A global phylogeny of Elysia Risso, 1818 (Gastropoda: Heterobranchia): molecular systematic insights focusing on European taxa and description of a new species

The genus Elysia comprises about one-third of the species richness in Sacoglossa. However, the species diversity in the genus remains poorly characterized in some areas like the north-eastern Atlantic and Mediterranean waters. To clarify the systematics of this genus and to characterize the species diversity in undersampled regions, we performed an integrative study based on a thorough literature review, molecular and morphological analyses, and species delimitation approaches. We conducted phylogenetic analyses of partial sequences of two mitochondrial genes (COI and 16S) and two nuclear genes (H3, 28S) using Bayesian inference and maximum likelihood methods, which confirmed the presence of five of the recognized European Elysia species: Elysia viridis, E. timida, E. flava, E. margaritae, and E. rubeni. Moreover, a new species (Elysia azorica sp. nov.) was identified in the Azores, and E. gordanae, currently considered a junior synonym of E. margaritae, was recovered as a distinct species. In addition, we consider E. hetta as a junior synonym of E. gordanae, and E. translucens as a taxon inquirendum. Finally, the tropical E. evelinae is recorded along European coasts for the first time. Our results demonstrate the value of integrative approaches in resolving taxonomic uncertainty surrounding polymorphism and unravelling potential cases of cryptic or pseudocryptic species complexes.