The crown-of-thorns starfish is a native coral predator on the Great Barrier Reef. Its many arms and venomous spines make it recognisable. At high densities, it can consume live coral faster than the reef replaces it. Managers survey priority reefs and use trained dive teams to suppress outbreaks. Visitors can help by photographing and reporting sightings from a safe distance.
Identification and feeding
The Pacific crown-of-thorns starfish found on the Reef is referred to as Acanthaster cf. solaris. Adults can have up to 21 arms and grow as large as about 80 cm across. They can shelter under ledges or among corals, so a diver may spot a pale patch on a coral before seeing the animal responsible. A recent feeding scar can expose white skeleton where the starfish removed tissue. Bleached coral can still have living tissue over its skeleton. A pale patch alone gives an observer limited evidence of its cause. AIMS crown-of-thorns starfish profile
An adult pushes its stomach out over a patch of coral and digests the tissue externally. It commonly targets fast-growing branching and plate corals, including Acropora. AIMS estimates a large adult can eat up to 10 square metres of coral in a year. At ordinary densities this predation is part of reef ecology. During an outbreak, concentrated feeding can remove large areas of living coral, including colonies that survived a recent heatwave. AIMS starfish biology and feeding · AIMS outbreak research
Why outbreaks occur
The life cycle helps explain how numbers can rise quickly. Adults release eggs and sperm into the water during summer. The larvae spend weeks in the plankton, where they feed on microscopic algae and can drift between reefs. Newly settled starfish are small and concealed in rubble. Juveniles initially feed largely on algae before switching to coral. A favourable period for larval survival can therefore precede a visible outbreak by years. AIMS life cycle research
| Factor | How it may influence an outbreak |
|---|---|
| Food for larvae | Nutrients can stimulate phytoplankton growth. More food at the right time may improve larval survival. Catchment runoff and upwelling from deeper water can both contribute nutrients. |
| Predation | The giant triton can eat adult starfish. Researchers also study fish and invertebrates that eat juveniles, when the starfish are hard to observe. The importance of different predators in regulating outbreaks remains under study. |
| Reef connectivity | Currents can carry larvae from reefs with many spawning adults to other reefs. Local conditions affect whether those larvae settle and survive. |
Outbreaks have occurred naturally. Their frequency, intensity and spread may reflect several interacting causes. In 2026, AIMS reported detecting larval DNA in samples collected as deep as 30 metres, where upwelling may help supply food-rich water. That finding adds evidence for a possible mechanism. It leaves open how much runoff, upwelling or predation contributes at any particular reef. AIMS causes of outbreaks · AIMS 2026 field research
Why small starfish are hard to count. In a laboratory experiment, juveniles fed on crustose coralline algae for 6½ years before switching to coral when researchers offered it. They stayed roughly 16–18 mm across while eating algae and grew rapidly on coral. The experiment shows that small size is a poor guide to age and that some juveniles can wait out a period of low coral availability. How long they typically remain hidden on a natural reef is still uncertain. Deaker and colleagues’ juvenile feeding experiment
Monitoring and control
The Reef Authority uses broad surveys to identify possible outbreaks. A manta tow lets an observer scan large stretches of reef from behind a small boat, recording coral cover, visible starfish and feeding scars. Dive teams then search more closely, including under ledges. Researchers are testing environmental DNA as another way to detect larvae or hidden animals. Each technique measures something different, from signs of feeding to the presence of genetic material. Reef Authority control programme · AIMS detection research
Trained teams use targeted injections to reduce numbers on priority reefs and return to see whether coral has sustained protection. Reef selection considers coral value, outbreak risk and how currents link reefs. The programme reported surveying or treating 220 reefs in 2025–26 and removing 64,879 starfish. Those figures describe programme activity. Managers also track whether starfish density stays low and coral survives. AIMS’s annual monitoring identified signs of a fifth recorded outbreak wave emerging in the north. Reef Authority programme dashboard · AIMS 2025–26 report
What the evidence says about culling. A 2024 evaluation compared two outbreak waves using 20 years of observations from 207 reefs. In some regions, timely and sustained control was associated with starfish densities as much as six times lower than in the earlier wave. The study also found coral-cover benefits in areas that received enough control effort. These comparisons help managers decide when to send crews back and assess whether the programme protects coral beyond individual dive sites. AIMS evaluation of the control programme
What to do if you see one
Keep your hands, fins and equipment clear. If you can do so safely, take a photo, note the reef or GPS location and tell your tour crew. Submit the observation through Eye on the Reef, including date, location, approximate number and depth if known. Leave removal to trained, authorised teams. If a spine punctures your skin, get out of the water safely, notify your crew and seek urgent medical care. Eye on the Reef sightings · Reef Authority control permits · AIMS starfish profile · St John marine sting guidance