The new Arctic side-gilled sea slug genus Boreoberthella (Gastropoda, Opisthobranchia): Pleurobranchoidean systematics and evolution revisited

Most of the ca. 100 known pleurobranchoid side gilled sea slug species are confined to temperate and warm coastal waters worldwide. Exceptions were five enigmatic Antarctic and southern deep sea species. Herein the first Arctic deep water pleurobranchid, Boreoberthella augusta gen. et spec. nov., is anatomically described. To reveal the origin, relationships and phylogeography of Boreoberthella, a cladistic analysis of 24 pleurobranchoid taxa representing all genera and traditional higher groups was prepared. The morphological data set includes 72 characters; many of them, such as details of the copulatory apparatus, were explored for the first time. The monophyly of both Pleurobranchoidea and Pleurobranchaeidae is confirmed. In contrast to an earlier study, the southern Ocean deep-water species Tomthompsonia antarctica results as basal offshoot of the monophyletic though poorly supported Pleurobranchidae which show a secondary, internal shell. The traditional genus Berthella with worldwide temperate and tropical members may represent an evolutionary grade rather than a clade. The monotypic genera Parabathyberthella and Polictenidia are synonymized with Bathyberthella according to taxonomic and phylogenetic evidence. The basal pleurobranchaeid genus Pleurobranchella, together with Tomthompsonia, Bathyberthella, and Boreoberthella are discussed as potential relics of a cold-water related early nudipleuran radiation in Antarctica, with subsequent dispersal through the depths of the world oceans. The ancestor of Pleurobranchus species, the closest relatives of Boreoberthella, colonized coastal and warmer waters and radiated there.