Behavioral mechanism for an associational refuge for macroalgae on temperate reefs

Abstract

Temperate reefs are sensitive to fluctuations in grazing intensity and, while some species of marine algae possess chemical and structural defenses, many species lack adaptations to withstand high grazer densities. The presence of foliose macroalgae in the face of high grazing pressure can have important consequences for higher trophic levels, in part because macroalgae harbor prey for fish and macroinvertebrates. The present study explores a biotic interaction between the sea urchins Strongylocentrotus purpuratus and S. franciscanus and the colonial anemone Corynactis californica, and describes the behavioral underpinnings of an associational refuge for benthic macroalgae on heavily grazed rocky reefs. Field surveys of heavily grazed reefs revealed that macroalgae is more abundant within C. californica colonies, and that sea urchins touch far fewer C. californica polyps than would be expected by chance. In laboratory studies, the movement of urchins was impeded when urchins withdrew their tube feet upon contact with C. californica. This was probably the behavioral mechanism explaining results of field experiments where recruitment of macroalgae was relatively high among C. californica colonies transplanted within a sea urchin barren. C. californica may play an underappreciated role on temperate rocky reefs by altering the behavior of a dominant herbivore and thus providing an associational refuge for important biogenic structure. © Inter-Research 2008.