Orange is the new white: taxonomic revision of Antarctic Tritonia species (Gastropoda: Nudibranchia)

Abstract

Among nudibranch molluscs, the family Tritoniidae gathers taxa with unclear phylogenetic position, such as some species of the genus Tritonia Cuvier, 1798. Currently, 35 valid species belong to this genus and only three of them are found in the Southern Ocean, namely T. challengeriana Bergh, 1884, T. dantarti Ballesteros & Avila, 2006, and T. vorax (Odhner, 1926). In this study, we shed light on the long-term discussed systematics and taxonomy of Antarctic Tritonia species using morpho-anatomical and molecular techniques. Samples from the Weddell Sea and Bouvet Island were dissected and prepared for scanning electron microscopy. The three molecular markers COI, 16S, and H3 were sequenced and analysed through maximum likelihood and Bayesian methods. The phylogenetic analyses and species delimitation tests clearly distinguished two species, T. challengeriana and T. dantarti, being widely-spread in the Southern Ocean, and endemic to Bouvet Island, respectively. Coloration seemed to be an unreliable character to differentiate among species since molecular data revealed both species can either have orange or white colour-morphotypes. This variability could be explained by pigment sequestration from the soft coral species they feed on. Morphological analyses reveal differences between Antarctic and Magellanic specimens of T. challengeriana, thus, we suggest the resurrection of T. antarctica Martens & Pfeffer, 1886 to encompass exclusively the Antarctic species. To progress further, additional molecular data from Magellanic specimens are required to definitely resolve their taxonomy and systematics.