Articulated coralline algae provide a spatial refuge to juvenile sea urchins from predatory crabs

Coralline algae provide settlement cues, food, and biotic structure for benthic marine invertebrates in intertidal and subtidal habitats. Here, we present evidence from laboratory experiments that juvenile sea urchins Strongylocentrotus droebachiensis (1.0–3.0 mm test diameter) are chemically attracted to articulated coralline algae Corallina vancouveriensis in the San Juan Archipelago (Washington, USA), resulting in movement of sea urchins into the algae. This behavior significantly reduced sea urchin mortality in the presence of predatory crabs Pagurus spp. and Cancer oregonensis as compared to treatments with no algae. In addition, we tested predation rates of various benthic predators from intertidal and subtidal habitats, and found that crabs were the most voracious predators of juvenile sea urchins. Our results indicate that C. vancouveriensis provides a spatial refuge to juvenile sea urchins from predatory crabs, and may facilitate recruitment into sea urchin populations by enhancing juvenile survival.