The most unusual diplostomoideans from atypical hosts: Pipatrema mirabilis n. sp., n. g. (Digenea: Diplostomidae) from Pipa pipa (Amphibia: Pipidae) in Brazil and a related species from fish in Argentina

The Superfamily Diplostomoidea is a major lineage of digenean trematodes characterized by the presence of a ventral holdfast organ. Its members parasitize the intestines of tetrapod vertebrates worldwide, with only a single genus reported from fishes and none from amphibians. During a biodiversity survey in the Brazilian Amazon (Pará State), we discovered highly unusual diplostomoidean digeneans lacking a holdfast organ in the fully aquatic anuran Pipa pipa (Pipidae). We describe these digeneans as a new species and genus Pipatrema mirabilis n. sp., n. g. Additionally, a single specimen of the same genus was found in P. pipa from Amapá State, Brazil and a single specimen belonging to a related genus and species was collected from the South American catfish Rhamdia quelen (Siluriformes: Heptapteridae) in Argentina. The molecular phylogeny based on the 28S rRNA gene revealed a secondary loss of the holdfast organ, a defining morphological feature of diplostomoideans. Furthermore, the phylogenetic analysis positioned both taxa lacking a holdfast organ within the proterodiplostome clade among digeneans parasitic in South American crocodilians. These results strongly suggest that parasitism in fish and amphibians is the result of secondary evolutionary host-switching events. This study provides the first record of adult diplostomid trematodes from amphibian and fish hosts. Our findings highlight the importance of integrative approaches combining morphology, molecular phylogenetics, and ecological context to understand parasite diversification and host-parasite evolution in complex aquatic systems. • First diplostomoideans without a holdfast organ are reported. • A new genus and species is described from Pipa pipa in Brazil. • This is the first report of adult diplostomids from fishes and amphibians. • Molecular phylogeny suggests secondary loss of the hodfast organ. • Transition from reptiles to fishes and amphibians is a secondary host-switching event.