Larval parasite occurrence in two killifish species, Argolebias guarani and Argolebias adrianae (Cyprinodontiformes: Rivulidae) inhabiting two ephemeral ponds.

Ephemeral ponds provide unique opportunities to study host-parasite interactions under contrasting environmental conditions. We assessed the occurrence of larval parasites in two closely related rivulid fishes, Argolebias guarani and Argolebias adrianae, inhabiting geographically proximate but hydrologically isolated ponds. A total of 121 individuals of A. guarani and 30 of A. adrianae were examined. Parasites were recorded only in A. guarani, including a nematomorph larva, a proteocephalidean metacestode, a telorchiid metacercaria, and a diplostomid metacercaria, whereas A. adrianae showed no detectable parasitism. Morphological examination combined with molecular analyses using Internal Transcribed Spacer and Cytochrome c Oxidase subunit I sequences supported the taxonomic placement of proteocephalidean, telorchiid, and diplostomid larvae, revealing distinct phylogenetic lineages and highlighting gaps in knowledge of larval stages in South American killifishes. The observed differences in parasite occurrence likely reflect structural and ecological features of the ponds rather than intrinsic physiological differences between hosts. Open, structurally complex ponds with abundant invertebrates appear to facilitate transmission of multiple parasite taxa, whereas smaller, shaded ponds with limited access to definitive hosts may constrain parasite establishment. These findings provide novel insights into the role of ephemeral pond environments in shaping larval parasite presence and underscore the value of integrative morphological and molecular approaches for understanding host-parasite interactions in understudied freshwater systems.