Molecular phylogeny endorses the relationship between carnivorous and filter-feeding tunicates (Tunicata, Ascidiacea)

Abstract

The phylogeny of the tunicates (animals considered the closest relatives to the vertebrates)is not yet completely defined, especially the evolutionary relationships within the class. Molecular studies do not include particular benthic deep-sea species that show morphological changes in the evolution from filter feeding into a carnivorous-feeding habit. Accordingonly to morphological features, these animals are considered as a part of the Class Ascidiacea (Family Hexacrobylidae), but also as a different class, Sorberacea, belonging tothe Phylum Tunicata. In this study, we present a phylogenetic analysis based on 18SrDNA sequences, which clearly included these animals in Ascidiacea but in the Family Molgulidae, faster-evolving ascidians with a high evolution rate. This finding supports the idea that carnivory in Molgulidae represents a more recent adaptation to life in the ocean deep bottoms, where organisms have to adapt themselves to a less plentiful particulateorganic carbon supply. Based on molecular and morphological evidence, we propose the following new synonymy: Hexacrobylidae Seeliger 1906 = Molgulidae Lacaze-Duthiers,1877.