Molecular data on the genus Faustula Poche, 1925 (Trematoda: Faustulidae) resolves a 100-year taxonomic intrigue

Faustulidae sensu lato is a polyphyletic assemblage of digeneans that parasitise marine or euryhaline fish as adults in the gastrointestinal tract. In previous phylogenetic reconstructions, this family appeared to be represented by two groups nested within the Gymnophalloidea and Microphalloidea. However, the choice of a correct name for these groups was impossible without assessing the phylogenetic position of the type genus of the Faustulidae, Faustula Poche, 1925. In the current study, we provide morphological and molecular data on Faustula gangetica (Srivastava, 1935), which represents the first molecular data for the genus Faustula. Specimens of F. gangetica were found in Tenualosa ilisha (Hamilton) from Shatt Al-Arab River, southern Iraq. We report the presence of discernible muscle fibres between the pharynx and the oral sucker, as well as a zone of distinctly differentiated transversely oriented muscle fibres located anterior to the common genital pore in this species for the first time. All molecular markers used in our study (ITS2 region, 28S rRNA gene) indicated that the genus Faustula is nested within the clade of gymnophalloid faustulids. Thus, Faustulidae sensu stricto is confirmed to belong to the Gymnophalloidea.