What is a Crown-Of-Thorns Starfish?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 12 October 2019
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The crown-of-thorns starfish is a large starfish native to tropical and subtropical waters, where it prefers to live on and around coral reefs. These starfish have a remarkable appearance, thanks to their multitude of spines, and they have greatly increased since the 1970s. The jump in the population of this starfish is attributable to several factors, including a decline of natural predators and an increase of nutrient-rich runoff near the coasts.

This starfish is formally known as Acanthaster planci, and it is the second-largest starfish in the world. A crown-of-thorns starfish can grow to be the size of a car tire, with 12-19 arms radiating from its center, and the starfish sometimes join forces to become especially formidable. The distinctive spines attached to this starfish contain a neurotoxin which can cause numbness, swelling, and infection if the spines are not completely removed, making the starfish very irksome to divers.

However, the spines are only the tip of the iceberg. The preferred diet of this starfish is coral, and thanks to the growing population, these starfish are seriously damaging already vulnerable coral reefs around the world. Many biologists are concerned that the proliferation of crown-of-thorns starfish could lead to widespread and irreparable damage, and in some regions, reefs have been almost totally destroyed by these voracious sea creatures.


Controlling the population of this starfish is very difficult. These creatures tend to do well in nitrogen-rich runoff such as that carried down many rivers in the industrialized world. This runoff also feeds algae blooms, which tend to choke out other marine species, creating a dead zone in which crown-of-thorns starfish can thrive. In addition, the natural predators of this starfish are on the decline, thanks to extensive hunting and habitat damage.

In some regions, divers have taken the problem into their own hands, running monitoring teams and attempting to control the population by killing off the starfish periodically. These groups have also promoted protections for the predators of the starfish, and they offer education to members of the public and scientific organizations who are interested in the problem.

If you happen to be diving in the tropics and you notice a particularly large, spiny starfish, be careful. You should avoid contact with the spines, and if you do accidentally step on or touch a crown-of-thorns starfish, surface immediately to get medical attention.


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Post 5

@Mor - The worst part is that the more the crown of thorns destroy the reefs, the more they are destroying the habitat of the fish that would usually be eating them and keeping their numbers down. So it becomes a feedback loop. More starfish, less coral, less predators, even more starfish.

It's especially bad since corals are already under huge amounts of stress from pollution and climate change. They can't survive in oceans above or below a certain temperature, and they take so long to grow it's not like they can just shift their habitat like birds seem to be doing.

I think the people who go out hunting crown of thorns starfish are doing us all a favor, and should be commended.

Post 4

@anon152881 - It's just specifically this kind of starfish (or sea star, which they are more often called these days) which is a problem. Unfortunately, crown of thorns sea stars munch on corals and they can eat a huge amount.

They chew up the hard bits to get at the soft, living polyps and then spit out the coral as gravel afterwards.

This was not harmful to the ecosystem when they were held in check by predators, but since the predators are being overfished, the crown of thorns has grown in numbers.

Corals take many years to grow, so if they are eaten this quickly they can potentially become more and more endangered.

This is bad news for the fish that need them to survive, as well as humans who want to be able to explore beautiful corals reefs into the future.

Post 1

hey guys do you know what the problem is if there are too many starfish?

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