A notoriously ravenous predator of coral polyps, the crown of thorns starfish feeds primarily on scleratinian, or reef-building, corals, particularly fast-growing Acropora species. During severe outbreaks when competition for food is scarce, it will eat almost any coral species present on the reef (2) (5) (6) (7). Young crown of thorns starfish typically feed on encrusting algae, which are common among rocks and rubble on the reef (2).
The crown of thorns starfish is generally thought to be a nocturnal species, although some larger individuals do feed during the day (2) (8). It feeds by a method known as ‘eversion’, where the gastric folds of the stomach membrane are forced through its mouth and turned inside out, smothering the coral. It then secretes digestive enzymes onto the coral, absorbing the digested tissues of its prey externally (4) (5) (8) (10). Although the crown of thorns starfish will generally feed alone, during outbreaks many individuals will aggregate in large groups (7) (10).
Breeding typically occurs between December and April. The crown of thorns starfish reproduces sexually, releasing eggs or sperm into the water through pores on top of the central disk. Often, groups of the crown of thorns starfish on the same reef may spawn at the same time. A single female can produce up to 60 million eggs in a breeding season. The crown of thorns starfish has one of the highest rates of fertilisation recorded in any invertebrate, meaning that even small populations have the potential to produce a large number of offspring (2) (3) (7).
The crown of thorns starfish larvae typically spend between two and four weeks drifting in ocean currents (2) (5) (12). The larvae have small hairs, called cilia, which propel them through the water and produce feeding currents that trap the plankton on which they feed (7) (11). The larvae then settle on shallow reefs, eventually becoming five-armed juvenile starfish which feed on coralline algae. After between four and six months the juveniles switch their diet and begin to feed on corals (5) (7) (12). The crown of thorns starfish becomes sexually mature after two years (7).
Some species of carnivorous fish, such as the humphead wrasse (Cheilinus undulatus) and the starry puffer fish (Arothron stellatus), are known to prey on adult crown of thorns starfish, although in general it has very few predators once it has reached maturity (3) (13). Juvenile crown of thorns starfish are much more vulnerable to predation as they lack the characteristic spines of the adult, being preyed upon by crabs, shrimps, annelid worms and fish (3).
Several species of cardinalfish and damselfish are known to shelter within the venomous spines of the crown of thorns starfish (14).
At low densities, the crown of thorns starfish is a ‘normal’ part of coral reef ecosystems in the Indo-Pacific region. However, when the numbers of crown of thorns starfish dramatically increase it results in a major disturbance to the whole system, as the starfish consume coral faster than it can grow thereby drastically reducing coral cover (3). The large expanses of dead coral are subsequently colonised by algae, with a knock-on effect on other reef organisms that rely on coral colonies for food, shelter and nesting (3) (5).
The reasons behind outbreaks of the crown of thorns starfish are poorly understood, despite considerable research being carried out (3) (6). Although evidence suggests that outbreaks have occurred relatively regularly for thousands of years, the frequency and severity of these outbreaks appears to be increasing in recent decades (5).
Human activities are thought to be likely to be exacerbating natural cycles in the crown of thorns starfish populations. Overfishing, which is a major problem on many unprotected reefs, may have removed predators of the crown of thorns starfish, such as the humphead wrasse. Similarly, another natural predator of the crown of thorns starfish, the triton snail (Charonia tritonis), is threatened in many areas by collection for its highly prized shell. Agricultural runoff may also contribute to the increasingly frequent and persistent crown of thorns starfish outbreaks, as nutrients washed into the oceans can cause phytoplankton ‘blooms’, providing a more abundant food resource for the starfish larvae (3) (5) (6) (15).