Riding South American rivers: a multilocus assessment of Creptotrematina batalhensis (Digenea: Allocreadiidae).

The Allocreadiidae comprise a diverse group of digeneans infecting freshwater fishes worldwide, with South America harbouring a particularly rich fauna. Within this family, Creptotrematina includes five valid species parasitizing primarily characid fishes, most of which have been recorded from the La Plata Basin. Despite the large geographic extent and hydrological complexity of this basin, the genetic structure and evolutionary relationships of Creptotrematina species across its sub-basins remain poorly understood. The inclusion of a new ITS sequence of Martorellia liciae recovered the same topology observed in the 28S phylogeny, supporting Martorellia as the sister genus of Creptotrematina. In the present study, specimens of Creptotrematina spp. collected in the Paraná and Iguazú rivers were analysed using partial sequences of 28S rDNA, ITS, and mitochondrial COI genes. Phylogenetic analyses based on individual markers recovered the new material within Creptotrematina batalhensis with strong support. Genetic divergence between Argentine and Brazilian populations of C. batalhensis was extremely low or absent across all markers, despite the large geographic distances and differences in hosts infected. These results indicate basin-wide genealogical cohesion and the absence of detectable phylogeographic structure. The observed pattern suggests that hydrographic subdivision within the La Plata Basin has not acted as an effective evolutionary barrier for this species. Instead, the strong association of Creptotrematina spp. with closely related characid hosts supports the hypothesis that host phylogeny and host-mediated dispersal may play a central role in shaping parasite diversification, with hydrological dynamics modulating connectivity rather than driving lineage fragmentation.