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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 …