Can portunid crabs protect massive coral against the attack by long-spined sea urchins?

Sea urchin grazing can damage corals, but little is known how such damage can be remediated. We conducted three caging experiments in the field to determine the relationships among the portunid crab Thalamita prymna, the long-spined sea urchin Diadema setosum and the massive coral Platygyra carnosa. Experiment 1 showed that urchins caused coral tissue loss and reduced coral weight, and such effects were stronger at higher urchin densities. Experiment 2 showed that crabs could be effective urchin predators. Experiment 3 revealed that, when a rock with fouling organisms (an alternative food source) was available, some urchins managed to survive, but they did not cause appreciable coral damage. Together, our results indicate that, crab predation, an overlooked relationship in coral reefs, can be exploited to help control urchin corallivory and bioerosion. Prohibiting fish trapping in reef areas could reduce the by-catch of these crabs and protect reefs against urchin attack.