Integrative taxonomy and phylogeography of the genus Acanthodoris (Gastropoda: Nudibranchia) in the Russian seas, with the description of a new genus

The nudibranch genus Acanthodoris Alder et Hancock, 1845 includes 15 recognised species, most of which are distributed in the temperate marine waters of both hemispheres, but only two species were known from the Russian seas: the widely distributed trans-Arctic A. pilosa (Abildgaard in Müller, 1789) and A. uchidai Baba, 1935, which is found from in the Kuril Islands. The main aim of this study is to revise the species composition of Acanthodoris in Russian waters, evaluate the genetic population structure of the trans-Arctic A. pilosa in the context of connectivity among its distant populations, and infer the phylogenetic relationships between the Russian species of Acanthodoris and other representatives of the family Onchidorididae. The material was collected from eight localities in the subarctic and North-West Pacific waters. The methods employed included morphological analyses, particularly, scanning electron microscopy and computer microtomography, as well as molecular analyses of partial fragments of genetic markers: cytochrome c oxidase subunit I, 16S rRNA, histone H3, 28S rRNA, and 18S rRNA. The genus Acanthodoris is represented in the Russian seas by two species: the North Atlantic and subarctic A. pilosa and the North Pacific A. atrogriseata O’Donoghue, 1927, which is recorded here for the first time from Russia. Acanthodoris uchidai represents a separate clade that is not related to other Acanthodoris; therefore, a new genus Acanthomira gen. nov. is proposed herein, with a single representative species Acanthomira uchidai, comb. nov. We also provide evidence for the existence of cryptic biodiversity within North Atlantic A. pilosa. Our current phylogenetic hypothesis suggests an independent loss of the rachidian teeth in both Acanthodoris and Acanthomira gen. nov., although this should be further tested with a larger taxon sampling. The results of this study emphasise the necessity of incorporating molecular analyses to confirm species identity and to prevent the description of new Nudibranchia taxa based solely on subtle morphological differences.