Phylogeny of Chaetonotidae and other Paucitubulatina (Gastrotricha: Chaetonotida) and the colonization of aquatic ecosystems.

Chaetonotidae is the largest family within Gastrotricha with almost 400 nominal species represented in both freshwater and marine habitats. The group is probably non-monophyleticand suffers from a troubled taxonomy. Current classification is to a great extent based on shape and distribution of cuticular structures, characters that are highly variable. Wepresent the most densely sampled molecular study so far where 17 of the 31 genera belonging to Chaetonotida are represented. Bayesian and maximum likelihood approachesbased on 18S rDNA, 28S rDNA and COI mtDNA are used to reconstruct relationships within Chaetonotidae. The use of cuticular structures for supra-specific classificationwithin the group is evaluated and the question of dispersal between marine and freshwater habitats is addressed. Moreover, the subgeneric classification of Chaetonotus is tested in aphylogenetic context. Our results show high support for a clade containing Dasydytidae nested within Chaetonotidae. Within this clade, only three genera are monophyletic followingcurrent classification. Genera containing both marine and freshwater species never form monophyletic clades and group with other species according to habitat. Marine membersof Aspidiophorus appear to be the sister group of all other Chaetonotidae and Dasydytidae, indicating a marine origin of the clade. Halichaetonotus and marineHeterolepidoderma form a monophyletic group in a sister group relationship to freshwater species, pointing towards a secondary invasion of marine environments of these taxa. Ourstudy highlights the problems of current classification based on cuticular structures, characters that show homoplasy for deeper relationships.