Outbreaks of crown-of-thorns starfish Acanthaster planci have been a major issue on the Great Barrier Reef and other Indo-Pacific reefs for nearly 40 years. The outbreaks have generated great concern among the community and considerable debate among scientists.
Outbreaks generally occur at regular intervals with coral cover returning to pre-outbreak levels in the intervening years. In 2003, there is an outbreak on reefs between Cairns and the Whitsundays which is costing tourism operators, and the Queensland and Commonwealth Governments about $3 million a year for control measures.
Despite more than 30 years of research on this very complex problem, and the commitment of millions of dollars on research over the last 17 years, the ultimate causes of the outbreaks are still not clear. However, there is now a much greater understanding of the biology of the starfish, the nature of the outbreaks, and the pattern of recovery of the reefs.
Biology of crown-of-thorns starfish
The crown-of-thorns starfish is an unusually large starfish and can grow to more than 1 m in diameter. It has up to 21 arms, with the entire upper surface of its body covered in long venomous spines. The starfish can move up to 20m in an hour.
Crown-of-thorns starfish spawn from December to April when water temperature is about 28°C. The starfish release eggs and sperm into the water through pores on the top of their central disc. When the eggs are fertilised, they develop into larvae which spend from two to four weeks drifting as plankton in ocean currents. The juveniles settle (or recruit) onto the reef when they are about 1-2 mm across. They live among rocks and rubble on the reef and are almost invisible until they are about six months old.
A crown-of-thorns starfish first breeds when it is 2-3 years old and breeds for five to seven years. Each female can produce up to 60 million eggs during a single spawning season. The starfish tend to gather together to spawn which increases the chance of fertilising the eggs. Indeed, fertilisation rates in the field for crown-of-thorns starfish were the highest measured for any invertebrate. Therefore, a small population of crown-of-thorns starfish could potentially produce a very large number of offspring.
Feeding and growth
Young crown-of-thorns starfish eat encrusting (coralline) algae which are common among rocks and rubble on the reef. At about six months of age, they start to eat coral and begin to grow more rapidly. During the next two years, the starfish grow from about 1cm to about 25cm in diameter.
Crown-of-thorns starfish spend about half their time feeding. When there are few crown-of-thorns starfish, they are very cryptic and tend to hide in the reef and under corals during the day. Larger starfish (more than 40 cm) usually feed during the day while smaller starfish (less than 20 cm) usually feed at night. Crown-of-thorns starfish feed mainly on tabular coral species particularly Acropora spp. and may only eat a portion of the entire coral colony. As a result, the reef can recover quite rapidly from low levels of predation by crown-of-thorns starfish. Some reefs seem to support small populations of crown-of-thorns starfish for many years, with only a small reduction in coral cover. Scientists estimate that a healthy coral reef with about 40-50% coral cover can support about 20-30 crown-of-thorns starfish per hectare (10,000 square metres).
But when starfish are in large numbers, there is intense competition for food and most types of corals will be eaten, including species such as Porites spp. that are not usually eaten by the starfish. During a severe outbreak, there can be many crown-of-thorns starfish per square metre, even piling on top of each other. They can eat so much that they can kill most of the living coral in that part of the reef, reducing hard coral cover from the usual 25 - 40% of the reef surface to less than 1%. Such a reef can take 10 years or more to recover its coral cover. Results from fine-scale surveys indicate that coral cover of more than 10% is needed for juvenile starfish to survive and grow.