Molecular Identification of Clinostomum chaacci (Digenea: Clinostomidae) Metacercariae Reveals Two New Siluriform Hosts in the Peruvian Amazon

Clinostomum chaacci Sereno-Uribe, López-Jiménez, González-García, Ortega-Olivares & García-Varela, 2025 is a recently described clinostomid trematode whose metacercariae have been molecularly identified in freshwater fishes from Mexico and South America. In October 2025, 29 siluriform fishes, comprising 18 Pimelodella cristata (Müller & Troschel, 1849) (Heptapteridae) and 11 Rineloricaria morrowi Fowler, 1940 (Loricariidae), were examined from the Nanay River, Loreto, Peru. Eight metacercariae were found encysted in the cephalic integument. The specimens were characterized morphologically and identified through Bayesian phylogenetic analyses of the mitochondrial Cox1 gene and the nuclear 28S rDNA. The new Cox1 sequences from both hosts were identical and nested within the C. chaacci clade, differing by 0.42–5.27% from conspecific isolates, whereas the 28S rDNA sequence differed by only 0–0.08% from reference sequences of C. chaacci. Morphometric data partially overlapped with those of C. chaacci metacercariae previously reported from Peru. The records from P. cristata and R. morrowi represent two new second intermediate hosts and add Loricariidae to the known host spectrum of C. chaacci, providing further evidence that a taxonomically diverse assemblage of fishes participates in its transmission in the western Amazon Basin.