Abstract
Clinostomum Leidy, 1856 is a cosmopolitan genus of digenean trematodes whose metacercariae commonly infect freshwater fishes and amphibians as second intermediate hosts. In South America, the diversity and taxonomy of Clinostomum metacercariae remain poorly understood, due in part to the morphological similarity of larval stages and the scarcity of molecular data. This study provides the first integrative evidence of two phylogenetically distinct Clinostomum lineages infecting the callichthyid catfish Hoplosternum littorale (Hancock, 1828) (Siluriformes: Callichthyidae), a benthic catfish of commercial and subsistence importance in the Peruvian Amazon. Morphological analyses (light and scanning electron microscopy) combined with molecular data from mitochondrial (cox1) and nuclear (28S rDNA) market revealed two lineages with clear genetic differentiation. Specimens from Loreto were conspecific with Clinostomum chaacci Sereno-Uribe, López-Jiménez, González-García, Ortega-Olivares & García-Varela, 2025, previously reported from several fish hosts across Central and South America, whereas those from Ucayali formed a distinct and highly divergent lineage (11.9-12.7 % cox1 divergence) closely related to Clinostomum L1, representing a potentially undescribed species. The detection of C. chaacci in H. littorale constitutes a new host record and expands the known distribution of this species to the western Amazon Basin. The detection of Clinostomum metacercariae in fish muscle tissue raises potential zoonotic concerns, especially in regions where raw or undercooked fish is consumed. These findings underscore the need for integrative taxonomic approaches to unravel hidden diversity in Clinostomum and emphasize the need for broader parasitological surveys and public health awareness in Amazonian communities.