To reduce predation risk, many prey species alter their behavior or morphology in situations where risk of injury or death is imminent. Responding to predators often incurs costs, such as reductions in growth or fecundity, but how these costs affect organisms long-term has rarely been investigated. Oysters grow heavier, stronger shells in the presence of predators. It is assumed such changes increase energy allocation to shell production and reduce allocation to the production of soft and gonadal tissue, potentially producing long-term effects on oyster populations and oyster reef health. We investigated the relationship between shell and soft tissue production in oysters, including gonad tissue. We characterized oyster shell morphology and collected somatic and gonadal tissue weights for oysters collected from local reefs. Regression analysis was used to build models of somatic tissue and gonad index using shell thickness and density.
Results/Conclusions
Shell characteristics showed a strong inverse correlation, so that oyster shells fell along a gradient from thin with high dense to thick with low density. Gonad index decreased significantly as shells characteristics initially moved from dense (undefended) to thick (partially defended) but this cost leveled off as thickening further increased (fully defended). Thus, characteristics of oyster shells which are influenced by ambient levels of predation risk influence oyster fecundity. Results suggest shell thickening initially involves increased production of the energy-expensive shell protein matrix. However, additional thickening results solely from increased production of inexpensive calcium carbonate, despite its weaker mechanical properties. As fishing practices alter oyster reef communities, these shell and soft tissue relationships could have important consequences for oyster population dynamics. Results from this study are valuable for understanding the relationship between shell and soft tissue in oysters and in guiding future studies to investigate this relationship under conditions of predation risk.