Parasites significantly influence the genetics of a host population, cause changes in behavior and lead to changes in host physiology and morphology. By altering the ecology of hosts, parasites shape the interactions within and the maintenance of entire communities. Trematodes are widespread and have significant implications for hosts in marine nearshore communities. Using this ubiquitous yet poorly understood group of parasites, I investigate how host-parasite interactions scale up to community dynamics. Parasites can mediate the success of an invading species which have escaped parasites or competitors in their native range. Littorina littorea is a snail native to Northern Europe which invaded the East Coast of North America within the last 200 years. It has since become the dominant snail in intertidal habitats, displacing native. I found that there are two morphologically distinct trematode species infecting this host in the Cape Cod area: Cryptocotyle lingua and Cercaria parvicaudata. While C.lingua has been described as the most common and abundant trematode infecting L.littorea in North America, I found that a second parasite species, C.parvicaudata, predominates in the Cape Cod region. Because trematodes that infect snails have been shown to have cryptic diversity, I am developing novel molecular methods using the reconstruction of the full mitochondrion genome to investigate the diversity of trematode parasites in Littorina littorea. Preliminary results suggest that Cercaria parvicaudata may be Renicola roscovita, a previously described species from other locations. Future analysis of mitochondrial DNA will allow to determine the population structure of the trematode parasites, as well as design species specific DNA probes that allow for a more effective identification and examining of trematodes at different stages of the life cycle.