Mapping alien Mollusca distribution in the Mediterranean Sea: the Lessepsian immigrant Retusa desgenettii (Audouin, 1826) reaches Turkey

The opening of the Suez Canal in 1869 has led to the colonisation of the Mediterranean Sea by a large number of tropical/subtropical species, including several sea slugs and shelled relatives. Among them, the Red Sea taxon Retusa desgenettii (Audouin, 1826) is a well known Lessepsian invader, being first recorded from the Suez Canal and subsequently from the Mediterranean waters of Egypt and Israel. We hereby review R. desgenettii Mediterranean literature and offer an updated map of its Mediterranean spreading. On the basis of the analysis of a bioclastic sediment from Bozburun, we also considerably extend the known invaded Mediterranean range, first reporting its presence from Turkey, presumably reached via natural dispersal. R. desgenettii shell is redescribed and compared with the most similar native Retusa sensu stricto and alien Cephalaspidea recorded from the Mediterranean Sea, as to facilitate possible future records in the wide area between previous records and present ones. Within this framework, Retusa candidula (Locard, 1892), Retusa pellucida (T. Brown, 1827) and Retusa obtusa (Montagu, 1803) are also excluded from the Mediterranean fauna.