Crown-of-thorns starfish
Encyclopedia
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 venom
ous thorn-like spines
that cover its body. It is endemic to tropical coral reef
s 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 is the second largest sea star in the world. Only the sunflower starfish is larger.
The crown-of-thorns starfish normally range in size from 25 to 35 cm. The sharp spines on the sides of the starfish's limbs resemble thorns and create a crown-like shape, giving the creature its name. The thorns are very sharp and are capable of piercing through standard wetsuits and other clothing. They are also venomous. When the crown-of-thorns feed, they release a chemical which is known to attract more starfish to the area.It is also has very vibrant colours.
which can be released through its spines. Not only are the wounds themselves serious, but the neurotoxin can cause a sharp stinging pain
that can last for hours, as well as nausea
and vomiting
.
Frequently, the area around the wound turns a dark blue and begins to swell (edema
), which may persist for weeks.
The spine(s) themselves may break off and become embedded inside the skin. This can lead to infection, and increased toxicity.
predator that preys on reef coral polyps. They climb onto reef structures, and then extrude their stomach onto the coral. This releases digestive enzymes that allow the starfish to absorb nutrients from the liquefied coral tissue. The crown-of-thorns also preys on brittle star
s in a similar fashion.
They are voracious predators. An individual starfish can consume up to 6 square metres (64.6 sq ft) of living coral reef per year.
During times of food shortage the crown-of-thorns can survive on energy reserves for over six months.
(a mollusc) and the harlequin shrimp attack and feed on crown-of-thorns starfish. Some large reef fish, particularly humphead wrasse
, may also prey on the starfish.
Sea star larvae are plankton
ic, so the major population control of the species comes from planktonic predation of junior species members.
In an interesting reversal, a large solitary coral polyp of the genus Pseudocorynactis
has been observed attacking, and then wholly ingesting a crown-of-thorns starfish of similar size.
Decline in predator populations (through overharvesting, habitat destruction
) has also been offered as an explanation for increasing outbreaks of crown-of-thorns starfish.
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 bloom
s 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.
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. To control areas of high infestations, teams of divers have had kill rates of up to 120 per hour per diver. The practice of dismembering them was shown to have a kill rate of 12 per hour per diver and the diver performing this test was spiked 3 times. Therefore, it is for this reason and not rumors that they might be able to regenerate that dismembering is not recommended.
An even more labor intensive route, but less risky to the diver, is to bury them under rocks or debris. This route is only suitable for areas with low infestation and if materials are available to perform the procedure without damaging corals.
published in September 2008 suggest that the crown-of-thorns starfish is actually constituted of four species (or subspecies – given that the genus
Acanthaster is otherwise monotypic
, treatment as allopatric species is preferrable), with distinct distributions in the Red Sea, Pacific, Northern and Southern Indian Oceans.
Differences between these putative species in behaviour, diet, or habitat may be important for the design of appropriate reef conservation strategies.
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...
that preys upon coral polyps. The crown-of-thorns receives its name from venom
Venom
Venom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite or a sting...
ous thorn-like spines
Spine (zoology)
A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...
that cover its body. It is endemic to tropical coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
s in the Red Sea
Red Sea
The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...
, the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
, and the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
. As solitary animals, they feed alone and maintain constant distance between themselves and other members of their species.
The crown-of-thorns is the second largest sea star in the world. Only the sunflower starfish is larger.
Physical description and physiology
Venom
The crown-of-thorns produces a neurotoxinNeurotoxin
A neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...
which can be released through its spines. Not only are the wounds themselves serious, but the neurotoxin can cause a sharp stinging pain
Pain
Pain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...
that can last for hours, as well as nausea
Nausea
Nausea , is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It often, but not always, precedes vomiting...
and vomiting
Vomiting
Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...
.
Frequently, the area around the wound turns a dark blue and begins to swell (edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...
), which may persist for weeks.
The spine(s) themselves may break off and become embedded inside the skin. This can lead to infection, and increased toxicity.
