Crown Of Thorns

Picture credit : Simon Dowling

The Crown Of Thorns Starfish (Acanthaster planci) is native to coral reefs in the Indo-Pacific region.

They tend to have a lot more arms than your average Starfish (between 14 and 21). On healthy coral reefs, the Starfish plays an important role, being Corallivores (they munch on coral polyps) They tend to feed on the fastest growing corals such as staghorns and plate corals, allowing slower growing coral species to form colonies. This helps increase coral diversity. However, since it does feed on coral, too many Crown Of Thorns can mean a big problem.

According to research by the Australian Institute of Marine Science, coral cover on surveyed reefs has declined by about 50 per cent over the past 30 years. Crown-of-thorns starfish were responsible for almost half of this decline.

Crown-of-thorns starfish spawn during the warmer months of the year, with large females capable of producing up to 65 million eggs over the spawning season. Predators of adult Crown of Thorns starfish include the giant Triton Trumpet, Starry Pufferfish and the classic Koh Tao fish that everyone loves to hate…. the Titan Trigger Fish.

In Koh Tao, we have certain dive sites that have an over abundance of them, so we have to operate a removal strategy, although this is not an easy task since stress can cause the release of eggs from the Starfish resulting in an even bigger outbreak. Other in-water strategies are vinegar injections which has a 100% kill rate. Both techniques have to be carefully done, because those spines pack a powerful venom.

In Australia due to the size of the outbreaks that are experience they have started to develop an underwater robot to detect the Crown of Thorns:

With Crown of Thorns outbreaks, we monitor numbers of individuals on each dive site and take care that they do not overtake the area. It doesn’t mean that they are not important but everything needs to be balanced. Have a look at some of our other projects and get involved!!