21. Do crown-of-thorns starfish prefer to feed on particular types of corals?

It would appear that crown-of-thorns starfish generally prefer to feed on staghorn and plate corals (the majority of these are of the genus Acropora) which are relatively fast growing. They seem to least prefer feeding on large, hemispherical (termed massive) corals (such as the coral Porites) (see Fig. 11 ) and certain types of branching colonies which contain small crabs and shrimps (termed commensals) that defend the coral from predators. Sometimes these are the only adult colonies that survive outbreaks of crown-of-thorns starfish. During large outbreaks even these corals may be killed.

Figure 11 A massive coral of the genus Porites, often not eaten by crown-of-thorns starfish.

Massive colonies grow at a considerably slower rate (about 1 cm in diameter per year) than most other types of corals. It is thought that they are often not preyed upon because they are smooth and slippery and the starfish are unable to hold onto the coral when feeding. Like all corals (which are related to jellyfish) massive corals have stinging cells (containing nematocysts) which prevent them from being preyed upon or overgrown and which are used to immobilise small animals when feeding. The stinging cells of corals normally don't appear to affect the crown-of-thorns starfish although those of some massive corals have been shown to produce an adverse reaction. This possibly explains why these types of corals are least preferred as a source of food by crown-of-thorns starfish.

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