Size matters for risk assessment and resource allocation in bivalves

Abstract

Nonlethal predator effects can significantly influence trophic interactions, and in this study we examined how size relationships between predators and prey would influence the expression 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.

Eastern oysters Crassostrea virginica deter crab predators by altering their morphology in response to crab cues

“…Prey size often affects vulnerability to consumers (Wong et al 2010), and crabs prey more heavily on smaller bivalves possibly because they require less energy to open (Nakaoka 2000, Johnson & Smee 2012 and are less likely to cause injury to crab claws (Juanes 1992). We therefore elected to focus on oyster spat for this experiment because they are readily consumed by many organisms, including mud crabs (Newell et al 2000, Johnson & Smee 2012.…”

“…We therefore elected to focus on oyster spat for this experiment because they are readily consumed by many organisms, including mud crabs (Newell et al 2000, Johnson & Smee 2012. Our results suggest that oyster spat detect and respond to predation risk by altering their morphology, that they can perceive differences in predator species, and that their responses to predators can increase survival even when they are small (< 5 mm).…”

“…Like other bivalves, eastern oysters are heavily preyed upon by a suite of predators in cluding fish, crabs, and gastropods (Brown et al 2003, Newell et al 2007, Lord & Whitlatch 2012, and their vulnerability to consumers is inversely related to body size (Newell et al 2000, Johnson & Smee 2012. Three recent studies have shown that oysters react to gastropod and crustacean predators by producing thicker, heavier shells (Newell et al 2007, Johnson & Smee 2012, Lord & Whitlatch 2012 and in doing so, produce less soft tissue (Johnson & Smee 2012, but see Lord & Whitlatch 2012). Here, we sought to build upon these studies by assessing how growth and shell morphology of newly settled oysters, the size class most vulnerable to crab predation, was influenced by 2 common predatory crabs: blue crabs Callinectes sapi dus and Atlantic mud crabs Panopeus herbstii.…”

Phenotypic plasticity in oysters (Crassostrea virginica) mediated by chemical signals from predators and injured prey

“…In the presence of predatory crabs, oysters are known to alter the morphology of their shells in ways that reduce their susceptibility to crabs, including changes in thickness, mass, and strength (Newell et al. ; Johnson & Smee ; Lord & Whitlatch ; Robinson et al. ).…”

Temporal Structure and Trends of Parasites and Pathologies in U.S. Oysters and Mussels: 16 Years of Mussel Watch

“…If increased mortality is the proximate cause of the decline in length in Gulf oysters, the agent would not likely be a predator, as most predators target the smaller size classes (Powell et al 1997, Linton et al 2007, Johnson & Smee 2012. Fishing targets the larger size classes, as most states have a 7.6-cm size limit.…”