Crown Of Thorns Starfish, Acanthaster planci

The crown-of-thorns starfish (Acanthaster planci) is a species of starfish that is classified within the Acanthasteridae family. This species has a large range that extends from the Red Sea to the African coasts in the east and from the Indian Ocean to the western coasts of Central America. It prefers a habitat in coral reefs, which can be harmed if population numbers are too high. Damage occurs when filamentous algae covers bare skeletons of coral and the starfish move in to feed, stripping the coral of its layers. In order to control the populations of this starfish, many practices have been applied including poisoning, which kills high numbers at a quick rate, and manual removal, which is a slow method and not preferred.

The crown-of-thorns starfish can reach an average diameter between 9.8 and 14 inches and can have up to 21 arms. Its coloring is typically dull, ranging between greyish green to light brown, but brighter colors can be common in some areas of its range. Although this species is similar to other starfish, it is unique in many ways including holding a larger stomach to body ratio and having more flexibility in movement.

This movement is attained by the two rows of tube feet under the starfish’s arms and the amount of arms that it can grow. The spines that occur along the upper body and arms resemble thorns, which are typically long and very sharp, varying drastically from the shorter oral spines that extend around its mouth. If the starfish is taken from the water, its body ruptures and loses its fluids, making the sea star flat and stretched out. Its spines also flatten out, but the starfish may return to normal if placed in the water quickly enough.

Adult crown-of-thorns starfish consumes reef coral polyps by climbing on coral reefs, easily sticking to the coral’s rough edges using its many tube feet.  Once a prey item has been found, the starfish will turn its stomach inside out, engulfing its prey and releasing a digestive enzyme that breaks the polyp down. Once the polyp is removed, a blank space of coral skeleton appears that is quickly filled with filamentous algae. One individual is known to consume up to sixty-five square feet of coral reef per year. Studies have shown that coral damage increases in warmer areas, where the starfish have higher metabolic rates. This species prefers hard corals like flat corals and branching corals in the Acropora genus.

Like other starfish species, the crown-of-thorns starfish breeds by releasing gametes into the water column. After eggs are fertilized, they float in the water and undergo many developmental changes. Once the larvae are large enough, they float to the sea floor and develop onto young starfish. Because of the varied range of this species, its breeding season and lifecycle stages can vary. There many species of sea creature that hunt this starfish including the triton, a mollusk, and a species of the Pseudocorynactis genus, which is said to consume starfish whole. The crown-of-thorns starfish can ward off these predators with its spines, which serve as a mechanical and chemical defense. These spines release saponins, chemicals that irritate the tissues of both humans and predators and have a foul taste.

Image Caption: Crown-of-Thorns starfish near Qamea Island in Fiji. Credit: Matt Wright/Wikipedia (CC BY 2.5)