Nemerteans from the deep-sea expedition KuramBio II with descriptions of three new hoplonemerteans from the Kuril-Kamchatka Trench

Abstract

The benthic and plankton samples collected during the KuramBioII expedition (August–September 2016) along the Kuril-Kamchatka Trench contained 117 benthic and 6 pelagic specimens of nemerteans from seven groups: (1) palaeonemerteans of the family Carininidae; (2) tubulanid palaeonemerteans; (3) lineid Heteronemertea; (4) pelagic Polystilifera; (5) reptantian Polystilifera; (6) cratenemertean Hoplonemertea; (7) eumonostiliferous Hoplonemertea. The maximum depth at which nemerteans were found in the expedition is 9577 m. Systematic positions of 26 specimens were established based on five gene markers (16S, 28S, 18S, COI, and H3). All the collected benthic nemerteans are attributed to species new to science. None of the species occurred both in the abyssal and hadal samples and, apparently, among benthic nemerteans eurybathic species with a wide range of habitat depths are not as common. Nemertovema norenburgi sp. nov. from depths of 8220 and 8271 m is closely related to N. hadalis Chernyshev & Polyakova, 2018 from the Puerto Rico Trench: the p-distances between the COI gene sequences of the two species are only 8.1–8.5% despite the huge geographic gap between the two trenches. Proamphiporus crandalli gen. et sp. nov., collected from a depth of 5495 m, possesses an unusual for Eumonostilifera position of cerebral organs and is very similar to Amphiporus rectangulus Strand et al., 2014 from the coastal waters of Norway. Galathenemertes giribeti gen. et sp. nov., found in the tube of Galatheanthemum sp. from depth of 7256 m, is the deepest known symbiotic nemertean and the second known species associated with Actinia. According to a phylogenetic analysis, six species of the abyssal and hadal Carininidae collected in the Northwest Pacific form a sister clade to the shallow-water species of the Carinina. Reptantia is a polyphyletic group, because Uniporus belongs to the Cratenemertea clade, but other reptantians are a paraphyletic group where the pelagic Polystilifera is sister to the subclade of three hadal reptantian species from the Kuril-Kamchatka Trench. A giant free-swimming juvenile,attributed to the same species as theundescribed sublittoral cratenemertid nemertean from the coastal waters off Iturup Island (Cratenemertidae sp. IZ-45644), was collected at a distance of more than 100 km from the nearest of the Kuril Islands, which indicates swimming by juveniles as another dispersal mechanism for nemerteans. Our phylogenetic analysis has shown that the suborder Eumonostilifera includes two large sister clades, Amphiporina and Oerstediina.