New species from the cradle of biodiversity inventories: cryptic diversity within Lamellariinae in the European Atlantic and Mediterranean (Gastropoda: Velutinidae)

The Mediterranean, despite a long tradition of naturalist observation dating back to Aristotle, is still home to undisclosed cryptic species complexes. The gastropod subfamily Lamellariinae d'Orbigny, 1841 (Velutinidae Gray, 1840) is composed of benthic sea slug-like species with a vestigial internal shell covered by the mantle. The high morphological plasticity of this group hampers species delimitation and identification. In the Mediterranean Sea, two - considered well-known - species are present: Lamellaria latens (O. F. Müller, 1776) and Marsenia perspicua (Linnaeus, 1758). This study assesses the real diversity of this group using an integrative taxonomy approach that combines molecular (COI, 16S rRNA, ITS2, 28S rRNA), morphological (body size, shell, radula, internal anatomy) and ecological data (geographic distribution, depth). The genus Marsenia Oken, 1823 is recognised as a synonym of Lamellaria Montagu, 1816 and the new genus Marseniotta n. gen. is established for Marsenia sensu authors. Two cryptic species complexes are recognised, in the European Atlantic and Mediterranean, with a total of six species, three here described as new: Lamellaria alba n. sp., Lamellaria violacea n. sp., and Marseniotta corsicana n. sp. Additionally, Eirenika n. gen. is proposed as a replacement name for the lamellariine genus Pacifica Fassio, Bouchet & Oliverio, 2022, which is a junior homonym. Our results suggest that characters traditionally used for species identification in this group, such as body size, shell shape and mantle colouration, are unreliable when examined across a broader dataset. Some recurrent chromatic patterns, protoconch and radula shape, can be used, in combination with geographic distribution, to attempt a first genus-level identification. However, given the high degree of plasticity and overlap in morphological characters, genetic data emerge as essential for a correct species-level identification in this group.