Preliminary approach to the phylogenetic relationsships of Polycladida (Platyhelmintha)

Abstract

The Polycladida is a group of marine flatworms with more than 900 species and a world wide distribution. This group was firstly described in 1881, and since then, there have been several conflicting classifications, some of them still coexist nowadays. These previous and current taxonomic disagreements have made difficult the understanding of evolutionary relationships, as well as the identification of geographical distributional patterns of many species. The order Polycladida was firstly divided in 1885 in two suborders Cotylea and Acotylea. Cotylea comprises four families, two of which are well represented, while the other two are monospecific and dubious. Within Acotylea there are three well represented superfamilies, but some genera and families are monospecific and their determination is based only on external characters. This study pursues a deep investigation into the relationships between the different groups. For this purpose we have analyzed morphological (internal and external) information, as well as molecular (genes 16S and COI) data. We provide a phylogenetic framework that facilitates the understanding of the evolution of morphological traits and provides a more stable hypothesis for the systematics.