The term “crown-of-thorns fish” is a misnomer. This organism, known to scientists as Acanthaster planci, is not a fish but a large species of starfish native to Indo-Pacific coral reefs, including the Great Barrier Reef. While a natural part of the reef, its reputation is primarily associated with its capacity to consume vast quantities of coral, which has significant consequences for its environment.
Physical Characteristics and Habitat
The crown-of-thorns is one of the largest starfish in the world, with adults measuring 25 to 35 centimeters, though some have been observed at nearly a meter in diameter. Unlike the familiar five-armed starfish, this species is multi-armed, possessing 12 to 21 arms that radiate from a wide, disc-shaped body. This flexible structure allows the animal to move over complex reef structures at speeds up to 20 meters per hour.
Its entire upper surface is covered in long, sharp spines, giving it the appearance of a biblical crown of thorns. These spines, up to 4 centimeters long, are not just a physical defense; they are also coated in toxins. The starfish’s coloration can vary significantly by region, appearing in shades of purple, red, green, or brown. This animal is found on coral reefs throughout the tropical and subtropical waters of the Indo-Pacific.
Feeding Behavior and Ecological Role
The diet of the crown-of-thorns starfish consists of living coral polyps. The starfish uses its numerous tube feet to climb onto a coral colony, where it then everts its stomach out through its mouth over the coral’s surface. Secreted digestive enzymes liquefy the coral’s soft tissues, which are then absorbed before the starfish retracts its stomach. This process leaves behind a white coral skeleton.
An adult starfish can consume up to 10 square meters of coral per year, showing a preference for faster-growing coral species like Acropora. In balanced, low-density populations, this predation is not entirely destructive. By consuming fast-growing corals, the starfish helps prevent them from monopolizing the reef, which allows slower-growing species the space and light to thrive.
The Threat of Outbreaks
The relationship between the starfish and the reef changes during population explosions, known as outbreaks. An outbreak is defined as a density of 15 or more starfish per hectare, a threshold at which coral consumption outpaces the reef’s ability to recover. During these events, the starfish can strip up to 90% of the living coral tissue from an affected reef in a matter of months.
The precise causes of these outbreaks are still under investigation, but scientists believe human activities may be increasing their frequency. One hypothesis points to increased nutrient runoff from land, linked to agriculture and coastal development. These excess nutrients can cause blooms of phytoplankton, the food for crown-of-thorns larvae, boosting their survival rates and leading to a population surge. The overfishing of natural predators, such as the giant triton snail and certain triggerfish, is another potential factor.
Human Interaction and Management
Direct human interaction with the crown-of-thorns starfish carries risks due to its venomous spines. A puncture wound from a spine is painful and may cause swelling, nausea, and more severe reactions as the toxins can damage liver and red blood cells. First aid involves carefully removing any broken spine tips and immersing the affected area in hot water for 30 to 90 minutes to break down the venom.
To combat outbreaks, management programs employ targeted control methods. Early attempts to control populations by cutting the starfish into pieces were counterproductive, as the animals can regenerate from severed limbs. Current strategies involve manual culling by trained divers who administer a lethal injection of household vinegar or a solution of bile salts. This method causes the starfish to decompose within about 24 hours without harming the reef.