Gyrodactylidae from Fundulus heteroclitus (Fundulidae, Cyprinodontiformes) in South Carolina, USA, and insights on the historical ecology of the gyrodactylids of Fundulidae.

Mummichog (Fundulus heteroclitus) hosts a diverse array of parasites, including several species of Gyrodactylidae. We surveyed populations of mummichogs across three estuarine systems in South Carolina, USA, for gyrodactylids and studied their taxonomy, phylogenetics, and evolutionary history. Molecular analysis using concatenated sequences of 18S, ITS, and 5.8S rDNA fragments and morphology support the synonymization of Fundulotrema with Swingleus, as the former was shown paraphyletic. Five species of Gyrodactylidae were encountered. Gyrodactylus kritskyi n. sp. is described and four other species are redescribed: G. stephanus, Swingleus polyclithroides, S. prolongis n. comb., and S. porterense n. comb. Analyses of the phylogeny, tanglegram, and Euler diagram of gyrodactylids of Fundulidae reveal a complex mosaic of historical processes, including ecological fitting, oscillation, taxon pulses, and putative hybridization events driving host-parasite associations observed, fully compatible with the Stockholm Paradigm. These findings underscore the dynamic nature of gyrodactylid evolution within Fundulidae and highlight the utility of this model system for studying host-parasite diversification and evolution of host repertoire.