The crown-of-thorns starfish, also known as Acanthaster planci, is an unusually large starfish and can grow to more than 1 m in diameter. It has up to 21 arms, with the entire upper surface of its body covered in long venomous spines. The starfish can move up to 20 m in an hour.
The Crown-Thorn-Starfish has been given a lot of notoriety do to the affect they are having on the Great Barrier Reef and other Indo-Pacific reef ecosystems for nearly 40 years. Despite the extensive research the ultimate cause of the regularly occurring outbreaks is still not clear. The starfish has a few predators such as: Giant Triton Snail, The Humphead Maori Wrasse, Starry Puffer Fish, Triton Trigger Fish.
Due to the increased use of the coastal zone by humans nutrients is continuously flowing to the sea and improves the survival rate of the starfish larvae. This further allows for an increase in the number of adult starfish.
The first documented case of the Crown Thorn Starfish was in the 1962 as there was a large population of the starfish present on the Great Barrier Reef. The outbreak continued up until 1975
When starfish are in large numbers, there is intense competition for food and most types of corals will be eaten, including species such as Porites spp. that are not usually eaten by the starfish. During a severe outbreak, there can be many crown-of-thorns starfish per square metre, even piling on top of each other. They can eat so much that they can kill most of the living coral in that part of the reef, reducing hard coral cover from the usual 25 – 40% of the reef surface to less than 1%. Such a reef can take 10 years or more to recover its coral cover.
Some human influences, such as overfishing and poor water quality, have been suggested as playing a role in crown-of-thorns outbreaks.
Natural controls on starfish populations include high mortality of the larvae, predation of small starfish and diseases. Adult starfish have few predators because of their tough and toxic ‘skin’ and long spines. There is little evidence of any major diseases in crown-of-thorns starfish.
The recommended control method involves trained divers injecting sodium bisulfate (dry acid) solution into the starfish, which kills them within a few days. This chemical is non-toxic to other marine life. This control method is extremely costly. Some tourism operators in the Cairns region spend up to $300,000 each per year in crown- of-thorns starfish control. During active outbreaks, operators may need to inject 200 to 500 starfish every day in an effort to keep selected sites free of starfish.
This starfish produces a neurotoxin which can be released through its spine. The wounds are serious and the neurotoxin can cause a sharp pain that stings and lasts for hours. It will cause nausea and vomiting as well. Divers beware, the spines of these starfish can break off and may become embedded in inside the skin which would lead to infection and increase toxicity.