Filter feeding by oysters reduces disease transmission in a marine host-parasite system

As a mechanism of the dilution effect, predation and filter feeding on parasitic propagules are hypothesized to reduce transmission to susceptible hosts and alter host-parasite interactions. In marine systems, the effect of other community members on the disease dynamics of microparasites in their suitable hosts is poorly known. In a coastal estuarine host-parasite system, we examined how eastern oysters, Crassostrea virginica, affect the transmission of a parasitic dinoflagellate, Hematodinium perezi, to juvenile blue crabs, Callinectes sapidus. We deployed juvenile blue crabs in custom mesh bags that were sandwiched by oysters into holo-endemic areas, or areas with high endemic transmission for the parasite in juvenile hosts. Controls consisted of juvenile crabs deployed with an equivalent number of oyster shells to test for the effect of rugosity on transmission and crabs deployed alone. Deployments lasted 7-13 days and were done over different temporal and spatial scales. Results from the field deployments suggest that oysters, not shells, reduced the probability of infection to crab hosts. To investigate consumption in the laboratory, single oysters in 1 L aquaria were fed dinospores of H. perezi released from infected crabs. Oysters reduced parasite densities in the water at rates similar to those observed for a common phytoplankton, Tetraselmis chui, that is grown specifically as oyster food. Our results jointly support that oysters benefit adjacent community members through feeding on transmissive stages of their pathogens and highlight the need for additional field-based approaches addressing environmental heterogeneity in pathogen transmission.