Owing to the small size and cryptic morphology, runcinids are among the most difficult marine heterobranchs to study and consequently one of the groups about which little is known. Only recently were molecular tools and phylogenetics first employed to study the systematics. The charismatic European reddish-brown species Runcina ferruginea Kress, 1977, first described from Plymouth, UK, is a paradigm of the challenges facing the taxonomy of these slugs. Due to similarities between R. ferruginea and the Croatian species R. zavodniki described by Thompson, 1980, the latter has been considered as a junior synonym. However, molecular phylogenetics revealed the occurrence of a complex of four species masked under the name R. ferruginea. Through an integrative approach, combining multi-locus (COI, 16S rRNA and histone H3) molecular phylogenetics and morpho-anatomical characters (shape and colouration of body, radula, gizzard plates and reproductive systems) based on specimens from south-western UK, and the central and western Mediterranean Sea (Spain, France, Italy and Croatia), we redescribe R. ferruginea proper and confirm R. zavodniki as a valid species. Also, two new species are described, one belonging to the genus Runcina (R. lupiaensis sp. nov.) and the other to the new genus Pseudoruncina gen. nov. (Pseudoruncina marinae gen. et sp. nov.). Our study exposes the occurrence of cryptic diversity among runcinids and stresses the need for additional work to understand the diversity of this group of minute molluscs.

ZooBank: urn:lsid:zoobank.org:pub:2611EF7D-7762-40CE-BD04-B6D9AB70F2AA.