Crown of thorns is a starfish that eats the flesh of coral. The starfish has from 8 to 21 arms arranged around a central disk. The animal’s mouth is on its underside. The body is covered by large venomous (poisonous) spines. These spines protect the starfish from being eaten by most animals. The color of the body may range from red to orange to purple. Adults typically measure about 10 to 14 inches (25 to 35 centimeters) across, including the arms.
Swarms of crown of thorns starfish have attacked many coral reefs in the Indian and Pacific oceans. For example, many areas of the Great Barrier Reef off northern Australia have been damaged by crown of thorns starfish. Scientists do not know why crown of thorns have become abundant in some areas. Some researchers think overfishing of the few animals that do feed on the starfish may be the main cause. Others think that water pollution mainly from agricultural runoff may provide food for young crown of thorns, enabling more of them to survive. People have attempted to control the numbers of these starfish in some areas. Divers can kill crown of thorns by injecting them with a poison. However, these control efforts have had little success, partly because each female crown of thorns can produce millions of eggs.
