Crown-of-Thorns Are Not All Bad


If you have ever seen a crown-of-thorns sea star on a dive, it probably left an impression. The multi-legged invertebrate is covered in large spikes, something straight out of the nightmares of sailors. Through our anthropomorphic eyes these things look evil; a spiky, slinky, creepy-crawly of the deep.  You may have also seen your dive guide attempt to do battle with this crazy creature, trying to kill it or send it to the deepest depths they can find. If you’ve witnessed such behavior, it hopefully made you think—after all the environmental pre-dive briefings about not touching or hurting sea life in any way, why are the guides behaving like this?

The reason these armored foes are targeted is because of their diet. Crown-of-thorns sea stars, known as COTs, eat coral. Like all sea stars, they eat by extending their digestive organs outside of their bodies and onto their prey. The digestive process breaks down the coral tissues so that the sea star can absorb them. Once they devour the coral beneath them, they retract their organs back into their bodies and move on to the next buffet. Crazy, right!? 

Why the heck does such a thing even exist? A crazy, spiky, gut-extruding, coral-eating monster roving the ocean floor? The answer lies in the finely balanced work of art that is an ecosystem driven by the ever-churning engine of evolution. All creatures have predators. Those predators help maintain the balance of the ecosystem, preventing any individual part from growing out of control. The COTs are simply a predator for coral species.

Research has shown that COTs are deterred by the symbiotic algae that live in the corals and give them their color, preferring instead to feed on bleached corals. This behavior suggest that the COTs are preying on the weakest and sickest of the corals, potentially allowing more space for healthier corals to thrive and grow. 

There are also schools of thought that believe the COTs predation encourages coral species diversity, which in turn increases the overall health of the reef, or that COTs feed on faster growing corals, allowing slower growing varieties to gain more territory on the reef. These slower growing corals tend to have more sturdy, less breakable structures, increasing the reef’s durability.

Crown of Thorns Sea Star

So… are COTs good then?

Any scientist will tell you that the concept of good and bad is purely human, and I would encourage us all to try to look at the situation with our scientific eyes. COTs are neither good or bad—the problem is that they are out of balance. Natural predators of COTs include the giant triton snail, the humphead Maori wrasse, the starry puffer fish, and the titan trigger fish. 

All of these species are overfished by humans, which creates an overabundance of COTs. This imbalance in the ocean ecosystem has lead to increased predation of corals. Combine this phenomenon with the other issues plaguing the ocean (ocean acidification, temperature increases, unsustainable fishing practices, etc.) and the corals are going through a real rough patch. 

Got any good news? 

There’s loads of good news! First, corals are incredibly resilient. They can recover from bleaching incidents, they can propagate both asexually and sexually (they have two methods of reproducing), and there are thousands and thousands of species of corals. The chances that a few of them will adapt and overcome are pretty good.

Second, you don’t have to hate COTs! Not hating things is always great news. You can view them with knowledge of their complex role in the underwater ecosystem and feel smart. Feeling smart is fun. 

Third, you learned something! Hopefully. And isn’t learning just it’s own reward? No? Fine, then you can impress all your boat buddies on your next dive trip. You’re welcome. 


by Guest Blogger, Sacha Belter

Sacha developed a love of the ocean at an early age, despite being born and raised in Colorado. In her career she has spent time working in sustainability, tourism, and scuba education.

Sacha is passionate about marine conservation and education. In her spare time Sacha loves exploring the outdoors and having adventures with her friends and family, she’s been to over 40 countries and counting. Sacha serves as Secretary of the Board of Directors of Mother of Corals.

Sacha Belter

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