Refugia and top-down control of the pencil urchin Eucidaris galapagensis in the Galápagos Marine Reserve

Although sea urchins can strongly influence the structure of benthic communities and are abundant in the Galápagos Islands, factors mediating predation on urchins have not been studied experimentally. Here, we examine how habitat structure and behavioral patterns of prey influence predation on the pencil urchin Eucidaris galapagensis, an abundant grazer in rocky subtidal habitats. Results indicate that the distribution, abundance and body sizes of E. galapagensis vary predictably by habitat in the central Galápagos. Urchins were five times more abundant and significantly smaller in rubble than in exposed ledge habitats. We thus hypothesized that rubble habitats provide a refuge from predation, and conducted tethering manipulations using small and large urchins as prey. Predation by the hogfish, Bodianus diplotaenia, triggerfishes, and the sea star Pentaceraster cumingi, was significantly higher in exposed than in rubble habitats for small urchins, indicating that rubble habitats represent a refuge. In addition, urchin activity over a 24-hour period indicated that E. galapagensis were significantly more abundant on exposed substrate at night than during the day as they emerged from refugia at dusk. Since the fish that prey on E. galapagensis are predominantly diurnal, we suggest that the nocturnal activity patterns of the urchins represent a predator avoidance strategy. These results underscore the importance of considering spatial refugia and prey behavior in investigations of top-down control of sea urchins in the Galápagos Marine Reserve.