Research Thirty to fifty percent of known animals are parasites, and parasites play a central role not only in human health, but also in the functioning of whole ecosystems. Many important human parasites, such as liver flukes and parasites causing schistosomiasis, belong to the same group of parasites (trematodes) as those studied in this project. Obtaining information about substrate specificity and aggregation behavior of these parasites may inform public health policy by helping predict where parasites may be found and how to prevent infection. This project investigated a previously unknown portion of the life cycle of a trematode parasite. These parasites form cysts on hard objects in the environment, which are consumed by their final host to complete transmission. I combined data from experiments and natural surveys to show that this parasite preferentially forms cysts on snail shells, and that it is attracted to shells that already have cysts on them. These results contribute not only to our understanding of the life cycle of this parasite, but also to our understanding of parasite host specificity. Though some parasites may have the ability to infect (or form cysts on) many different hosts (or objects), they may (and likely will) still show a strong preference for the substrate(s) that maximize their chances of getting into their final host. The results of this project also contribute to our understanding of aggregation behavior. Because these parasites show a preference for shells that already have cysts, some shells will likely end up with many more cysts than others. Many parasites (perhaps the most well-known being ticks) show similar patterns, where some hosts have many more parasites than others, and this pattern has often been attributed to differences in host susceptibility, exposure or attractiveness. However, the results of this project suggest that parasite behavior, rather than host characteristics, may cause aggregated patterns. This result lends support to the hypothesis that there may be a benefit to parasites in clumping, such as a greater availability of mates in the final host. EAPSI Science does not progress in isolation, but through the combined efforts of and communication among scientists internationally. The East Asia and the Pacific Summer Institute (EAPSI) program encourages collaboration between US graduate students and prominent scientists internationally. This program enabled me to work with one of the leading ecological parasitologists worldwide, to gain new skills and perspectives, and to build relationships that will hopefully lead to future collaboration. As a graduate student just beginning my career in science, the experience was valuable in helping to encourage my participation in the greater scientific community through collaboration, which will surely have a large impact on my ability to address globally important questions in my future career. Participation in EAPSI also gave me experience working with trematodes, a group of parasites that should be very accessible to younger students. Trematodes are common, infect snails (which are easy for younger students to collect and handle), and some are even visible with the naked eye. Since returning from my summer abroad, I have begun looking for snails in this area and have contacted a local high school biology teacher about starting a program with high school students. I think the integration of research and education is important, and hope to use the skills gained during my participation in EAPSI as an educational tool now and in the future.