Evidence for local adaptation of oysters to a within-estuary gradient in predation pressure weakens with ontogeny

Abstract

Consistent spatial variation in phenotypes within a species can reflect local adaptation to gradients in selective pressures, or plastic responses to variable conditions. In benthic marine foundation systems with long, dispersive pelagic larval stages, the usual assumption is that the plastic strategy should dominate, yet surprising examples of local adaptation are accumulating. We tested the potential for local adaptation to an environmentally-driven spatial gradient in predation pressure on the Eastern oyster (Crassostrea virginica) in a Florida estuary by reciprocally transplanting juvenile oysters between two sites. Experimental units included oysters in predator exclosures and controls that were exposed to both consumption by predators and nonconsumptive predator cues. One month after deployment, juvenile oysters from the upper-estuary site where more predators of adult oysters are present exhibited thicker shells in both the upper and lower estuary deployment sites, a possible signal of local adaption to a more intense predation regime. However, nine months after deployment, oysters sourced from both the upper and lower demes had thicker shells in predator-exposed treatments than in predator exclosures, regardless of location. Additionally, there was no evidence that the thicker shells in those oysters led to changes in survival or growth, as one would expect. These results suggest that while oysters from high-predator sites may initially exhibit an apparently adaptive antipredator trait, over the course of an oyster's life, trait expression is a plastic response to the perceived predation threat environment.