Hitch-hikers of the sea: concurrent morphological and molecular identification of Symplegma brakenhielmi (Tunicata: Ascidiacea) in the western Mediterranean Sea

We report here one of the first records of the non-indigenous colonial ascidian Symplegma brakenhielmi in the western Mediterranean Sea. Colonies of this invasive species were collected in 2014 and 2018 along the North-eastern Sardinia coasts (Olbia, Italy). Further colonies were observed in 2016 in the Mar Piccolo basin (Gulf of Taranto, Italy). Both areas are strongly influenced by anthropogenic activities such as commercial shipping and aquaculture and these human-mediated pathways are the most likely vectors of introduction. In both areas, the colonies present two different color phenotypes, the yellow and the red type, with the yellow coloration never previously found in the Mediterranean Sea. Morphological and DNA barcode analyses of the collected specimens show that both these color types belong to the same species. Phylogenetic and species delimitation analyses based on the DNA barcode confirm our identification as S. brakenhielmi, but also indicate a surprisingly high similarity with published sequences of two other species, including the co-generic species Symplegma rubra Monniot, 1972. Morphological and molecular examination of a large number of samples of these species is needed in the near future to clarify this issue