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Reef Report for Sunday, July 30, 2006

Creatures of the Reef: Crown of Thorns Starfish

Every so often we profile interesting animals found on the Great Barrier Reef, and this week have decided to cover the Crown of Thorns Starfish.

These creatures are both beautiful and notorious at the same time. © Wet Image Their purple/blue/green colors are very handsome, their long spines toxic, and their eating habits are at times considered a threat to the health of coral reefs all over the Indian and Pacific Oceans. They can grow to a size of up to metre in diameter, can have up to 21 legs, and are able to move up to 20 metres per hour, a very fast rate of movement for a starfish.

Like other sea stars, Crown of Thorns are echinoderms; closely related to other sea stars, and more distantly to sea cucumbers, sea urchins and sand dollars. They are found on coral reefs in the tropical waters of the Indian and Pacific oceans, not just on the Great Barrier Reef.

Their spines contain a toxin, and can inflict a painful sting. Most divers contact them inadvertently with their hands and feet. You can avoid being stung by simply following the wise rule of avoiding contact with the reef while diving and snorkeling, which not only protects you, but protects the reef at the same time.

While the larval stages of these organisms each phytoplankton, once they take on their adult shape Crown of Thorns starfish eat coral. Periodically the populations of these animals build up to the point where they can do damage to coral reefs, reducing the coral cover on a reef from a healthy 30% to just 1%. During outbreaks of Crown of Thorns various dive operators and the Australian government will spend money on reducing these populations in key areas, especially around key dive sites used by tourism operators.

Scientists are just beginning to understand the biology and ecology of Crown of Thorns, and itís hoped that this will lead to more effective ways of reducing the populations of these animals, and the damage they can do.

At this writing very few of the some 3,000 reefs making up the Great Barrier Reef have outbreaks of Crown of Thorns. Itís been 10 years since the last significant outbreak, and most reefs have recovered from that event. Scientists are now studying how and where the populations of Crown of Thorns grow, with the idea of developing a model they can use to predict and control outbreaks before they become threats to the health of the reef.

Scientific research has also demonstrated that there is a connection between wet weather, nutrient runoff into the ocean, and the increased survival of Crown of Thorns larvae. Despite the abundance and diversity of life on coral reefs , the ocean water over the reefs is very low in nutrients, which is why the water is so clear. Increased nutrients eroded from soils, forests and farmlands by wet weather cause an increase in phytoplankton growth. Phytoplankton is the main food for larval crown of thorns starfish, and increased food means increased numbers of them growing up. The Australian and Queensland governments in the past few years have made major efforts at managing watersheds to reduce nutrient sources, and many local farmers have also joined up with this effort.

For More Reading:

A primer on Crown of Thorns Starfish

Nutrients and Crown of Thorn Starfish (COTS) Survival

Current population levels of COTS on various reefs

Crown of Thorns Starfish Control Program

An interesting database of Reef Health surveys

(C) DiveTheReef The weekly reef report is written by Joel Groberg of DiveTheReef.com, who compiles them from the many conversations he has with dive staff in the area, as well as† many other local sources in the dive community.

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