Size matters for risk assessment and resource allocation in bivalves

Nonlethal predator effects can significantly influence trophic interactions, and in this study we examined how size relationships between predators and prey would influence the ex pression of nonlethal predator effects. We assessed how size and vulnerability to predators would influence nonlethal effects in bivalve species common to oyster reefs. We used 2 size classes of mussels Ischadium recurvum, clams Mercenaria mercenaria, and oysters Crassostrea virginica as prey and compared energy allocation and growth of small vs. large bivalves in the presence of Atlantic mud crabs Panopeus herbstii, a common, resident reef predator. After a 45 d field experiment, we observed significant differences in growth among bivalves in response to mud crabs, but the effects were size and species dependent. In the presence of mud crabs, small clams and small oysters grew significantly less soft-tissue, small mussels grew more shell mass, large clams grew less shell mass, and large mussels grew less tissue and shell mass. Significant differences in the growth of larger oysters were not found. Changes in growth reflect resource allocation differences in response to predators and most likely resulted from costs associated with feeding reductions to minimize release of metabolites attractive to predators and/or allocation of additional energy for morphological defense to minimize predation risk. Fecundity is positively correlated with size in bivalves, and the ability to detect predation risk and appropriately allocate resources may be important for future reproductive output of these species. Smaller bivalves were more vulnerable to mud crab predators in laboratory feeding assays. Since size is inversely related to bivalve susceptibility to mud crabs predation, slower growth may lengthen the time that these bivalves are vulnerable to mud crabs and therefore increase their mortality. Results from this study suggest that mud crabs can affect the growth and fecundity of commercially important bivalves by nonlethal interactions and that size is an important consideration when investigating the propagation of nonlethal predator effects.