The Mediterranean Sea as a gateway for invasion of the Red Sea: the case of the Indo-West Pacific head-shield slug Chelidonura fulvipunctata Baba, 1938

The number of tropical species established in the Mediterranean Sea has risen at an unprecedented rate in the last 40–50 years, often entering the basin from the Red Sea and to a lesser extent by shipping or mariculture. The taxonomic identity of these alien species can be difficult to determine and DNA barcoding can help to clarify the status of questionable species. One of these cases is the W-shaped mark Aglajidae slug usually identified as being the Indo-West Pacific species Chelidonura fulvipunctata Baba, 1938 but was first described in the Mediterranean Sea as a native species under the name C. mediterranea Swennen, 1961. A Bayesian phylogeny using the barcode marker cytochrome c oxidase subunit I and an ABGD species delimitation analysis unequivocally showed that specimens from the eastern and central Mediterranean Sea are conspecific with specimens from the Indian and Pacific Oceans. In this work, we hypothesize that C. fulvipunctata (presently occurring in the entire Mediterranean Sea and Indo-West Pacific; only once recorded in the Red Sea) has entered the Red Sea through the Mediterranean Sea. Thus the Red Sea can also be a receiver of tropical species that have arrived in the Mediterranean by other routes than Lessepsian immigration, with the Suez Canal acting as a “revolving door” allowing both species of Indo-Pacific origin to enter the Mediterranean and species established in the Mediterranean resilient to tropical/subtropical environmental conditions to move into the Red Sea.