Crown-of-thorns starfish (COTS for short) feed on coral. These spiky and spineless marine creatures occur naturally on reefs in the Indo Pacific region, including the Great Barrier Reef.

In normal numbers on healthy coral reefs, COTS are an important part of the ecosystem. They tend to eat the faster growing corals which gives the slower growing species a chance to catch up, enhancing the coral diversity of our reefs.

However, when the coral-eating starfish appear in outbreak proportions, the impact on coral reefs can be disastrous.

A 2012 study by the Australian Institute of Marine Science revealed that crown-of-thorns starfish and tropical cyclones were the two leading causes of coral cover loss on the Great Barrier Reef over the previous 27 years.

Just the facts​ – Acanthaster planci (Scientific name)

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Think big

It’s the world’s second largest starfish, usually measuring between 25 cm and 35 cm though they can grown to 80 cm with reports of some even spanning more than one metre!

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Night moves

Nocturnal by nature, they can move at speeds of up to 20 km an hour.

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Eat it

COTS prey on nearly all corals, and can eat their way through 10 square metres of it a year.

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Inside out

They eat their coral prey by extruding their stomachs out from their bodies and covering the corals. Then they release enzymes and digest the coral polyps.

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Hunger games

An adult Crown of Thorns Starfish can live up to 9 months without eating!

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All arms

This starfish can have between 7 and 23 arms.

Sea Star


Like most starfish, if it loses one of its arms, a COTS can regrow a new one in around six months.

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Baby boom

Female COTS can produce as many as 65 million eggs in one spawning (breeding) season.

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Prey day

Giant triton snail, titan trigger fish, starry pufferfish, humphead Maori wrasse, yellow margin trigger fish, harlequin shrimp and lined worm are natural predators of the adult COTS while parrot fish, spangled emperor and red emperor, annelid worms, crabs and shrimps like to feed on the juvenile COTS before they grow their spikes.

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Toxic touch

The COTS spikes or spines contain toxins that are poisonous to both humans and marine creatures. They can pierce through a wet suit and give a nasty puncture wound laced with venom which can cause vomiting, nausea and severe pain.

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If 30 Crown of Thorns Starfish are found in a one hectare area, it’s called an outbreak. (A normally occurring population is less than one per hectare.) In the 1970s, the northern Great Barrier Reef saw an outbreak of the scale of 1,000 COTS per hectare.


Just cause

Factors that influence outbreaks of COTS include excess nutrients from run-off in the ocean and overfishing or removal of the natural predators of COTS.