The Crown of Thorns Sea Star Epidemic

Silent killers of coral reefs

By jon hanson on flickr. — https://www.flickr.com/photos/jonhanson/89930167/, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=665552

Meet the Crown of Thorns Sea Star — an animal that seems to come from a horror sci-fi movie. What are these creatures? And why are they causing destruction to reefs worldwide? First, let’s try to understand these alien-looking creatures.

The most distinct feature of these sea stars is, of course, the numerous spines covering their body. These spines contain venom and are not only sharp but brittle, meaning that they can break off and continue pumping venom into whatever is bothering the sea star. Like other sea stars, the Crown of Thorns also has regenerative powers. These sea stars are capable of losing whole arms and regenerating them completely. On top of that, if the severed arm is not too damaged, it can grow into a completely new sea star! Another interesting fact about the Crown of Thorns is their unusual feeding method — they first eject their stomachs out through their mouth and digest their meal externally before sucking their stomachs (along with their digested food) back into their body. So what do they eat? Crown of Thorns mostly consumes hard coral. Normally, this would be beneficial to the environment and the reefs, as these sea stars clear the way for softer and more fragile coral species to grow. However, as we will soon learn, human activities and climate change have turned this once beneficial organism into a weapon of mass destruction.

By Matt Wright — Own work, using a Canon PowerShot S410, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=379049

How these sea stars threaten reefs

Nature often has a way to balance itself — a population usually wouldn’t be able to grow past a certain point due to a lack of resources or a predator that keeps its numbers in check. However, human intervention has continued to disrupt this balance, and as we see here, it can lead to devastating consequences. In recent years, the Crown of Thorns sea star population has surged to unprecedented numbers. This imbalance in the ecosystem has turned the sea star from hard coral regulators to voracious predators that ravage all the hard coral in their habitat, turning the once colourful reefs into trails of white skeletons. The surge of the Crown of Thorns is mainly attributed to three factors: Overfishing of the Triton snails, warming waters, and terrestrial runoff.

Triton snails are the main predators of the Crown of Thorns. Studies by the Australian Institute of Marine Science show that even the smell of triton snails in the water is enough to cause the sea stars to panic and disperse. Unfortunately, these snails are often overfished for their elaborate shells, and since there aren’t many other predators that are capable of hunting Crown of Thorns, this has led their numbers to rise unchecked.

A triton snail: By NOAA — the National Oceanic and Atmospheric Association, U.S. ; http://www.photolib.noaa.gov/htmls/reef2590.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=544085

Female Crown of Thorns usually lay about 20 million eggs per spawning cycle. Usually, only a small fraction of these larvae would reach adulthood, as there won’t be enough nutrients and plankton to sustain all the larvae. However, as the climate continues to change, warming sea waters and nutrient-rich soil runoff from agricultural practices have been causing algae blooms and a surge in phytoplankton numbers. This increased availability of food leads to an increase in survival of sea star larvae into coral munching adults, leading to a significant increase in Crown of Thorns outbreaks.

Culling the Crown of Thorns

We all know that coral reefs are important: not only do they provide shelter and food for a wide variety of marine life, but they also generate a significant amount of revenue for the tourism industry. Naturally, both marine biologists and the tourism industry want the Crown of Thorns gone. However, it is easier said than done. The venomous spines makes handling them difficult and dangerous as the spines are capable of penetrating wetsuits and cause excruciating pain. Their ability to regenerate from broken-off limbs means that fragmentation could actually end up ironically increasing their population. If exterminators wanted to poison the sea star, they would have to inject the poison into every single limb, which is not only time-consuming but costly. Furthermore, the Crown of Thorns usually only comes out to feed at night and hide in crevices within the reefs during daytime, making it even harder for divers to efficiently eradicate these creatures.

The main method of culling Crown of Thorns currently is with either bile salts or vinegar. One injection of either substance into the body of the sea star is enough to cause the cells to break down, and after a few hours, the sea star will fall apart with no chance at regeneration. Tourism sites often employ teams of sea star exterminators to inject Crown of Thorns in popular reef sites. While one extermination session can cull up to thousands of sea stars, this strategy has yet to make a dent in the Crown of Thorns population, which may number up to millions at this point.

By Michel Dammeron — Given with written rights release., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=28115871

Organizations such as the Australian Institute of Marine Science have been looking into alternative methods to help curb the Crown of Thorns problem. Some solutions that have been explored include replacing exterminators with drones, so that the extermination can take place safely at night, when the sea stars are more active. Another potential solution is to spray the waters with Triton snail smell during the Crown of Thorns mating season so that the sea stars would disperse and there would be a lower chance of successful mating.

Behind rising water temperatures directly killing corals, outbreaks of Crown of Thorns are one of the leading causes of coral devastation, yet this is a problem that has evaded the attention of the general public for far too long. While the prospect has been improving, only time can tell whether these strategies are enough to manage the Crown of Thorns outbreaks and save our corals.

Thank you for reading!

If you would like to know more about coral reefs collapsing, do check out: https://medium.com/something-simple/corals-are-beautiful-creatures-f4b606aaeca1

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