The processes that affect the pattern and persistence of marine communities are receiving increasing attention in the ecological literature. In particular, intertidal studies on temperate rocky shores have been the major source of information, not only contributing assessments of interactions among species, but also serving as a focus for discussions about testing hypotheses and ecological experimentation (cf Strong, Simberloff, Abele & Thistle, 1984). This attention to intertidal areas is hardly surprising given their accessibility, the gradient of physical conditions in the relatively short distance between tide marks, and the large number of algae, herbivores, and predators of many phyla that inhabit most shores (Stephenson & Stephenson, 1949, 1972). In contrast, subtidal areas have been studied comparatively less, even though rocky reefs on most shores are usually much more extensive subtidally than intertidally. Only in recent years, however, has there been routine access to shallow subtidal habitats, allowing descriptive and experimental evaluation of some of the processes that structure communities there.