The genus Ancistrohaptor was proposed to accommodate monopisthocotylans flatworms parasitic on the gills of species of the genus Triportheus in Manaus, Amazonas state, Brazil. Its main characteristics are (a) an accessory piece of the male copulatory organ composed of two distinct parts; (b) dextral or dextroventral vaginal openings; and (c) large ventral anchors with elongated shafts. A new species of Ancistrohaptor was found to parasitize the gills of Triportheus signatus collected from the Salgado River, Ceará State, Brazil. A new species of Monopisthocotyla was collected and described. Ancistrohaptor forficata sp. n. is primarily characterized by having a male copulatory organ with less than one turn, the presence of an articulated accessory piece with a concave rod-shaped termination, and a free accessory piece that is clamp shaped and bifurcated, as well as a dorsal bar with shading present in its medial part. This is the fourth species description of the genus Ancistrohaptor for fish of the genus Triportheus and the first record for T. signatus and the aquatic ecosystems of the Caatinga domain.

The present study aimed to inventory the parasitic fauna of Leporinus piau from the Lima Campos weir, Salgado River basin, municipality of Icó, Ceará, Brazil. A total of 35 host specimens were collected, whereas all were parasitized by at least two metazoan taxa. A total of 2,910 parasite specimens belonging to five taxonomic groups were identified such as: Myxozoa (Henneguya sp. 1 and Henneguya sp. 2), Monogenea (Jainus beccus, Jainus radixelongatus, Tereancistrum flabellum, Tereancistrum paranaensis, Tereancistrum parvus, Urocleidoides digitabulum, Urocleidoides paradoxus, Urocleidoides sp., Dactylogyridae gen. sp. 1, Dactylogyridae gen. sp. 2 and Dactylogyridae gen. sp. 3), Digenea (Diplostomum lunaschiae and Clinostomum sp.), Nematoda (Procamallanus (Spirocamallanus) inopinatus) and Copepoda (Gamispatulus schizodontis). Seventeen parasite taxa were found, with the class Monogenea being the most abundant group. The main site of infestation was the gills, presenting 14 taxa, including myxozoans, monogeneans and digeneans. This study highlights seven new parasitic associations for the host and 16 for the studied locality, as well as new records of diversity indices for the parasitic taxa found. It was observed that the class Monogenea exhibited a broad diversity of species, contributing to the understanding of the distribution patterns of fish parasites in the Brazilian semiarid region.

Tereancistrum is a common genus of Neotropical monopisthocotylans; however, information on its diversity and phylogeny remains limited. In this study, we describe four new species of Tereancistrum parasitizing the gills of Prochilodus brevis (Characiformes: Prochilodontidae) from a weir in the state of Ceará, Brazil. Tereancistrum spiralocirrum n. sp. and Tereancistrum scleritelongatum n. sp. are characterized by a dextro-ventral vaginal pore and a Y-shaped dorsal bar. Notably, Tereancistrum spiralocirrum n. sp. is the first species in the genus to possess a male copulatory organ (MCO) with multiple rings (16 to 18). In contrast, Tereancistrum ancistrum n. sp. and Tereancistrum kritskyi n. sp. are distinguished by a sinistral vaginal pore, a sclerotized MCO in the form of a coiled tube with slightly more than one clockwise ring, and an accessory piece that is non-articulated with the base of the MCO. However, Tereancistrum ancistrum n. sp. is unique in lacking a dorsal bar. Sequences of the LSU rDNA obtained from seven species of Tereancistrum parasitizing P. brevis and Leporinus piau, along with published sequences of other Dactylogyridae members, were included in the molecular analyses. Phylogenetic reconstructions supported the monophyly of Tereancistrum.

This study describes seven new species of Anacanthorus (Monopisthocotyla, Dactylogyridae) parasitizing gills of Triportheus spp. (Characiformes, Triportheidae) from two aquatic ecosystems in Brazil. These new species exhibit unique characteristics, such as variations in the morphology of the male copulatory complex and the morphology of the hooks, corroborating a high degree of host-parasite specificity. Despite the broad distribution of Anacanthorus in Characiformes, both molecular and morphological data suggest close coevolution, with parasite lineages often associated with specific host groups. This research expands the known diversity of Anacanthorus within the family Triportheidae, provides the first record of the genus in Triportheus signatus and Triportheus trifurcatus, and emphasizes the need for integrative studies to fully understand the evolutionary patterns of the group. The findings highlight the importance of investigating the parasite fauna in the Salgado River and Tocantins-Araguaia River basin ecosystems, which remain largely understudied, providing a solid foundation for future phylogenetic analyses.