Host specificity is an important component of parasite ecology, determining the extent to which hosts are susceptible to infection by a particular parasite species at any given stage in its development. It can greatly influence the ecology of any stage in a parasite's life cycle plays and plays a significant role in parasite evolution. A parasite with broad host-specificity can be subdivided by host use into subpopulations. This in turn may affect the genetic diversity and the fate of the alleles under selection in a component parasite population, all individuals of a particular life stage in all hosts. If populations remain isolated for long periods of time, each sub-population may evolve to be a new species. Thus, understanding the relationship between host specificity and genetic diversity can help us to understand both the ecology and evolution of any parasite species with a complex life cycle.

The larval stages of trematode parasite Acanthoparyphium spinulosum infect various invertebrates in Californian estuaries. Previously, A. spinulsoum have been considered to be a single species despite the broad host-use pattern. Together with non-overlapping distributions of second intermediate hosts within a wetland could result in population segregation by host use. To this end, genetic data was gathered for both mitochondrial cytochrome c oxidase subunit 1 (COI) and the nuclear internal transcribed spacer 1(ITS1) loci from first intermediate hosts. Results suggest that A. spinulosum morphospecies represents a complex of cryptic species, which is differentially distributed between sites in Northern versus Southern California. Similarly, genetic data was obtained for parasites from second intermediate host species for COI and ITSI loci. Results from this larval stage suggest population segregation by second intermediate host use, such that metacercariae from Cerithidea californica all belonged to one clade. Metacercariae data from a clam and polychaete second intermediate host belonged to another clade. Finally, experimental infections were conducted to test the compatibility of the described genotypes (Chapter 2 and 3) for different second intermediate hosts. Fluorescent dye was used to distinguish between pre-existing and experimental infections. Experimental data suggest a relationship between compatibility of different parasite genotypes to different host species.