Eastern oysters Crassostrea virginica deter crab predators by altering their morphology in response to crab cues

Many prey species alter their behavior and/or morphology in response to exudates from predators and injured con- or heterospecifics to alleviate predation risk. Yet, few studies have assessed the effectiveness of risk aversion in prey in terms of decreasing mortality. Recent studies have shown that eastern oysters Crassostrea virginica produce heavier shells in the presence of predators, but the benefits of this morphological change have not been evaluated. We performed an indoor laboratory experiment and exposed newly settled oysters to chemical cues from 2 common predators: blue crabs Callinectes sapidus and Atlantic mud crabs Panopeus herbstii. After 8 wk, we compared shell diameter, shell mass, shell breaking force, and susceptibility to predation between juvenile oysters in these predator treatments to those in controls without predators. Oyster shell diameter and mass were significantly greater in blue crab treatments than in controls, and mud crabs and controls were not significantly different in these parameters. Yet, in both mud crab and blue crab treatments, oysters produced shells that required more force to crush as measured with a hand-held force transducer. Oysters reared in the presence of blue and mud crabs were less susceptible to predation than those maintained in no-predator controls in feeding assays performed after the 8 wk induction. Although oysters reacted differently to mud crabs and blue crabs, changing their shell morphology was an effective deterrent against predators. Future studies are needed to determine the long-term effects of induced responses to predators on oyster growth, fitness, and reef formation.