Cue specificity of predator-induced phenotype in a marine snail: is a crab just a crab?

A wide range of taxa have been shown to display inducible, phenotypically plastic responses to known predators. Most studies of inducible defenses include only known predators but not non-predatory species in experimental designs, precluding tests of specificity for these responses. We tested the specificity of predator-induced defenses in the marine snail Nucella lamellosa, when exposed to chemical cues from potential crab predators as well as more distantly related non-predatory crabs that co-occur with this snail. Surprisingly, all crabs tested, even those that are not predators, triggered the common induced response of a reduction of soft-tissue mass relative to control animals, likely reflecting a reduction in snail feeding activity. In contrast, only N. lamellosa's major predator, Cancer productus, triggered the production of a thicker apertural lip. Increased thickening of the apertural lip may be an adaptive response specific to C. productus, which uses shell-breaking at the apertural lip (i.e., shell-peeling) as their main form of attack. Apertural lip thickening appeared to be due to reallocation of shell material (i.e., a change in shell shape) rather than an increase in shell deposition. Our findings demonstrate the importance of determining the specificity of cues triggering inducible responses in prey, and the mechanisms that underlie these plastic responses, as the responses to general versus specific cues may limit the adaptive value of an inducible defense.