Molecular phylogeny of Didemnidae (Ascidiacea: Tunicata)

Abstract Didemnidae is the largest family of tunicates within Aplousobranchia, with about 578 species. This family comprises eight genera: Atriolum, Clitella, Didemnum, Diplosoma, Leptoclinides, Lissoclinum, Polysyncraton, and Trididemnum. Morphological and molecular data suggest that Didemnidae is monophyletic, but the monophyly of each didemnid genus and their phylogenetic relationships are still poorly understood. This study aimed to evaluate the monophyly of six of the eight didemnid genera and assess their phylogenetic relationships based on mitochondrial (COI) and nuclear (18S) sequences. All genera were recovered as monophyletic except Trididemnum. Didemnum comprises two clades that differ one from the other by the presence of an atrial lip, the number of testicular lobes, and the number of ampullae in larvae. These morphological differences indicate that Didemnum could be split into two genera. Morphological evidence and previous taxonomists have suggested a close relationship between Didemnum and Polysyncraton as well as between Diplosoma and Lissoclinum. We re-evaluate these hypotheses, which are supported by our 18S sequences and concatenated data.