Inducing defenses to deter predators is a necessary process theorized to incur costs. Although studies have investigated defense trade-offs, quantifying trade-offs is challenging and costs are often inferred. Additionally, prey employ strategies to reduce costs, making costs difficult to predict. Our purpose was to investigate induced defense costs by characterizing the defense mechanisms and costs in eastern oysters (Crassostrea virginica). In the field (summer 2014; 28.13°N, 96.98°W), newly-settled oysters reared under field conditions and assigned to control or predator (blue crab, Callinectes sapidus) conditions, were tested for shell weight and crushing force (a proxy for shell strength; both metrics are known to increase in response to exudates from crab predators), and amino acid content. Amino acid content indicates the quantity and type of organic material present in shells and informs our understanding of the physiological mechanism of defense in oysters. Oysters exposed to blue crab exudates grew stronger shells containing less percent organic material than oysters in controls. In oysters collected from natural populations (spring 2014 and 2017; 28°N, 97°W), we tested the correlation between shell density and shell thickness to determine natural patterns of oyster shell morphology. We also performed regression analyses of soft tissue mass and gonad investment (gonad index, calculated as gonad tissue mass/soft tissue mass) to assess the relationship between shell morphology and other biologically valuable processes (growth and reproduction respectively). Shell density was negatively correlated with shell thickness, further suggesting oysters thicken their shells by increasing low-density calcium carbonate. Reproductive investment showed an increasingly negative relationship with thickness as density decreased (and induction increased). In a lab experiment (Texas A&M University-Corpus Christi; summer 2013), oysters were exposed to a temporal gradient in risk and tested for shell weight and strength to indirectly test hypotheses regarding the mechanism and costs of oyster defenses suggested by the above experiments. Oysters grew heavier shells in all crab treatments, but only grew stronger shells under constant exposure. Collectively, these results suggest oysters initially react to predators by adding inexpensive calcium carbonate to their shells to quickly outgrow risk. However, in high risk environments, oysters may increase production of costly organic material to increase shell strength. Thus, oysters demonstrate a two-tier mechanism allowing them to cheaply escape predation at lower risk but to build stronger shells at greater expense when warranted. These results illuminate the complex strategies prey deploy to balance predation risk and defense costs and the importance of understanding these strategies to accurately predict predator effects.
