The Pisaster‐Tegula Interaction: Prey Patches, Predator Food Preference, and Intertidal Community Structure

Abstract

The herbivorous gastropod Tegula funebralis is not highly ranked in a food preference hierarchy of its major predator, the starfish Pisaster ochraceus, and exhibits a persistent broad overlap with it in the rocky intertidal zone at Mukkaw Bay, Washington. Observations on Tegula over a 5—yr period indicate that it settles high intertidally, lives there for 5—6 yr, and then tends to migrate lower into contact with Pisaster. Tegula lays down an annual growth line permitting it to be aged and a growth curve constructed. Analysis of relative growth and reproduction indicates that beyond a certain size (16 mm) large individuals perform less well in the upper than those in the lower intertidal zone. Pisaster consumes 25—28% of the adult Tegula per year in the area of spatial overlap, based on analysis of the age structure of 6—17 yr old Tegula, and by direct estimates of the percentage of the standing crop consumed annually. The relationship between Pisaster and sex ratio, relative energy limitation and reproductive output (fitness) of Tegula is discussed for three subpopulations. It is suggested that the implied results of the interaction is typical of that between a major predator and one of its less preferred prey. The prominent zonation exhibited by preferred prey, the observed intimacy of association of predator and less preferred prey, and the zoogeographic homogeneity of the Pacific rocky coastline community are discussed in relation to three intermeshing ecological processes.