Long-Spined Crown-Of-Thorns

The body form of the crown-of-thorns starfish is fundamentally the same as that of a typical starfish, with a central disc and radiating arms. Its special traits, however, include being disc-shaped, multiple-armed, flexible, prehensile, and heavily spined, and having a large ratio of stomach surface to body mass. Its prehensile ability arises from the two rows of numerous tube feet that extend to the tip of each arm. In being multiple-armed, it has lost the five-fold symmetry (pentamerism) typical of starfish, although it begins its lifecycle with this symmetry. The animal has true image-forming vision.

The adult crown-of-thorns is a corallivorous predator that usually preys on reef scleractinian coral polyps, as well as encrusting sessile invertebrates and dead animals. It climbs onto a section of living coral colony using the large number of tube feet, which lie in distinct ambulacral grooves on the oral surface. It fits closely to the surface of the coral, even the complex surfaces of branching corals. It then extrudes its stomach out through its mouth over the surface to virtually its own diameter. The stomach surface secretes digestive enzymes that allow the starfish to absorb nutrients from the liquefied coral tissue. This leaves a white scar of coral skeleton that is rapidly infested with filamentous algae. An individual starfish can consume up to 6 square metres (65 sq ft) of living coral reef per year.

A. planci is one of the most efficient predators on scleractinias (stony corals or hard corals). Most coral-feeding organisms only cause tissue loss or localized injuries, but adults of A. planci can kill entire coral colonies.

In 1960-1965, marine scientist Robert Endean reported high densities of A. planci on the Great Barrier Reef.. This, along with 1970s book Requiem for the Reef, suggesting that there was a cover-up of the extent of damage, prompted the alarming idea that the coral and with it whole reefs were being destroyed by the starfish. In fact, as described above, the starfish preys on coral by digesting the surface of living tissue from the coral skeletons. These skeletons persist, together with the mass of coralline algae that is essential for reef integrity. The initial change (first-order effect) is loss of the veneer of living coral tissue.