Today I Learned: Why We Should Get Rid of These Strangely Beautiful 'Sea Stars'

The approximately 27,000 square kilometers of coral reefs of the Philippines are the country’s biodiversity crown jewel, and there are incredibly thriving and heart-warmingly intact coral reefs in the country. Other coral reefs are suffering from environmental problems, and the mass outbreaks of crown-of-thorns sea stars (COTS) are one such problem.

Coral reefs are finely tuned ecological systems, and if the population of one animal all of a sudden grows to a size multiple times what it is under unperturbed, pristine conditions, the ecosystem as a whole can suffer. This is what happens with the crown-of-thorns sea star, a species with the scientific name Acanthaster solaris, living in this part of the Pacific Ocean. As a side-note, other parts of the Pacific Ocean and the Indian Ocean are home to other, closely related species of the genus Acanthaster. These are all large, spiny, somewhat scary-looking sea stars. And they eat hard corals. The same hard corals which are the cornerstones of the eponymous coral reefs, which build the limestone structures at their bases.

Now, under normal conditions there are a handful of crown-of-thorns sea stars on a large coral reef. They will eat some hard corals, but the corals will grow back fast enough, and the coral reef remains intact. The female crown-of-thorns sea stars produce millions of eggs when they spawn, and the larvae often spend many weeks adrift in the open ocean before finding a new reef to settle on and start munching on corals. With so many eggs, only a very, very small fraction of the crown-of-thorns larvae ever make it to adulthood.

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Sometimes though crown-of-thorns mass population outbreaks occur. This happens when the larvae survive their time in the open ocean much better, because they have more phytoplankton (single-cellular algae in the sea water) to feed on. The increased phytoplankton in turn usually comes from poorer water quality, due to human-caused sedimentation or influx of sewage or agricultural fertilizer into the ocean. Once the larvae have more phytoplankton to feed on, a very small fraction instead of a very, very small fraction of the crown-of-thorns larvae grows up to be adults. The surviving fraction is still tiny, but it’s a tiny fraction of a huge number of eggs, and it’s now big enough that the coral reef is overburdened with the coral-eating sea stars.

Once such an outbreak happens, the large numbers of crown-of-thorns sea stars can eat so many of a reef’s hard corals that the ecosystem as a whole degrades and can’t provide a home anymore for the thousands of fish and invertebrate species found in a healthy coral reef.

A video showing a crown-of-thorns sea star outbreak, and explaining the stunning reproductive biology of the animal

There are a number of predators of the crown-of-thorns sea stars, such as the Triton’s horn sea snail (see the image in this article). This snail used to be a popular souvenir, but is now legally protected in the Philippines (let’s hope that the legal protection works!).  The presence or absence of predators can make a difference to the crown-of-thorns sea star population, though the more important factor is the survival of larvae.

I recently had the opportunity to contribute to a reef survey, led by Ms. Venice M. Hulleza of Northern Negros State University, where we looked for crown-of-thorns sea stars, and for the damage they had done. We went to Carbin, Panal and Macahulom reefs, all less than an hour by bangka from Sagay City at the northernmost tip of Negros Occidental.

The three reefs looked quite different from each other, with varying degree of damage done to their hard corals. Oddly, Macahulom reef, with the most healthy corals, also had the most crown-of-thorns sea stars, possibly because they still had the most coral food on this reef.

Our expedition to the Northern Negros Reef System off Sagay

Crown-of-thorns sea star outbreaks are not linear, straight-forward affairs. Environmental stresses, ocean currents and biological factors such as food (coral) availability and sea star predators all interact to shape the events.

What is there to do when encountering a big group of crown-of-thorns sea stars on a coral reef? An important step is to notify the experts, such as DENR and nearby marine biologists in a college or university. As a second step, ideally coordinated with aforementioned experts, you can eliminate the excessive crown-of-thorns sea star population. If you pick them up, do so with barbecue prongs, because the multiple spines of the sea stars are quite venomous.

However, it’s not ideal to pick them up and discard them on land. While the sea stars are not the Einsteins of the underwater world (they don’t even have a brain at all, but a diffuse nerve net), they still notice that they are being picked up from the reef. The sea stars then often “panic spawn” and release all the eggs and sperm they have stored in their bodies. Such mass-panic-spawning can cause secondary outbreaks, where reefs down-current from the one where the removal took place end up with large numbers of crown-of-thorns sea star larvae.

Hence, it’s better to inject the crown-of-thorns sea stars with vinegar. This causes them to disintegrate, without spawning before death. The method sounds somewhat gruesome, but in the absence of a brain (see above) they probably don’t feel any pain in a human sense, and the whole procedure saves the coral reef ecosystem as a whole.

It’s of course of importance to remove the root cause of the outbreaks if possible. In Australia, where crown-of-thorns sea stars have caused damage to the Great Barrier reef, improved irrigation and land fertilization technologies are aiming to both reduce the nutrient and sediment flow into the ocean, as well as save the farmers cash for excess water and fertilizer which doesn’t benefit their crops (but ends up in the ocean).

All of these activities should be done in coordination with the experts. And, show these experts some love! There are many skilled and knowledgeable marine biologists in the Philippines, who don’t always have all the resources they would want to survey the country’s coral reefs. While it’s clear that funding marine science is not the most burning political issue, when looking at issues like the crown-of-thorns sea star outbreaks, the value of these experts should not be discounted.