In addition to impacts from the surrounding ocean environment, sometimes threats to coral health can come from other living reef inhabitants. One of the greatest biological disturbances to coral reefs is an outbreak of crown-of-thorns sea stars. These sea stars are found throughout the Indo-Pacific region, from the Red Sea to the western coast of Panama. Reaching up to a half meter in diameter, a crown-of-thorns sea star is the largest tropical sea star and its favorite prey is corals.

Crown Of Thorns

Crown-of-thorns sea stars consume the coral tissue cover the surface of a colony, leaving behind the bare coral skeleton. In low densities, crown-of-thorns have little effect on coral reefs. However, when they aggregate and form large densities or outbreaks, crown-of-thorns can decimate the amount of live coral on the reef, changing reef structure, freeing space for algal growth, and affecting fish populations. Although smaller and less destructive, snails and other small gastropods also feed on coral reefs and can cause scars on coral tissue. Evidence of fish predation, from small blenny bites, to the trademark white scraps left behind on the coral’s surface from grazing parrotfish, are also commonly observed in Pacific reefs.

Crown-of-thorns sea stars feed on coral tissue, leaving only the skeleton of this Acropora coral at Alamagan Island (Photo: NOAA Fisheries/Keisha Bahr).