Crown of thorns starfish (Acanthaster planci)

KingdomAnimalia
PhylumEchinodermata
ClassAsteroidea
OrderValvatida
FamilyAcanthasteridae
GenusAcanthaster (1)
SizeLength: up to 1 m (2)

The crown of thorns starfish has yet to be assessed by the IUCN.

A voracious predator of corals, the crown of thorns starfish (Acanthaster planci) is an unusually large starfish which may grow to more than one metre in diameter (2). Named for the dense covering of long, sharp, venomous spines on the entire upper surface of its body (2) (3) (4), the crown of thorns starfish has amassed a formidable reputation because of its devastating impact on coral reef ecosystems (5). Although it is naturally present on coral reefs, in some years the crown of thorn starfish may undergo huge population ‘booms’. During these outbreaks there may be a ten-fold increase in starfish numbers, causing dramatic damage to the reefs they inhabit (5) (6).

A rather flattened sea star, the crown of thorns starfish has between 7 and 23 arms that radiate from a large central disk (7) (8). The disk and arms are covered in a soft, membranous skin, which is armed with hundreds of stout, hinged, elongate spines that may grow to five centimetres in length. Although the body of the crown of thorns has a stiff appearance, it is able to bend and twist to fit around the contours of the corals on which it feeds. The underside of each arm has a series of closely fitting plates which form a groove and extend in rows to the mouth (7).  

The crown of thorns starfish is highly variable in its colouration, but it is usually brightly coloured above and lighter below (9) (10).

The crown of thorns starfish is found throughout the Indian and Pacific Oceans (6) (7) (11).

The crown of thorns starfish inhabits coral reefs, especially where scleratinian (stony) corals, such as corals in the genus Acropora or Montipora, are dominant (5) (6) (7) (11).

A notoriously ravenous predator of coral polyps, the crown of thorns starfish feeds primarily on scleratinian, or reef-building, corals, particularly fast-growing Acropora species. During severe outbreaks when competition for food is scarce, it will eat almost any coral species present on the reef (2) (5) (6) (7). Young crown of thorns starfish typically feed on encrusting algae, which are common among rocks and rubble on the reef (2).

The crown of thorns starfish is generally thought to be a nocturnal species, although some larger individuals do feed during the day (2) (8). It feeds by a method known as ‘eversion’, where the gastric folds of the stomach membrane are forced through its mouth and turned inside out, smothering the coral. It then secretes digestive enzymes onto the coral, absorbing the digested tissues of its prey externally (4) (5) (8) (10). Although the crown of thorns starfish will generally feed alone, during outbreaks many individuals will aggregate in large groups (7) (10).

Breeding typically occurs between December and April. The crown of thorns starfish reproduces sexually, releasing eggs or sperm into the water through pores on top of the central disk. Often, groups of the crown of thorns starfish on the same reef may spawn at the same time. A single female can produce up to 60 million eggs in a breeding season. The crown of thorns starfish has one of the highest rates of fertilisation recorded in any invertebrate, meaning that even small populations have the potential to produce a large number of offspring (2) (3) (7).

The crown of thorns starfish larvae typically spend between two and four weeks drifting in ocean currents (2) (5) (12). The larvae have small hairs, called cilia, which propel them through the water and produce feeding currents that trap the plankton on which they feed (7) (11). The larvae then settle on shallow reefs, eventually becoming five-armed juvenile starfish which feed on coralline algae. After between four and six months the juveniles switch their diet and begin to feed on corals (5) (7) (12). The crown of thorns starfish becomes sexually mature after two years (7).

Some species of carnivorous fish, such as the humphead wrasse (Cheilinus undulatus) and the starry puffer fish (Arothron stellatus), are known to prey on adult crown of thorns starfish, although in general it has very few predators once it has reached maturity (3) (13). Juvenile crown of thorns starfish are much more vulnerable to predation as they lack the characteristic spines of the adult, being preyed upon by crabs, shrimps, annelid worms and fish (3).

Several species of cardinalfish and damselfish are known to shelter within the venomous spines of the crown of thorns starfish (14).

At low densities, the crown of thorns starfish is a ‘normal’ part of coral reef ecosystems in the Indo-Pacific region. However, when the numbers of crown of thorns starfish dramatically increase it results in a major disturbance to the whole system, as the starfish consume coral faster than it can grow thereby drastically reducing coral cover (3). The large expanses of dead coral are subsequently colonised by algae, with a knock-on effect on other reef organisms that rely on coral colonies for food, shelter and nesting (3) (5).

