Abstract

Freshwater fish have been model hosts in the study of the community and evolutionary ecology of parasites for decades. Most studies have dealt only with adult parasites, although larval stages often dominate fish parasite communities. In addition, few studies include replicates of both host species and sampling localities. For this thesis, both larval and adult parasites were surveyed in six fish species ( Notemigonus crysoleucas, Pimephales notatus, Perca flavescens, Etheostoma nigrum, Lepomis gibbosus, Ambloplites rupestris ) collected from six localities in the St. Lawrence River and molecular techniques were used to distinguish species of strigeid metacercariae (Platyhelminthes: Trematoda). Novel primers were developed to sequence the barcode region of the cytochrome c oxidase I (COI) gene and sequences of internal transcribed spacer (ITS) regions of ribosomal DNA were also obtained. Both markers indicated unexpectedly high numbers of species in strigeid metacercariae, but resolution between species was clearer with COI than with ITS sequences. Strigeid species inhabiting the lens of the eye of fish were significantly less host specific than species inhabiting other tissues, possibly due to limited immune activity in the lens. Patterns of host specificity were consistent across the separate fish communities, which included fish species that are ecologically distinctive but closely related. Together, these findings suggest that physiological incompatibility between host and parasite is a more important determinant of host specificity than the ecological availability of host species to parasites in strigeid metacercariae. The high host specificity of most strigeid species had important effects on the parasite communities as a whole. Closely related fish species showed a significant tendency to have similar parasite communities that was much stronger than the tendency of parasite communities in spatially proximate fish to be similar. Geographic distance was strongly associated with parasite community similarity only when comparisons were limited to fish of the same species. Host taxonomy explained much more structure in parasite communities than spatial categories, suggesting that host phylogeny is more important than habitat in determining parasite community composition and abundance.