Hidden in the frog: morphological and molecular characterization of Halipegus ovocaudatus (Platyhelminthes: Trematoda), type species of the genus Halipegus Looss, 1899.

The type species of the genus Halipegus Looss, 1899, Halipegus ovocaudatus (Vulpian, 1859) is a generalist parasite of different species of amphibians, mainly anurans. To date, the phylogenetic position of this species has not been established. Specimens of H. ovocaudatus, were collected from three species of water frogs, Pelophylax ridibundus (Pallas), P. esculentus (Linnaeus), and P. lessonae (Camerano) inhabiting the Middle Volga region (European Russia). In our study, we provide detailed morphological redescription of these digeneans and combine it with molecular phylogenetic analyses based on partial sequences of the 28S rDNA and coI mtDNA genes. The phylogenetic position of H. ovocaudatus among the derogenids has been established as a sister clade to other representatives of the Halipeginae. We compared pairwise distances based on partial sequences of coI mtDNA genes of H. ovocaudatus and other representatives of Derogenidae. It was shown, that adults of H. ovocaudatus from different hosts are representatives of the same species. Specimens parasitizing three frog hosts from several localities differ slightly in body shape and size, which probably reflects host-induced intraspecific variability.