Found throughout the Indo-Pacific the crown of thorns starfish, Acanthaster planci is one of the largest sea stars in the world (up to 45 cm across). Unlike the typical starfish with five arms, the crown of thorns starfish is disc-shaped with multiple arms (up to 21) covered in poisonous spines. These unique features gave rise to this starfish's commonly referred to name of the crown of thorns.
Acanthaster planci is a corallivore, which means that it feeds on live coral tissue. After climbing on a coral colony, the crown of thorns starfish extrudes its stomach out through its mouth and spreads it over the surface of the corals. The stomach secretes digestive enzymes that liquefy the coral tissue which is then absorbed. After this process, the coral colony is left devoid of living tissue and only the white skeletons of the colony remain. On the skeletons, the starfish leave behind observable feeding scars which mark the paths of their destruction. Without the living tissue, the skeletons become more vulnerable and brittle, and are often colonized by non-reef building organism such as algae.
Known as voracious predators, a single crown of thorns starfish can consume approximately 6-10 square meters of living reef per year. Low densities of Acanthaster planci naturally occur on reefs with little impact to the ecosystem. However, under certain conditions periodic population explosions of these starfish can occur on reefs, generating alarm due to the extensive damage they can unleash to the corals and the reef ecosystem.