Where do I come from? Phylogenetic analysis of Bursatella leachii Blainville, 1817 (Mollusca: Gastropoda: Heterobranchia) from the Catalonian coast (NE Spain)

Climate change and other human activities have contributed to the dispersal of species from tropical to temperate waters. This is the case of the sea hare Bursatella leachii, a species of unknown type locality, considered pantropical, which adapts very well to shallow bays and the less favorable conditions within ports and marinas. This species was first reported in the Mediterranean Sea in the 1940s, with increasing observation records ever since. The most accepted idea, based on molecular data, is that this species entered the Mediterranean through the Strait of Gibraltar and not through the Suez Canal (Lessepsian migration) as postulated in some previous works. To confirm these ideas, we collected B. leachii specimens from two locations (Delta del Ebro, Tarragona and Forum bathing area, Barcelona) on the coast of Catalonia, Spain (NW Mediterranean) and carried out both ML and Bayesian molecular analysis (MrBayes), in order to compare their sequences with specimens from the Atlantic (East and West), Indo-Pacific and other areas of the Mediterranean, obtained from GenBank; the morphology of different species of the genus was also compared. Our results show that specimens from Catalonia share haplotypes with specimens from both shores of the Atlantic (West and East) and from other locations in the Mediterranean Sea, but not with those from the Indo-Pacific even though all B. leachii from the different regions are morphologically the same. This confirms the hypothesis that B. leachii entered the Mediterranean Sea through the Strait of Gibraltar helped by favorable conditions such as warm currents, abundance of food and the adaptability of the larvae of the species, and not from the Red Sea by Lesepsian migration. It also indicates that the Atlantic Ocean is not a geographical barrier for the different species, because the larvae of B. leachii, being planktonic, can cross the Strait of Gibraltar and reach the Mediterranean Sea helped by warm currents such as the Algerian Current.