Crown-of-thorns starfish (COTS) sound like something out of a science-fiction movie. Their brightly coloured bodies have up to 21 arms, all covered in barbed spines, hence the name crown-of-thorns.
Growing up to 80cm in diameter, these starfish can be bigger than a dustbin lid.
How deadly are crown-of-thorns starfish?
You don’t want to get too close as COTS are the world’s most venomous starfish. In case their tangle of sharp spines aren’t enough to put off a would-be attacker, these sea stars reportedly exude a toxic slime.
“Their spines are covered in plancitoxins which can cause liver damage. When threatened they also release saponins – compounds which destroy red blood cells,” writes the Australian Institute of Marine Science (AIMS) on its website. “Together, these toxins can cause great pain to any animal (or human!) unfortunate enough to come into contact with it.”
Their venom can cause aches and pains, nausea, vomiting and sometimes paralysis.
When one woman fell onto a crown-of-thorns starfish, she ended up with “approximately 10 stab wounds of the left palm with pus, subcutaneous bleeding and many abrasions around them,” according to the study that documented the incident.
There is only one recorded human fatality from these large sea stars: the venom caused anaphylactic shock in the victim who died 13 hours after being stung.
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Why are crown-of-thorns starfish a problem?
But crown-of-thorns starfish are worrying for another important reason: they are devastating coral reefs. Conservation experts are particularly worried about outbreaks in the Great Barrier Reef
As juveniles, these animals eat algae. But adults have a voracious appetite for something else. “Adult crown-of-thorns starfish have an enormous appetite for eating hard coral,” explains the Australian Institute of Marine Science (AIMS) on its website. “An adult crown-of-thorns starfish can consume up to 10 m2 of coral a year.”
When COTS numbers get out of control, they can wipe out a reef in just a few weeks.
The way they eat the corals is pretty grim. “Like other starfish, they feed by pushing their stomach out through their mouth, covering their coral prey with digestive enzymes and converting coral tissue into a coral soup,” says AIMS. “When finished feeding they simply retract their stomach back into their body.”
Killing these starfish is difficult. If people try to manage outbreaks by chopping them in two, it can make the situation worse. These predators – somewhat horrifyingly – can regenerate from the stumps and grow back into two starfish.
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Instead, injecting them with vinegar or bile salts is much more effective.
Not all marine animals are put off by the COTS’ impressive self-defence measures. Their most famous predators are giant triton snails [MH1] which follow the sea star’s scent to locate their victim then pin it down with their strong foot and paralyse it with their saliva. Once it’s subdued, they can gobble down their meal.
“Researchers estimated that, without these [predatory] starfish devouring coral, there would have been a net increase in average coral cover,” says the Great Barrier Reef Foundation on its website.
At least 18 reef fish also eat COTS, according to a study in Scientific Reports that tested the poop and gut contents of different species to find which ones were eating the starfish.
Scientists are even finding ways to use these predators as a way of protecting reefs because COTS flee when giant tritons are around. “There is nothing more alarming to a COTS then the scent of a giant triton,” writes AIMS. “Indeed, the very presence of a giant triton in the vicinity of a COTS is enough to cause the starfish to rapidly move away.”
