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Meera Koojul discusses with Genevieve the Crown-of-thorns management program in Mauritius.
Photo : Chris Johnson

January 20, 2004
The Crown-of-thorns Starfish - a coral killer

Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey in Mauritius.

The other day the crew was invited to visit the Ministry of Fisheries in Albion. Several Fisheries staff spent time onboard Odyssey observing our research. The crew are keen to learn more about the many valuable research projects that they are conducting. Among the responsibilities of the Albion Fisheries Division is to monitor the coral reefs around the country, including the management of any threats to this fragile ecosystem. We spoke with Meera Koonjul about her work with one potential threat to the coral reefs of Mauritius, the Crown-of-thorns Starfish.

Meera Koonjul - Scientific Officer Albion Fisheries Research Centre:

    I am Meera Koonjul, Scientific Officer and I work in the Ecology Department of the Marine Science Division at Albion Fisheries Research Center.

Genevieve Johnson:

The Crown-of-thorns Starfish (Scanthaster planci) is a comparatively large starfish that can attain a size of more than 40 centimeters in diameter. It is a distinctive creature with an abundance of long dorsal spines covering its entire body resembling a crown of thorns, thus the origin of its name. The spines serve as an effective defense against potential predators and may cause severe injury if handled.

Starfish belong to a group of invertebrates called Echinoderms where asexual reproduction is the norm. Reproduction involves a division of the central disc whereby the animal breaks into roughly two halves. The edges of the break reseal and regeneration occurs. Depending on the species, it may take anywhere from a month to a year for the new animal to develop.

Crown-of-thorns Starfish - Scanthaster planci.
Photo : Courtesy of Meera Koonjul

Meera Koonjul:

    The Crown-of -thorns Starfish is a predator of corals, they eat live corals. It is found in may places all around the world, especially in the Great Barrier Reef of Australia and Okinawa in Japan. We keep monitoring the Crown-of-thorns Starfish at one of our reefs, one of the very pristine reefs that we see around Mauritius, we found about 35-40 large specimens that could cause a lot of damage to the reef and we donÕt have many reefs compared to other large countries. We thought of removing them at first and burying them in the sand - the same way it is usually done in Indonesia and Japan, but we thought of using another method - the injection of dry acid or hydrogen sulphate.

    This is very harmless to the environment and only kills the organism (Crown-of-thorns Starfish). What we did was dive on the area, took the injections and inserted injections into all the specimens and dove back there again after two days. All of the Crown-of-thorns were dead on the spot and it was really amazing that no harm was done to any of the other organisms or the ecosystem as such. This was a really great experience for us. We have been talking to many people like divers and fisherman who would like to give us information on this type of problem, like infestations in certain places. We can then go there and clean them up using this method, the injection of dry acid - it works really well. I think this is a very good measure we can take to protect our reefs from the Crown-of-thorns Starfish.

Genevieve Johnson:

The Crown-of-thorns is a veracious carnivore, an extra-oral feeder that preys on coral polyps. On finding a suitable piece of coral, it feeds by throwing its stomach out of its body, covering the coral colony. A few hours later when the digestive enzymes have dissolved the coral tissue, the resultant ŌsoupÕ is absorbed and the stomach withdrawn. After consuming the living coral animals, all that remains is a white coral skeleton. The coral bleaches out and begins to breakdown in the elements, creating an open space for another benthic creature to fill in. If a balance is maintained between the amount of coral being consumed and the number of starfish, this will create a healthy coral reef.

Recently, the population of crown of thorn starfish has exploded to numbers that disrupt that balance, having drastic effects on reefs throughout the tropical waters of the world.

The Crown-of-thorns is able to consume an area of prey as great as the size of its own disk in a single day. In one year, an individual animal is capable of destroying 5 square meters of coral. Alone, this is not a big problem, but when this species reaches plague proportions, these starfish cause destruction on a grand scale.

For reasons not clearly understood, this starfish is prone to cyclic population explosions, with high numbers destroying whole areas of coral reef.

Meera Koonjul:

    We have been talking about this with scientists from Japan and everybody seems to be amazed by it. At certain times of the year in Mauritius, September and October, we find a bloom, like an algae bloom - there is a bloom of the Crown-of-thorns. Basically, we feel it is there reproductive time, they want to breed and they are coming to places where they can have lots of food. That is a problem for us because they are going to destroy our corals, so we have to be on the move during these times.

    In the world there is only one species of the Crown-of-thorns Starfish and everybody is of that belief that they come only during certain times and there is infestation. Therefore, during these times, we have to use measures to control them.

Genevieve Johnson:

In order to control the population of Crown-of-thorn starfish in Mauritius, Divers inject them with dry acid. This kills the starfish without harming the reef.
Photo : Courtesy of Meera Koonjul

One theory behind the infestations is that larval survival rates have become exceptionally high as a result of runoff from agricultural practices. Chemical fertilizers used in farming increase the amount of phytoplankton in the water, the food source of the larval starfish, and may then increase the number of crown-of-thorn larvae that survive to juvenile or adult stages.

The depletion of the Crown-of-thorn Starfishes' primary predator may also be a contributing factor. The Giant Triton (Charonia tritonis), one of the few reef species known to prey upon the Crown-of-thorns Starfish, has been over harvested in recent years from the sea and sold in the lucrative souvenir shell trade.

It is very likely that both influences are working in conjunction with each other creating a two-fold effect.

I asked Meera how the Crown-of-thorns will be managed over the long-term and if she believes the species will become a bigger problem.

Meera Koonjul:

    For Mauritius, because it is a very small island, as we are reef monitoring all around the island and we are getting information from almost every part of the country, it is not a big problem. We have non-governmental organizations like MSDA and MCS, the Scuba Diving Association, the dive centers all in close contact with the Ministry of Fisheries. SO whenever there is a problem of Crown-of-thorns, we have informed everybody, they then inform us and we do the removal.

    There is a danger of being harmed with Crown-of-thorns because they do have spines that are poisoness and it is very difficult to remove if you don't have the proper equipment and know-how. So, when we are informed we can go to a place, do a survey and find out how many are there, and then we can easily eradicate them with the injection.

    I think for Mauritius this is a very good measure because we are getting information almost every time there is an infestation.


Log written by Genevieve Johnson.

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