A starfish that doesn't only destroy coral: a University of Sydney study shows that at low numbers the crown-of-thorns can support reef diversity

What the scientists found

On 1 November 2026 the popular science portal ScienceDaily reported on a University of Sydney study published in the journal Ecology and Evolution. The scientists studied crown-of-thorns starfish on the Great Barrier Reef, in protected areas where starfish numbers are low.

The work was led by University of Sydney researcher Shawna Foo. Observations were made at the One Tree Island research station. It turned out that in almost 75% of observed feeding events, living coral tissue survived.

What corals are

Corals aren't plants or stones but colonies of tiny animals called polyps, relatives of jellyfish and sea anemones. In the tissues of most reef-building corals live microscopic algae, which use sunlight to make nutrients and share them with the polyps. The polyps build a limestone skeleton, and over thousands of years these skeletons grow into reefs that are home to thousands of species of fish and invertebrates.

How the observations were made

The scientists worked in protected areas of the reef where crown-of-thorns numbers are low. They watched how the starfish feed, marked damaged parts of colonies and then tracked what happened to them: whether coral tissue survived at the edges, whether the spot grew over and which corals appeared there. These long-term observations made it possible to see the starfish's role not during a disaster but in the everyday life of the reef.

How the crown-of-thorns feeds

The crown-of-thorns is a large starfish covered in sharp spines. It feeds on corals: it climbs onto a colony, pushes its stomach out and digests the living tissue, leaving a white skeleton behind. During outbreaks, when there are very many starfish, they can destroy huge areas of reef.

Crown-of-thorns starfish appear to consume a relatively consistent amount of coral during each meal and move on.

Shawna Foo, researcher at the University of Sydney

The freed-up space

The most interesting thing the scientists saw was on the damaged patches. Fast-growing branching Acropora corals didn't regrow over the eaten areas. Instead, slow-growing corals of the genera Montipora and Porites grew there. According to Foo, at low density starfish "create space for slower-growing coral species to persist".

The intermediate disturbance theory

Co-author Professor Maria Byrne notes that the results are "consistent with the intermediate disturbance hypothesis". This holds that species diversity is greatest not where nothing happens or where everything is constantly destroyed, but at moderate levels of disturbance. Small amounts of damage stop the fastest and strongest species from taking over all the space and leave room for others.

Outbreaks remain a threat

The authors stress that their findings apply only to low starfish numbers. Outbreaks, when there are hundreds of times more starfish, are still considered a serious threat to the Great Barrier Reef. The reef has had four major outbreaks since the 1960s, roughly every 15 years. Along with ocean warming, coral bleaching and storms, they do great damage to the reef.

How to recognise an outbreak

On a healthy reef there are few crown-of-thorns starfish, and seeing one is a matter of luck. During an outbreak, hundreds and thousands can be counted on one reef, and white eaten areas of coral stretch for long distances. For scientists and reef protection services, it's important to spot the start of an outbreak, while starfish can still be removed from key sites.

Why outbreaks happen

The causes of crown-of-thorns outbreaks aren't fully understood. Possible explanations include fertilisers and other substances washed into the sea by rivers, which boost the growth of plankton that starfish larvae feed on, as well as the fishing of fish and other animals that eat starfish. So in Australia, outbreaks are tackled by killing starfish by hand at key reef sites and working on water quality.

What eats the crown-of-thorns

The crown-of-thorns has natural enemies. The best known is the giant triton, a large sea snail that hunts starfish. Some fish also eat starfish and their larvae. When these animals decline, for example through fishing, it's harder to keep starfish numbers in check, which is one reason why protecting the whole reef ecosystem is so important.

How Australia tackles outbreaks

During outbreaks, special teams of divers in Australia kill starfish by hand at important reef sites, giving them injections after which they die without breeding. The method is labour-intensive but makes it possible to protect the most valuable sites, such as those popular with tourists. Scientists are also looking for ways to predict outbreaks earlier, so that work can start in advance.

Bleaching is the main threat

Today scientists see ocean warming as the main threat to corals. When the water is too warm, corals expel the algae living in them and turn white, which is known as bleaching. If the water cools quickly, corals can recover, but in prolonged heat they die. The fewer additional pressures a reef faces, including crown-of-thorns outbreaks, the better the corals' chances of surviving hot years.

What this changes

The study doesn't mean the crown-of-thorns is harmless. It shows that an animal's role in an ecosystem depends on its numbers. At natural, low numbers, the starfish is part of the reef's complex system and may even support its diversity. This matters for conservation planning: it may be best to focus on outbreaks rather than every single starfish.

Why coral diversity matters

A reef with a variety of coral species is more resilient to change. Fast-growing Acropora recover well from damage but are especially sensitive to heat and bleaching. Slow-growing massive corals are hardier. If different species survive on a reef, it's more likely to get through hard times.

The crown-of-thorns is dangerous to people too

The spines of the crown-of-thorns are venomous, and a sting causes severe pain, swelling and sometimes nausea. Divers and snorkellers shouldn't touch these starfish with their hands. After a sting, carefully remove any fragments of spine, and see a doctor if there's severe pain or signs of allergy.

Why reefs matter to people

Coral reefs cover a tiny part of the ocean but are home to a huge share of marine species. They protect coasts from waves and storms, feed millions of people and attract tourists. For Australia, the Great Barrier Reef is one of its greatest natural treasures and an important part of the economy. So any study that helps us understand the reef better has practical value.

For tourists on the reef

If you're heading to a coral reef, don't stand on corals or touch them, as this damages living tissue. Use reef-safe sunscreens, don't feed the fish and don't take corals or shells home. Choose tour operators who follow reef protection rules. This will help preserve reefs for future generations.

What this means for marine aquarium owners

Crown-of-thorns starfish aren't kept in home aquariums, but reef aquarium owners know how hard it is to keep the balance between coral species. Fast-growing corals can shade out and crowd out slow-growing ones, and fishkeepers regularly trim and move colonies. The study on the reef shows that moderate changes can support diversity in nature too.

How to choose corals responsibly

1. Buy captive-grown corals: aquacultured frags rather than ones collected from reefs.
2. Check the paperwork: many corals are protected under the CITES convention.
3. Don't bring corals home from your travels: it's banned and harms reefs.
4. Research the species' needs: light, water flow, temperature and water quality.
5. Quarantine new corals to avoid bringing pests into your aquarium.

Pests in a reef aquarium

Home aquariums have their own "little crown-of-thorns": small starfish, snails and worms that feed on corals. They often get in with new colonies. So experienced fishkeepers inspect and treat new corals before putting them in the main tank, and watch for signs of damage.

What this means for Europe

Europe doesn't have coral reefs like Australia's, but Europeans are among the biggest buyers of marine aquarium animals and among the tourists visiting the world's reefs. Studies like this one from Sydney help us understand how reefs work and how to preserve them. For marine aquarium owners in Europe, the lesson is practical: choose aquacultured corals, follow CITES rules and remember that the balance of species matters in nature and at home.

When to see a vet

Fish and invertebrates in marine aquariums are treated by vets who work with aquatic animals and by experienced fishkeeping specialists. Contact them if corals lose their colour on a large scale, tissue pulls away from the skeleton, or fish become lethargic, breathe heavily or develop spots. Check your water parameters first, as many problems are linked to them.