Phylogenetic position of Setopus (Gastrotricha, Paucitubulatina) among planktonic Gastrotricha, with the description of a new species

The understanding of phylogenetic relationships within the phylum Gastrotricha is currently evolving, revealing significant discrepancies between the evolutionary interpretations based on morphology and molecular data. This inconsistency also applies to the families Dasydytidae and Neogosseidae, which include the remarkable planktonic gastrotrich species of the order Chaetonotida. The inter- and intra-relationships within these families remain unclear, mainly due to incomplete taxonomic sampling. In this study, we describe a new species of Setopus and provide its 18S, 28S, and COI genetic sequences. We also provide the sequences of Setopus tongiorgii (Balsamo, 1983), offering the first available molecular data for the genus. The new sequences were used in a phylogenetic analysis that encompassed 53 terminals, covering a broad taxonomic range. The results indicate that the two Setopus species are closely related to Ornamentula Kisielewski, 1991 and highlight a possible non-monophyletic nature of the family Dasydytidae, with members of the family Neogosseidae falling within it.