Crabs interpret the threat of predator body size and biomass via cue concentration and diet

Abstract

Greater predator body size is often associated with greater predation risk. According to the threat-sensitive predator avoidance hypothesis, prey should display graded responses to increasing predator body size; in turn, these differences in behaviour should also cause differing indirect effects. Yet, in aquatic systems, where prey often use chemical cues to judge predator threats, the role of chemically mediated perception of predator body size and the propagation of indirect effects are still unclear. To differentiate intraspecific predator size via chemical cues, prey must judge predator threat quantitatively (i.e. via concentration) or qualitatively (i.e. via differing cues and/or diets). We investigated the role of individual and aggregate predator body size (i.e. biomass, cue concentration) and qualitative diet cues in antipredator behaviour and indirect interactions by examining the behavioural responses of the mud crab Panopeus herbstii and the survival of oyster prey (Crassostrea virginica) in response to various blue crab, Callinectes sapidus, biomass and diet treatments. Mud crabs increased their refuge use and decreased foraging in response to chemical cues from large, but not small, individual blue crabs. The perception of predator size appeared to be related to predator biomass as multiple small blue crabs and large crabs elicited similar foraging responses in mud crabs. However, multiple small blue crabs failed to affect mud crab refuge use, indicating that some measures of behaviour may not always be predictive of indirect effects. Predator diet also influenced mud crab behaviour and foraging: predators fed mud crabs elicited a greater antipredator response than crushed conspecifics or predators fed oyster diets, suggesting that qualitative cues also influence intraspecific threat perception and indirect interactions. These experiments demonstrate that we cannot successfully predict indirect interactions without considering predator population size structure and measuring indirect effects.