine species of Palaeonemertea (one in Carininidae, eight in Tubulanidae), seven species of Pilidiophora (six in Lineidae, one in Valenciinidae), and 14 species of Hoplonemertea (six Monostilifera, eight Polystilifera). All palaeonemerteans, six pilidiophorans, and eight hoplonemerteans represented species that are highly likely undescribed and distinct from species known in deep-sea environments globally in the last decade. Two new hadal hoplonemerteans were described in this study: Nemertovema kojimai sp. nov. in JT at 6211 m, and Profundonemertes hakuhoae gen. et sp. nov., found in JT and southern KKT at depths of 6807–7654 m. The phylogeny of the 30 species, including the two newly described, was analyzed using 16S, 18S, and 28S ribosomal RNA, cytochrome c oxidase subunit I (COI), and histone H3 genes. Nemertovema kojimai sp. nov. is closely related to Nemertovema norenburgi Chernyshev and Polyakova, 2019, while Profundonemertes hakuhoae gen. et sp. nov. was nested within Oerstediina, sister to Abyssonemertes kajiharai Chernyshev and Polyakova, 2018a, but with low support. Four previously described species were also reported for the first time in Japanese waters: Baseodiscus profundus Kajihara, Abukawa and Chernyshev, 2022b, in southern KKT; Dinonemertes cf. arctica Korotkevich, 1977 in both trenches; N. norenburgi, in southern KKT; and Phallonemertes murrayi (Brinkmann, 1912), in both trenches as well. The genus Nemertovema Chernyshev and Polyakova, 2018a was reported for the first time in JT, represented by N. kojimai sp. nov. and one additional undescribed species. This study provided the first comprehensive survey of deep-sea nemertean diversity along JT and updated our understanding of species diversity in KKT.