A phylogeographic approach of three species of Clinostomum Leidy, 1856, (Trematoda: Clinostomidae) from the Neotropical region of Mexico, with the description of a new species from Ardea herodias (Ardeidae).

Clinostomum Leidy, 1856 is a globally distributed group of endoparasites that reside attached to the mucosa of the mouth cavity and oesophagus of fish-eating birds (mainly herons) in the adult stage, whereas larvae known as metacercariae ("yellow grubs") damage freshwater fish and amphibians that serve as secondary intermediate hosts. In the Nearctic and Neotropical regions of Mexico, species of the genus Clinostomum have been intensively studied, revealing seven species that share a common ancestor. In the present study, sequences of the ITS from nuclear ribosomal DNA and cox 1 from mitochondrial DNA were generated for C. tataxumui, C. cichlidorum, and an unidentified species identified as lineage 3 sensu Pérez-Ponce de León et al. 2026, which is distributed sympatrically in Mexico and Middle America. The objectives of the present study were to 1) describe a new species of Clinostomum, named herein as Clinostomum chaacci n. sp., which combines morphological and molecular characters; 2) analyse the population genetic structure of two previously described species in addition to C. chaacci n. sp.; and 3) infer phylogeographic patterns using cox 1 sequences. The inferred phylogeographic results indicated that the populations of C. tataxumui, and C. cichlidorum lack a geographic structure with high haplotype diversity, low nucleotide diversity, and low Fst values among the populations with negative values on the neutrality test, suggesting that the populations are expanding. In contrast, the populations of C. chaacci n. sp., are less connected and subdivided into smaller populations, leading to slight genetic differentiation, suggesting that the populations have undergone significant demographic changes, such as recent expansion or selection effects.