Geographic and host-associated morphometric variation in Gyrodactylus atratuli Putz and Hoffman, 1963 and Gyrodactylus mediotorus King, Marcogliese, Forest, Mclaughlin and Bentzen, 2013 (Monogenea: Gyrodactylidae) from leuciscid fishes, with first re

Abstract The present study reports the first records of the viviparous monogeneans Gyrodactylus atratuli and Gyrodactylus mediotorus from leuciscid fishes in Mexico, providing new insights into intraspecific haptoral sclerite variability across hosts and geographic regions in North America. Nuclear rDNA (ITS region) genetic analyses confirmed species identities; Mexican G. atratuli sequences showed a 0.7% divergence from Canadian specimens, while Mexican G. mediotorus sequences exhibited a 0.2–0.8% divergence from other North American populations, including those from Texas and Canada. Relative to original and previously published descriptions, morphometric analyses revealed a clear latitudinal trend, in which northern populations of both parasite species possess larger hamuli, marginal hooks and associated attachment elements, while southern Mexican populations exhibit smaller structures. While elements like hamulus points and marginal hooks remained highly conserved, parts of the connecting bars, particularly the ventral bar membrane, demonstrated greater phenotypic plasticity, potentially reflecting adaptive adjustments to temperature and host-specific microhabitats. Gyrodactylus atratuli was recovered from Rhinichthys cataractae , while G. mediotorus was found parasitizing Cyprinella lutrensis and Cyprinella cf. proserpina , confirming low host specificity and ecological host flexibility at the family level (Leuciscidae). These results demonstrate that host traits, latitude and environmental fluctuations jointly drive morphological variation, while ITS variability reflects natural allopatric population divergence across the Nearctic.