Predator effects on host−parasite interactions in the eastern oyster Crassostrea virginica

Both parasitism and predation may strongly influence population dynamics and community structure separately or synergistically. Predator species can influence host−parasite interactions, either by preferentially feeding on infected (or uninfected) hosts — and thus altering parasite prevalence patterns — or by affecting host behavior in ways that increase host susceptibility to parasites. In this study, we tested if predators (the mud crab Panopeus herbstii and the blue crab Callinectes sapidus) influence interactions between the eastern oyster Crassostrea virginica and 2 of its most prevalent parasites, Perkinsus marinus and Haplosporidium nelsoni. Using a combination of field and laboratory experiments, we tested for predatory effects on the prevalence and intensity of parasite infections and on oyster immune response (phagocytic activity). Our results consistently demonstrated that crabs do not influence parasite infections in oysters at either individual or population levels. Thus, even though predators often have strong top-down direct and indirect effects on marine communities, we found their influence on host−parasite interactions to be minimal in this system.