The crown-of-thorns starfish (Acanthaster planci) is a large nocturnal sea star that preys upon coral polyps. The crown-of-thorns receives its name from venomous thorn-like spines that cover its body.
The crown-of-thorns is endemic to tropical coral reefs in the Red Sea, the Indian Ocean, and the Pacific Ocean. As solitary animals, they feed alone and maintain constant distance between themselves and other members of their species.
The crown-of-thorns starfish has gained notoriety as a threat to the coral reef ecosystem, particularly in the Great Barrier Reef off the coast of Australia. Overpopulation of crown-of-thorns has been blamed for widespread reef destruction. Birkeland (1985) describes the starfish as one of the most influential species in the diverse biotic communities that make up tropical coral reefs.
Some ecologists point out that the starfish has an important and active role in maintaining coral reef biodiversity, driving ecological succession. Before overpopulation became a significant issue, crown-of-thorns prevented fast-growing coral from overpowering the slower growing coral varieties.
Other factors negatively affecting the reef ecosystem, such as coral bleaching or Black band disease, mean that outbreaks of the crown-of-thorns can now cause permanent and devastating damage. Increasing outbreaks are also thought to be caused by possible environmental pollution triggers. Algal blooms caused by agricultural run-off may supply predators of crown-of-thorn starfish larvae with plentiful alternative food sources. This seems the most logical explanation for the recent crown-of-thorns outbreak in the Tubbataha Reef, a UNESCO World Heritage Site. These explanations may also explain why massive outbreaks seemingly appearing out of nowhere, with no previous indication of an increasing population at the affected site.
The crown-of-thorns starfish may “promote transmission” of some coral diseases.
Population numbers for the crown-of-thorns have been increasing since the 1970s. However, historic records distribution patterns and numbers are hard to come by, as SCUBA technology, necessary to conduct population censuses, had only been developed in the previous few decades.
To prevent overpopulation of crown-of-thorns causing widespread destruction to coral reef habitats, humans have implemented a variety of control measures.
Injecting sodium bisulphate into the starfish is the most efficient measure in practice. Sodium bisulphate is deadly to crown-of-thorns, but it does not harm the surrounding reef and oceanic ecosystems.
When under stress the crown-of-thorns can create outbreaks and, if dismembered, can regenerate from each severed or damaged limb, creating more sea stars. Therefore, controlling the crown-of-thorns is difficult and much care is required.
QUESTIONS AND ANSWERS
Dr. Peter Moran
On-line Reference Series
This booklet contains answers to questions, which in the author’s experience, are most commonly asked about the crown-of-thorns starfish. While it is meant to be used by all members of the community it has been developed particularly for secondary school students in preparing essays and assignments. The booklet has been designed so that individual photographs and line drawings can be cut out and used without destroying the text or other figures. As many of the questions are connected in some way the information given in an answer to one may relate to another.
Additional sources of information are referred to in the following form; (see 10) ie. see question 10 which would be under-lined and provide a hypertext link to the question, the back button on the browser will return you to the point from where you linked.
Where practicable confusing scientific terminology has been avoided as the intention is to provide a simple and easily understood explanation to the question raised. This is not always possible as many questions relating to the crown-of-thorns starfish cannot be answered in a simple way. A glossary of the scientific terms used in connection with this booklet has been included to assist readers.
Most of the answers in this booklet are based on published scientific information. For more detailed information, readers are referred to the following publications:
Moran, P.J. (1988) The Acanthaster phenomenon. Australian Institute of Marine Science Monograph Series Vol. 7. I 78 p. (Contains recent scientific review, annotated bibliography and subject index).
Zann, L. and Eager, E. (ed.) (1987) The Crown of Thorns Starfish. Australian Science Magazine 3:14-55. (A general publication for secondary schools and for the public).