Substratum-dependent predator-prey dynamics: patch reefs as refuges from gastropod predation

The impact of the gastropod predator Cassis tuberosa (Linnaeus ) on the local distribution of the sea urchin Diadema antiOanml Philippi was investigated. Artificial reefs stocked with Diadema were established 5 m otTthe fringing reef of St John, US Virgin Islands. Cassis preyed upon 17% of the urchins after 30 days. All predation by Cassis occurred on sand rather than on artificial reefs or a nearby fringing reef. Arenas containing a single Diadema and Cassis were set up with sand, flat rock and piled rock habitats. This resulted in urchin mortality of70, 40 and 10% for the three treatment groups, respectively. The results suggest that Diadema distribution and foraging may be restricted to reef habitats by predation pressure. The risks of mortality from predation may outweigh the costs of remaining in high-density food-timited conditions.