An integrative taxonomic framework for the study of the genus Ciona (Ascidiacea) and description of a new species, Ciona intermedia

The genus Ciona is an interesting ‘taxonomic case’ because its evolutionary history and taxonomy have not yet been resolved completely. In this study, we present new findings, describing specimens of an unidentified Ciona species collected along the north-eastern coasts of Sardinia (Tyrrhenian Sea, Mediterranean Sea). Applying an integrative taxonomic approach, based on the joint examination of morphological and molecular traits, we identify these specimens as a new species, Ciona intermedia sp. nov. Morphological comparisons and peculiarities of the habitat first revealed that these Ciona specimens have intermediate characters compared with other Ciona species. Molecular characterization (based on three mitochondrial regions: two already used for discriminating Ciona cryptic species and a newly developed one) confirmed that our specimens could not be assigned to any previously molecularly-characterized species. Both molecular phylogenetic reconstructions and morphological data clearly indicate C. intermedia as sister clade of Ciona edwardsi. Our findings add further complexity to the taxonomy of Ciona, underlying the importance of an integrative taxonomic approach for the study of the evolutionary history of this enigmatic genus.