Eastern oysters alter inducible defense mechanism of shell strengthening with age

Eastern oysters, Crassostrea virginica, use inducible defenses in the form of strengthened shells to reduce their risk of predation. Inducible defenses often have trade-offs between the costs and benefits associated with the organism's fitness, as developing defenses requires energetic resources. Shell strength is a product of the amount of material laid by the animal (thickness) and the material properties of the shell (e.g. hardness and fracture resistance). Previous studies have suggested that oysters may trade off between shell thickness or shell hardness as a mechanism for increasing shell strength against predation, which are hypothesized to have different energetic requirements. The present study analyzed the shell structural (thickness of composite layers) and micromechanical properties (microhardness and crack propagation tested within individual shell layers) of predator-induced and non-induced juvenile oysters at 4 and 8 weeks post-settlement to determine which shell strengthening mechanism oysters use in response to predator cues. Younger juveniles did not display any significant differences in micromechanical shell properties or shell thickness as a result of induction, though some marginal differences were detected. In contrast, older juveniles use a combination of increased hardness and thickness in shell defense, where induced oysters were 33% thicker overall and 12% harder within their outer prismatic layer. This suggests that oysters alter shell strength using multiple defense mechanisms depending on age, and we postulate that animals switch mechanisms when necessary to balance the advantages and associated costs of defense with an individual's physiological needs for growth.