The nightmare of Okenia in the Mediterranean Sea: what are we facing?

Nine species of the small nudibranch genus Okenia Menke, 1830 have been reported from the western Mediterranean: Okenia aspersa (Alder & Hancock, 1845), Okenia cupella (Vogel & Schultz, 1970), Okenia elegans (Leucjart, 1828), Okenia hispanica Valdés & Ortea, 1995, Okenia impexa Er. Marcus, 1957, Okenia leachii (Alder & Hancock, 1854), Okenia mediterranea (Ihering, 1886), Okenia pulchella (Alder & Hancock, 1854) and Okenia zoobotryon (Smallwood, 1910). However, except for O. hispanica and O. mediterranea originally described from the Mediterranean Sea, most of the remaining records are doubtful. In this contribution we study and discuss several specimens of Okenia collected in La Grande Motte (France), Sabaudia Lake (Lazio, Italy) and Gallipoli (Lecce, Italy), in order to try to clarify their taxonomic status. All of them were initially identifed as O. zoobotryon but, after a detailed anatomical, molecular and ecological study and comparison with the neotype of this species designated by Pola (2015), it is clear that we are dealing with two other species, both different from O. zoobotryon