Crown of Thorns Starfish (Hungry Coral Predators)

Acanthaster planci is known as the Crown of Thorns Starfish. This sea star is an organism that has caused great concern all over the world. When the starfish come into a reef ecosystem in these plague proportions, they feed so heavily on corals that they can completely destroy a reef. Crown of thorns starfish outbreaks causes significant damage to coral reefs across large spatial scales. The damage from crown-of-thorns starfish outbreaks adds to the damage from other major causes of coral decline, tropical cyclones, and coral bleaching events.

Crown of thorns starfish usually eats the polyps of hard, relatively fast-growing stony corals, such as staghorn corals. If food is scarce, they will eat other coral species. Their feeding preferences and behavior patterns vary with population density, water motion, and species composition. They feed by extruding their stomach out of their bodies and onto the coral reef and then using enzymes to digest the coral polyps. This process can take several hours. After the coral polyps are digested, the sea star moves off, leaving only the white coral skeleton behind. An individual starfish can consume up to 6 square meters of living coral reefs per year.

The lack of predators of these crown-of-thorns starfish is due to overfishing. The most common crown-of-thorns starfish predators include the giant triton snail, the titan triggerfish, brilliant pufferfish, hump head Maori wrasse, yellow edge triggerfish, harlequin shrimp, lined worm. Small crabs living within the complex structures of branching corals may ward off the starfish as it seeks to spread its stomach over the coral surface. The crabs pinch the starfish’s tube feet or even its stomach lining. Through this symbiosis, the crabs protect the coral colony from potential predators, and in return, they receive a safe place to live and avoid their own predators.

When crown-of-thorns starfish populations are at healthy levels, they can be good for a reef. They can keep larger, fast-growing stony corals in check, allowing small corals to grow. They also can open space for more slower-growing corals to grow and increase diversity.

The crown of thorns starfish has a healthy enough population that there is no need to evaluate it for conservation. In fact, sometimes crown-of-thorns starfish populations can get so high, they devastate reefs. The starfish are emerging at night to feed. When the starfish are at high densities, they may move day and night, competing for living coral. One issue is runoff, which washes chemicals (for example, agricultural pesticides) from the land into the ocean. This pumps more nutrients into the water that causes a bloom in plankton, which in turn provides extra food for crown-of-thorns starfish larvae and causes the population to boom. Another cause may be overfishing, which has decreased the population of starfish predators. An example of this is the overcollection of giant triton shells, which are prized as souvenirs.

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