First record of a Didemnum carpet ascidian from the southwestern Atlantic Ocean

In the last decades, species of Didemnidae (Tunicata: Aplousobranchia) have attracted attention for overcoming marine barriers, invading coastal areas and causing negative impact to local biodiversity and human facilities. Six years ago, large colonies of a didemnid species were observed growing on artificial substrate in a port in the southeastern Brazilian coast. Samples were collected and morphological and genetic analysis were done. In morphology, the species is similar to D. vexillum in colour, tunic surface consistency and internal organisational system. The morphology of the zooids is similar to D. perlucidum but differs in the number of stigmata, coils of sperm duct, spicules size and larval aspects. Phylogenetic analysis based on the COI gene indicated the Brazilian samples form a monophyletic group distinct from other species, including D. perlucidum , D. vexillum and D. pseudovexillum . Didemnum sp. demonstrated high reproductive activity during the winter season and can grow over native and non-native species, showing high invasiveness potential. Didemnum sp. was not recorded anywhere else until now and based on our findings it should be categorised as a non-native species. We highlight the need of a monitoring program and the evaluation of plans for eradication of the species.