Are All Parasitic Infections Created Equal? An Examination of Differential Infection Effects and Responses in Native and Non-Native Hosts

Interactions between native and introduced species are complex, multifaceted, and can have cascading effects throughout an ecosystem. While scientists have taken great strides to understand how these dynamics change with the introduction of a novel species, less work has been done to explore how native and introduced species interactions change in the face of parasitism. I investigated these interactions using three intertidal crab species in the Western North Atlantic; the native rock crab Cancer irroratus, the introduced European green crab, Carcinus maenas, and the introduced Asian shore crab, Hemigrapsus sanguineus along with a parasitic trematode Micropahllus spp. C. maenas and C. irroratus are both hosts to microphallid trematode parasites in the Western North Atlantic, but little is known about microphallid infections in H. sanguineus in this region. In order to properly evaluate infection in these crabs, I conducted two surveys along the New England coast to determine prevalence of infection in these three species. I utilized genetic tools to determine if the microphallids infecting these crabs are native to Western North Atlantic, potentially introduced from Europe with C. maenas, or a combination of both. During these surveys, I found all three crab species to be infected with one or more trematode parasites along the Western North Atlantic. Genetic analyses show that there are at least four distinct lineages of trematodes infecting crabs collected in this region, two of which matched previously identified trematodes in the region, Microphallus similis and Gynaecotyla adunca. In conjunction with determining which parasites are infecting these crabs, I wanted to determine what effects infection had on these different host species. To understand the effects of infection I conducted both physiological assays on crabs collected during the survey, analyzing Hepatosomatic and Gonadosomatic indices (HSI and GSI) as well as infection experiments on the two introduced crabs, C. maenas and H. sanguineus, as I was unable to collect enough of the native C. irroratus for these experiments. I tested each species for individual behavioral responses to infection, and performed interspecific competition trials to determine how parasites may influence crab interactions. For my physiological assays, there was a trend for the HSI in both C. irroratus and C. maenas to be higher in infected crabs compared to uninfected crabs, with no difference in H. sanguineus crabs. There appeared to be a trend with lower GSI in infected H. sanguineus that was not seen in the other crab species, however a more robust sampling size would help determine if this trend exists in nature or is instead an artifact of small sample size. For my infection experiments, I saw that while my intensities of infection were low, there seems to be a trend for slower righting response in infected C. maenas with a similar trend seen in exposed H. sanguineus even though the parasite was unsuccessful in infecting H. sanguineus. During the competition trials I was able to detect a decreased level of exploratory by C. maenas exposed to the parasite when compared to control crabs, however there were very few overall differences between treatments. There were differences between species, with H. sanguineus conducting more exploratory behaviors overall and C. maenas conducting more food based behaviors. By using this multi-layer approach to investigate parasite effects, my study provides in-depth coverage of various changes that could be induced by infection. It also provides data on how infection by microphallid trematodes may affect competitive interactions and lay the groundwork for future studies exploring how those differences could have community-wide effects throughout the ecosystem