Abalones and sea urchins in an area inhabited by sea otters

Abalones (Haliotis spp.) and sea urchins (Strongylocentrotus spp.) are part of the subtidal fauna in the kelp bed off Hopkins Marine Station, Pacific Grove, California, USA, a protected marine reserve. Although these animals have been preyed upon by sea otters for over 10 years, their densities are gubstantial (Haliotis spp.: 0.21/m²; Strongylocentrotus spp.: 0.22/m²), and two species, H. rufescens (Swainson) and S. purpuratus (Stimpson), have wide size distributions indicating broad age ranges. Most of the adult abalones and sea urchins occur clustered deep in crevices, either as a direct result of sea-otter predation or because ample drift of algal food reduces foraging activities. Abalones tend to occur in larger crevices than sea urchins, and competition for suitable crevice space may exist among these large, invertebrate, algal-drift feeders.