The reasons behind outbreaks of the crown of thorns starfish are poorly understood, despite considerable research being carried out (3) (6). Although evidence suggests that outbreaks have occurred relatively regularly for thousands of years, the frequency and severity of these outbreaks appears to be increasing in recent decades (5).

Human activities are thought to be likely to be exacerbating natural cycles in the crown of thorns starfish populations. Overfishing, which is a major problem on many unprotected reefs, may have removed predators of the crown of thorns starfish, such as the humphead wrasse. Similarly, another natural predator of the crown of thorns starfish, the triton snail (Charonia tritonis), is threatened in many areas by collection for its highly prized shell. Agricultural runoff may also contribute to the increasingly frequent and persistent crown of thorns starfish outbreaks, as nutrients washed into the oceans can cause phytoplankton ‘blooms’, providing a more abundant food resource for the starfish larvae (3) (5) (6) (15). 

The crown of thorns starfish is not currently considered at risk of extinction.

There are no known conservation measures in place to protect the crown of thorns starfish.

The Australian Institute of Marine Science has implemented a comprehensive crown of thorns starfish monitoring programme to better understand the reasons behind outbreaks and to assess its long-term impacts on coral reefs (11). The Great Barrier Reef Marine Park Authority requires a permit for anyone wishing to conduct crown-of-thorns starfish controls on the Great Barrier Reef, which are limited to small-scale tactical measures in areas important to tourism or science (6).

Find out more about the crown of thorns starfish:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
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  1. ITIS (March, 2011)
    http://www.itis.gov/
  2. Lassig, B. (1995) Controlling Crown of Thorns Starfish. Great Barrier Reef Marine National Park Authority, Queensland. Available at:
    http://www.reef.crc.org.au/publications/explore/feat45.html
  3. Great Barrier Reef Marine Park Authority - Crown of thorns starfish (March, 2011)
    http://www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/latest_updates/cots
  4. Oceana - Crown of thorns starfish (March, 2011)
    http://na.oceana.org/en/explore/creatures/crown-of-thorns-starfish
  5. Clark, C. and Weitzman, B. (2006) Population Study Survey of Acanthaster planci, the Crown of Thorns Starfish on the Northwest Coast Moorea, French Polynesia. Undergraduate Student Paper, University of California Santa Cruz.
  6. Harriott, V., Goggin, L. and Sweatman, H. (2003) Crown of Thorns Starfish on the Great Barrier Reef. Current State of Knowledge (Revised Edition). CRC Reef Research Centre, Queensland.  Available at:
    http://www.reef.crc.org.au/publications/brochures/COTS_web_Nov2003.pdf
  7. Madl, P. (1998) Acanthaster planci: An Overview of the Crown of Thorns Starfish (CoT) As Observed on the Great Barrier Reef (Australia) – Development, Primary Food Sources, Predators and Toxicity. Colloquial Meeting of Marine Biology I, Salzburg. Available at:
    http://www.sbg.ac.at/ipk/avstudio/pierofun/planci/planci.htm
  8. Koonjul, M.S., Mangar, V. and Luchmun, J.P. (2003) Eradication of crown of thorns starfish (Acanthaster planci) infestation in a patch of reef in the lagoon off Ile Aux Cerfs, Mauritius. Proceedings of the Annual Meeting of Agricultural Scientists (AMAS), Food and Agricultural Research Council, Réduit, Mauritius.
  9. Narváez, K. and Zapata, F.A. (2010) First record and impact of the crown-of-thorns starfish, Acanthaster planci (Spinulosida: Acanthasteridae) on corals of Malpelo Island, Colombian Pacific. Revista de Biologia Tropical (International Journal of Tropical Biology and Conservation), 58(1): 139-143.  
  10. Campbell, A. and Dawes, J. (2004) Encyclopedia of Underwater Life. Oxford University Press, Oxford.
  11. Australian Institute of Marine Science - Crown of thorns starfish (March, 2011)
    http://www.aims.gov.au/docs/research/biodiversity-ecology/threats/cots.html
  12. Keesing, J.K. and Halford, A.R. (1992) Field measurement of survival rates of juvenile Acanthaster planci: techniques and preliminary results. Marine Ecology Progress Series, 85: 107-114.
  13. Randall, J.E., Head, S.M. and Sanders, A.P.L. (1978) Food habits of the giant humphead wrasse, Cheilinus undulatus (Labridae). Environmental Biology of Fishes, 3(2): 235-238.  
  14. Stier, A.C., Steele, M.A. and Brooks, A.J. (2009) Coral reef fishes use crown of thorns seastar as habitat. Coral Reefs, 28: 227.
  15. Sweatman, H. (2008) No-take reserves protect coral reefs from predatory starfish. Current Biology, 18(14): R589-599.