| Summary: | Cuthonella Bergh, 1884 is of one of the most neglected nudibranch groups, with a long history of taxonomic confusion with other aeolidacean genera. Owing to its predominantly Arctic distribution with cold water, its species are difficult to find and to collect, and thus to describe. In this study we revise the genus by presenting molecular and morphological data for a majority of the species, including the type, C. abyssicola Bergh, 1884. The material is based on a broad geographic sampling throughout the Northern Hemisphere. Particular emphasis is placed on the Kuril Islands, a diversity hotspot for the genus. Seven new species and two subspecies of Cuthonella are described from the Arctic and North Pacific regions. The number of species of Cuthonella is thus increased over threefold and now comprises 15 species plus two subspecies instead of five species. This work is the most substantial update of the genus Cuthonella since its description in 1884. To delineate taxonomic and phylogenetic limits of Cuthonella-like aeolidaceans, the molecular phylogeny of the wider traditional “tergipedids” is presented and shows that Cuthonellalike aeolidaceans form a distinct molecular clade as the family Cuthonellidae Miller, 1977, corroborated by reliable morphological apomorphies.
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