Predator cues facilitate oyster biofiltration by suppressing the density-dependent impacts of ectoparasites

Predators commonly regulate the feeding behavior of consumers through non-consumptive effects (NCEs) generating cascading indirect effects. While parasites are ubiquitous consumers in ecosystems, it is not known whether predator NCEs can generate similar cascading impacts by regulating parasite feeding on their hosts. Ectoparasites may be particularly sensitive to predator NCEs, yet this interaction remains unexplored. We focused on a common ectoparasitic snail, Boonea impressa , and examined how predator-associated olfactory cues impact feeding on its host, the eastern oyster Crassostrea virginica . Scent-exposure assays revealed that B. impressa reduced feeding on its host in the presence of predator (mud crab Panopeus herbsti i ) cues. A laboratory biofiltration experiment, in which we manipulated the density of ectoparasites and the presence of predatory crab olfactory cues, showed that ectoparasites reduced oyster biofiltration rates, and these effects are mediated by both parasite density and predator cues. Mud crab NCEs switched from having negative to positive effects on oyster biofiltration as ectoparasite densities increased. While this study presents the first evidence of a predator influencing ectoparasite feeding through NCEs, this phenomenon may be common in nature.