The crown-of-thorns starfish (Acanthaster planci) is a starfish with thorn-like spines sprouting all over its body for protection. These echinoderms grow to a diameter of up to 40 cm across and have 12 to 19 arms extending from their center. They are the second largest starfish in the world after the giant sunstar Pycnopodia helianthoides. Crown-of-thorns starfish are found on coral reefs in the tropics ranging from the Red Sea, throughout the Indian and Pacific Oceans, and all the way to the Pacific coast of Panama.
The starfish is a coral reef predator (a corallivore) and preys on the coral polyps by climbing onto them, extruding its stomach over them, and releasing digestive enzymes to then absorb the liquefied tissue. They feed alone at night, maintaining a constant distance between themselves and other crown-of-thorns starfish. During times of food shortage, these creatures can live on their energy reserves for over six months.
The crown of thorns can grow from the size of a grain of sand to the size of a dinner plate. The sharp thorns on the sides of the tentacles resemble thorns, making a crown shape, hence the name, Crown of Thorns. An exceptionally large crown of thorns can grow to be the size of a car tire. Divers kill these predators by injecting the starfish's own stomach acid into each of their many legs. If even one leg is missed the sea star can live on. Before overpopulation, crown of thorns kept the fast growing coral from overpowering the slower growing coral. It then proceeds to destroy the coral. It has become a dire threat to the Great Barrier Reef.
Outbreaks of huge numbers of these starfish are believed to be caused by agricultural runoff which causes algal blooms. The connection lies probably in the increased amount of algae supplying coral polyps (which eat the eggs of Acanthaster) with large amounts of food, thus reducing predation on the starfish's eggs. Since the eggs drift considerable distances in the plankton, the runoff would have to occur on the settling, not the spawning, grounds. This also explains the phenomenon of massive outbreaks seemingly appearing out of nowhere, with no previous indication of an increasing population at the affected site.
Acanthaster planci, the crown-of-thorns starfish, due to its notorious damage to reefs, has been described as one of the most influential species in the diverse biotic communities that make up tropical coral reefs (Birkeland, 1985). It is in fact a most important member of coral reef biodiversity, as it is the main force driving ecological succession where it occurs. Outbreaks, especially when occurring with increased frequency or in combination with other factors such as coral bleaching or Black band disease, may, however, cause permanent damage.