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What is a Crown-Of-Thorns Starfish?

Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon

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.

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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.

Frequently Asked Questions

What is a Crown-of-Thorns Starfish?

The Crown-of-Thorns Starfish (COTS) is a large, multi-armed starfish that preys on coral. Named for its venomous, thorn-like spines, it is one of the largest starfish species, growing up to 80 centimeters in diameter. This marine invertebrate plays a controversial role in its ecosystem, as outbreaks can devastate coral reefs, which are already under threat from climate change and pollution.

Where can you find Crown-of-Thorns Starfish?

Crown-of-Thorns Starfish are primarily found in the Indo-Pacific region, including the Red Sea, the Indian Ocean, and the Pacific Ocean up to the coast of Central America. They inhabit coral reefs, where they consume the coral polyps, and are often seen in shallow waters, although they can live at depths of over 65 meters.

Why are Crown-of-Thorns Starfish considered a threat to coral reefs?

Crown-of-Thorns Starfish are considered a significant threat to coral reefs due to their capacity to consume large amounts of coral tissue. During population outbreaks, which may be exacerbated by human activities, these starfish can strip a reef of its live coral cover at an alarming rate, potentially leading to reef collapse and loss of biodiversity.

How do Crown-of-Thorns Starfish reproduce?

Crown-of-Thorns Starfish have a prolific reproductive strategy. They are broadcast spawners, releasing millions of eggs and sperm into the water column where fertilization occurs. A single female can produce up to 60 million eggs in a breeding season, contributing to rapid population growth under favorable conditions.

What is being done to control Crown-of-Thorns Starfish populations?

Efforts to control Crown-of-Thorns Starfish populations include manual removal and injection with lethal substances like bile salts or vinegar. Additionally, research into biological control methods, such as introducing natural predators or pathogens, is ongoing. Management strategies also focus on reducing land-based pollutants that contribute to outbreaks by promoting coral growth and resilience.

Can Crown-of-Thorns Starfish have any positive effects on coral reefs?

In the absence of outbreaks, Crown-of-Thorns Starfish can play a positive role in maintaining coral reef diversity by preying on the fastest-growing corals, which allows slower-growing species a chance to flourish. This predation can help prevent any single species from dominating the coral community, thus maintaining a healthy and diverse ecosystem.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...

Discussion Comments


@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.


@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.


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

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