The combined impact of multiple environmental factors on the expression of phenotypic plasticity has rarely been studied. Theory predicts that organisms may respond to combined factors with an intermediate phenotype that balances the fitness costs from several sources or may simply respond appropriately to the factor which imposes the greateast fitness cost. Predator-induced defenses are an ideal sytem for testing these alternative hypotheses. Here I examine the influence of separate and combined presence of nonlethal shell-breaking (crabs, Cancer productus) and shell-entering (seastars, Pisaster ochraceous) predators on the defensive shell morphology and antipredator behavior of a marine snail (Nucella lamellosa).

Results/Conclusions

When exposed to each predator separately, snails responded with a combination of predator-specific defenses and generalized antipredator traits. Snails responded to crabs by producing a thicker, more rotund shell and increasing refuge use and retreat potential (i.e. the ability to withdraw into their shell). Snails responded to seastars with thinner, more elongate shells and increased refuge use and retreat potential. Predator-specific changes in shell shape and thickness that decreased susceptibility to one predator increased susceptibility to the other, indicating a functional tradeoff. When predators were combined, snails amplified retreat potential and produced a morphological phenotype similar to that expressed in the presence of the more dangerous crab predator alone, suggesting that inducible morphological defenses were prioritized according to predation risk. These results suggest that prioritizing conflicting predator-specific defenses according to predator danger may be a common strategy for prey responding to combined predators, particularly in combination with amplified general defenses that could reduce the overall risk of producing directional phenotypes in multiple predator environments. This highlights the importance of considering multiple traits when examining the phenotypic responses of organisms to multiple environmental factors.