The Crown of Thorns Starfish, Acanthaster planci, is one of the largest starfish found on the coral reefs of New Caledonia. This specimen is about 45-cm in diameter and they sometimes reach 65-cm in diameter. The Crown of Thorns is also the most famous starfish in the world for two reason, the first is they are one of the few starfish that are dangerous. Their long venomous spines that are so sharp they will slide right through a glove and into a finger with practically no pressure at all. The poison is exceedingly painful and a wound will take a long time to heal, often leaving the hand or foot numb for weeks or even months.

The second reason for their fame is because they eat the living tissues of stony coral. The biggest of the three starfish in this sphere image will probably devour one square metre of living coral in a week. Normally, the Crown of Thorns are rare, with just an occasional specimen found in locations where corals are abundant and able to cope with the starfish. But sometimes the Crown of Thorns undergo population explosions and hundreds of thousands of them infest the reefs, killing almost all the living coral, destroying the complex ecological balance of the reefs. The infestations are a kind of cancer of the coral reef system - where one organism gets out of control and destroys the whole larger ecosystem. These reef cancers have been getting more abundant over the past 30 years and many parts of the New Caledonian coral reefs have been severely damaged. There are many causes for these cancerous outbreaks, but in general they happen when the reef is severely stressed - from overfishing, breakage of the coral during major storms or by explosives, removal of important starfish predators like the giant triton shell,and even pesticides and herbicides that enter the lagoon from agricultural activities.

This reef had over 30 specimens in one area - not a good sign - and as you can see in the sphere image much of the coral is already dead - dark gray algae covered branches of coral instead of the multicolored branches of living corals. There have been attempts at controlling the infestations In some parts of the Pacific, where infestations have had hundreds of thousands of starfish. The controls have successfully protected limited parts of the coral reefs - for example around resorts and in important reserves - but the overall cure is something like finding ways to minimize the incidence of cancer. A healthier life style for the coral reefs requires good reef management systems, including the protection of important species like the Giant Triton shell and the Hump Head Wrasse, reducing fishing pressure on reef fish, and reduction of terrestrial run-off, especially from agricultural areas that use pesticides and herbicides. More on the Crown of Thorns, Acanthaster planci