Predator‐Mediated Microhabitat Partitioning by Two Species of Visually Cryptic, Intertidal Limpets

Abstract

Two species of limpets appear to partition microhabitats according to substratum color within the mixed—barnacle zone of the Pacific Northwest rocky intertidal. A predominantly light—shelled species, Collisella digitalis, occurs most commonly upon the light—colored skeletons of barnacles (Pollicipes polymerus and Semibalanus cariosus), while a predominantly dark—shelled species, Collisella pelta, occurs most commonly on the dark valves of mussels (Mytilus californianus). Field experiments revealed significantly higher mortality of limpets mismtched to their background, due to predation by both fish and birds. When given a choice, in the absence of predation, both species moved actively onto the substratum where they were most cryptic. Strong selection by visual predators appears to maintain the partitioning of space by these limpet species. Competitive interactions are not required to explain this microhabitat partitioning, although they cannot be ruled out as one of the original selection pressures.