A new species of Tereancistrum Kritsky, Thatcher & Kayton, 1980, collected from the gills of Brycon nattereri (Bryconidae) in the Cerrado, in the state of Goiás, Brazil, is described based on morphological and molecular data, based on the mitochondrial cytochrome c oxidase subunit 1 (COI) gene region. Tereancistrum campanum n. sp. is distinguished from all its congeners by the unique ‘bell-shaped’ accessory piece in the male copulatory complex. Molecular analysis revealed that the specimens analyzed formed a distinct group from other Tereancistrum sequences and that the sequences of Tereancistrum kerri showed genetic divergence when compared to T. campanum n. sp., confirming the molecular distinction between the two species. This study also expands the knowledge of parasitic diversity in the Tocantins-Araguaia River Basin and extends the geographical distribution of the genus Tereancistrum. The combination of morphological and genetic data was essential for delimiting this new species, highlighting the importance of integrative approaches in understanding the diversity of monogeneans in freshwater fish in Brazil.

During the study of ectoparasites (Platyhelminthes) of fish in the floodplain of the Upper Paraná River - Brazil, a new species of Urocleidoides, present in the gill filaments of Hemiodus orthonops (Hemiodontidae), is described using morphological description and molecular data from the mitochondrial region of cytochrome c oxidase, subunit 1 (COI) and the partial region of the 28S rDNA gene. Urocleidoides luquei n. sp. differs from all its congeners by the presence of a filament that joins the accessory piece to the base of the male copulatory organ and resembles U. paradoxus and U. surianoae. Phylogenetic analyses using molecular data revealed that U. luquei n. sp. forms a paraphyletic group concerning the other Urocleidoides species. In this way, as well as contributing to the description of a new species, we seek to encourage and contribute to the increase in research using integrative taxonomy, thus making it possible to elucidate some unresolved questions about the genus Urocleidoides.