Coral reefs are living organisms made up of tiny creatures known as polyps. The reefs have built up over the centuries by the accumulation of limestone secretions from the polyps, which are rather like sea anemones and live in partnership with microscopic plants. But by 1980, the coral reefs of the Pacific Ocean had come under serious threat. They were being steadily eaten by crown-of-thorns starfish in numbers reaching plague proportions.
This starfish grows up to 2ft (610 mm) across and can have up to 21 arms, each covered with the poison-tipped spines that give the creature its name. It eats by extruding its stomach through its mouth and enveloping the coral. Strong digestive juices then dissolve the soft-bodied polyps.
In 1969, in just two weeks, 13,847 starfish were collected from a reef off Western Samoa. And in 1980, at the height of the plague, the coral round just one island of the Great Barrier Reef, off Australia's north-east coast, had been invaded by 1½ million starfish. The effect was devastating -a 24 mile (39km) stretch of reef was almost totally destroyed in two and a half years.
The reason for such a dramatic explosion in the starfish numbers remains uncertain. One suggestion was that the predators of the starfish — such as tritons (marine snails) and large fish — had been killed off by water pollution or excessive fishing. Another suggestion was that nutrients washed into the seas during unusually wet years foster a large amount of plankton — microscopic plants and animals. This allowed more starfish larvae, which graze on minute plants, to survive than usual. Fossil records show that a boom in starfish numbers can occur at irregular intervals, possibly with periods as long as 1000 years between.
Although protected by its spines, the crown-of-thorns starfish is eaten by the triton, a giant snail with a shell up to 16 in (400 mm) high. In one study, 15 tritons ate 125 starfish in three months. Larger starfish are sometimes only partly eaten, and may regrow the lost parts. So a decline in predators alone may not account for any dramatic increase in starfish numbers.
By 1992 the plague of crown-of-thorns starfish seemed to have subsided. The decrease in numbers on the Great Barrier Reef may have been caused by a parasite that destroys the starfish from the inside. Scientists are not sure.
