Impact of Acanthaster planci (Crown-of-Thorns) Sea Star on Coral Reef Systems
Acanthaster planci, better known as the crown-of-thorns sea star, is an invasive species of echinoderm. This variety of sea star causes enormous amounts of damage to coral reefs in the Great Barrier Reef in Australia and other Indo-Pacific locales. Researchers and communities alike have shown great concern for the future of the affected reefs. Scores of tourists flock to areas of the Great Barrier Reef specifically for diving and snorkeling. Destruction of this reef would cause serious damage to the tourist industry in this area. Additionally, greater environmental issues such as fish migration and major erosion of beaches would occur.
The crown-of-thorns sea star breeds once per year during the summer months. Group breeding occurs in shallow waters in order to increase the likelihood of fertilization.
During this breeding cycle one female is capable of laying 250 million eggs! Once fertilized, these eggs become larvae and within 6 months move into the juvenile stage. A juvenile grows rapidly and is capable of becoming an adult within a month’s time. With 14-18 arms and a diameter of 25cm in only 2 years, the crown-of-thorns sea star is a formidable individual.
An adult crown-of-thorns sea star is capable of causing large amounts of damage to a coral reef over the course of a year by feeding on healthy corals. The crown-of-thorns sea star is an extraoral grazer, meaning that it is capable of turning its stomach outward, pressing it against the healthy coral and digesting it by excreting enzymes that cause the coral to become a soupy mixture. It then absorbs the coral soup through its stomach wall before retracting its stomach and moving on. This area of coral is bleached white in color, an indication that it is dead. In a mere 9 hours it can consume an area of 160cm2 of coral polyps before being satisfied. It then takes a 12-70 hour break before feeding again. Over the course of 1 year a crown-of-thorns sea star can destroy an area of approximately 5-6m2. If the population of the crown-of-thorns sea star is small this feeding pattern would not be an issue. However, if you consider the damage inflicted on a coral reef by a large population, you now have a recipe for disaster. All of this destruction can have a permanent effect on the reefs. A quick growing coral species may be capable of recovering in 10-15 years but others may never recover.
There are several factors that lead to a large population growth in the crown-of-thorns sea star. One main theory for the growth is that nutrient runoff from human use in the area increases food for the larvae of the species to consume, thereby feeding them consistently and giving them a better opportunity for survival. Another variation of this same theory is that fertilizer runoff from local farm areas also provides large amounts of nutrients for the larvae. The removal of natural predators at the hands of humans is another theory. The Triton (Charonia tritonis), a gastropod, is the main predator for the crown-of-thorns sea star. However, it has been hunted dramatically by humans for its beautiful shell. This decline in predators causes an imbalance in prey/predator numbers and as a consequence the population of sea stars goes unchecked. Lastly, outbreaks may just be a natural phenomenon and not something able to be or needing to be controlled.
In an attempt to help curtail the population growth of the crown-of-thorns sea star several methods have been used to attempt to keep the population in check. Some have been remarkably ineffective such as cutting the sea star into pieces. This method only increased population as sea stars are capable of regeneration of body parts as long as a piece of the central disk is intact. Methods that are helping include chemical injection of formalin, ammonia or copper sulphate injections, which kills the individual. An alternative method, human removal of individual sea stars by catching them and removing them from the area, can net up to 150,000 per day during a large outbreak. A different method is the introduction of predator species such as the triton. However, biological introduction and tampering with the natural order can cause even greater issues.
So what does this all mean when considering coastal management issues? The breakdown of healthy coral systems causes fish living in the reef environment to leave for a better hunting ground, as the ravaged coral no longer provides nourishment for the reef fish. This in turn causes fishing shortages for human consumption. Also, dead corals are no longer as strong as their living counterparts and break apart in deeper waves. The fragments are washed around the bottom of the shoreline area and are pulverized over time. This process eventually leaves flat areas where once strong reefs grew. These flat areas offer no protection for the coastline during storms. Waves that broke further offshore on the reefs now barrel over them and break closer to shore causing serious erosional issues for the coastline.
Scientists are still unsure if theories concerning the crown-of-thorns sea star outbreaks are cyclical or are due to other possibly preventable factors. Much debate and research is currently being done and will continue to be done in the future. What is sure is that the crown-of-thorns sea star is a destructive species and is capable of causing major coastal problems if gone unchecked. How this delicate system reaches a good balance will certainly need thought and research to make certain our reef populations are not lost forever.
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Crown-of-Thorns Starfish Questions and Answers. 12 Dec. 1997. Australian Institute of
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Crown of Thorns Underside. 2001. Jay Torborg’s Website. 08 Oct. 2004.
NOAA National Ocean Service. 15 Apr. 2003. < http://www.nos.noaa.gov/education/
Triton Trumpet in the Florida Keys. 01 Jan. 2004. Ray I. Doan Photographic Collection.
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Universität Salzburg. 13 Oct. 2004.