Risk recognition and variability in escape responses among intertidal molluskan grazers to the sun star Heliaster helianthus

Abstract

Escape responses are a common form of inducible defense in the presence of a predator, which can be species-specific and adaptive. While conditions for their evolution have been amply investigated, little is known about variability in prey responses over the range of variation in predation risks typical of most systems. We characterized escape responses of 4 intertidal grazers (2 chitons, 2 fissurellid limpets) to the sunstar Heliaster helianthus. Comparisons were made among sites of apparently distinct predation risks: high risk, where the grazers constitute a significant portion of H. helianthus diet; moderate risk, where H. helianthus primarily consume preferred mussel prey; no risk of predation, where prey exist beyond the southern geographic limit of H. helianthus. Prey responses were highly specific to the H. helianthus stimulus. Escape responses differed among grazer species, but these differences did not correspond to taxonomic relatedness, nor to their intertidal vertical distribution. No differences in escape responses were observed between sites where H. helianthus exhibit different diets, suggesting that these grazer species cannot ‘perceive’ and/or respond to this apparent change in predation risk. In contrast, a greater proportion of individuals of all 4 species responded to the H. helianthus cue, predator recognition was significantly faster, and distance traveled after stimulation was greater at sites of predator-prey coexistence than at the site beyond the predator’s geographic range. Thus, longer contact history with the predator appears to favor greater capacity to display escape responses in all species examined. We highlight the importance of evaluating phenotypic responses to predators under natural conditions and across natural gradients of predation risks.