COTS Q&A

Crown-of-thorns starfish, or COTS, occur naturally on coral reefs throughout the Indo-Pacific region. But with their enormous appetite for coral, they are responsible for over 40 per cent of coral loss on the Great Barrier eef.

COTS are responsible for 40% of coral loss on the Great Barrier Reef

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What are COTS?

Growing up to one metre in size, COTS are the world’s largest starfish. They have up to 21 arms, hundreds of toxin-tipped thorns and an enormous appetite for coral. An adult can eat up to 10 square metres of hard coral in a year.

Why are they a problem?

In small numbers, COTS can live in harmony with the reef. In fact, some might argue they play a role in maintaining biological diversity by eating faster-growing corals and allowing space for slower growing corals to become established. But when their numbers increase it leads to widespread and unsustainable coral loss.

When does this become an outbreak?

When 15 or more COTS are reported within an area of one hectare it is deemed an outbreak.
What causes outbreaks?

After several decades of research, scientists agree that there are several contributing factors. Two of these are over-fishing (it reduces the number of natural predators) and increased nutrient levels in the water from farming and from sewage runoff (causing plankton blooms that juvenile COTS feed on).

What are their natural predators?

Giant triton snails, titan trigger fish, starry pufferfish, humphead maori wrasse, yellow margin trigger fish, harlequin shrimp and lined worms are just some of their natural predators. COTS are prolific reproducers (adult females can release over 200 million eggs a year) so scientists from the University of Queensland are researching natural predators of juvenile COTS. After testing more than 100 species of crabs, shrimps, worms, snails and fishes, the standout winner is the red decorator crab.

What are we doing to control COTS?

COTS control programs are labour intensive. They rely on hundreds of divers from tourism operations and Indigenous Ranger groups to inject COTS with bile salts. The salts kill these starfish within 24 hours. Given the expansive size of the Great Barrier Reef, programs rely on solid survey information. A new research method using eDNA may enable scientists to detect COTS before they become a major problem.

Where are the current outbreaks?

There are no known outbreaks in the Wet Tropics region of the reef currently, thanks to lots of hard work!

How many COTS have been culled?

Over the past 10 years, nearly 25,000 COTS have been culled at reefs in the Wet Tropics region alone through the Great Barrier Reef Marine Park Authority’s Starfish Control Program. In 2021, high numbers of COTS were reported at Fitzroy Island and the Frankland Islands Group, resulting in 2,958 COTS being removed from Fitzroy Island and 6,831 from the Frankland Islands Group. Last year, the numbers had declined dramatically, with only 122 and 498 COTS culled.

How can you help?

Report sightings to the Great Barrier Reef Marine Park Authority via the Eye on the Reef app.