Behavior
The crown-of-thorns is a corallivore, a carnivorousCarnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
predator that preys on reef coral polyps. They climb onto reef structures, and then extrude their stomach onto the coral. This releases digestive enzymes that allow the starfish to absorb nutrients from the liquefied coral tissue. The crown-of-thorns also preys on brittle star
Brittle star
Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to in length on the largest specimens...
s in a similar fashion.
They are voracious predators. An individual starfish can consume up to 6 square metres (64.6 sq ft) of living coral reef per year.
During times of food shortage the crown-of-thorns can survive on energy reserves for over six months.
Natural predators
Venomous, sharp spines cover nearly the entire surface of the crown-of-thorns. These natural defenses make it a very unattractive target for most other reef predators. In spite of this, the Giant TritonTriton (mollusk)
Triton is the common name given to a number of very large sea snails, predatory marine gastropods in the genus Charonia. The name "triton" is also often applied as part of the common name, to other, much smaller sea snails of other genera within the same family, Ranellidae.Tritons are named after...
(a mollusc) and the harlequin shrimp attack and feed on crown-of-thorns starfish. Some large reef fish, particularly humphead wrasse
Humphead wrasse
The humphead wrasse is a wrasse that is mainly found in coral reefs in the Indo-Pacific region. It is also known as the Māori wrasse, Napoleon wrasse, Napoleonfish; or "So Mei" 蘇眉 and "Mameng" ....
, may also prey on the starfish.
Sea star larvae are plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic, so the major population control of the species comes from planktonic predation of junior species members.
In an interesting reversal, a large solitary coral polyp of the genus Pseudocorynactis
Pseudocorynactis
Pseudocorynactis is a genus of anthozoans similar in appearance to sea anemones and in body format to scleractinian stony corals. These animals are cnidarians in the family Corallimorphidae.-Species:Species so far described in this genus include:...
has been observed attacking, and then wholly ingesting a crown-of-thorns starfish of similar size.
Decline in predator populations (through overharvesting, habitat destruction
Habitat destruction
Habitat destruction is the process in which natural habitat is rendered functionally unable to support the species present. In this process, the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction by human activity mainly for the purpose of...
) has also been offered as an explanation for increasing outbreaks of crown-of-thorns starfish.
Ecological impact
Great Barrier Reef
The Great Barrier Reef is the world'slargest reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometres over an area of approximately...
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
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
, driving ecological succession
Ecological succession
Ecological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...
. 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
Coral bleaching
Coral bleaching is the loss of intracellular endosymbionts through either expulsion or loss of algal pigmentation.The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with unicellular flagellate protozoa, called zooxanthellae, that...
or Black band disease
Black band disease
Black band disease is characterized by complete coral tissue degradation due to a pathogenic microbial consortium that appears as a dark red or black migrating microbial mat. The mat is present between apparently healthy coral tissue and freshly exposed coral skeleton.-Appearance: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 bloom
Algal bloom
An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration...
s 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
Tubbataha Reef
Tubbataha Reef is an atoll coral reef located in the Sulu Sea of the Philippines. It is a marine sanctuary protected as Tubbataha Reef National Marine Park...
, 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 control
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 SCUBAScuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....
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. To control areas of high infestations, teams of divers have had kill rates of up to 120 per hour per diver. The practice of dismembering them was shown to have a kill rate of 12 per hour per diver and the diver performing this test was spiked 3 times. Therefore, it is for this reason and not rumors that they might be able to regenerate that dismembering is not recommended.
An even more labor intensive route, but less risky to the diver, is to bury them under rocks or debris. This route is only suitable for areas with low infestation and if materials are available to perform the procedure without damaging corals.
Taxonomy
The crown-of-thorns sea star has generally been considered as a single widespread species: A. planci. However, results from DNA analysesGenetic analysis
Genetic analysis can be used generally to describe methods both used in and resulting from the sciences of genetics and molecular biology, or to applications resulting from this research....
published in September 2008 suggest that the crown-of-thorns starfish is actually constituted of four species (or subspecies – given that the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Acanthaster is otherwise monotypic
Monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...
, treatment as allopatric species is preferrable), with distinct distributions in the Red Sea, Pacific, Northern and Southern Indian Oceans.
Differences between these putative species in behaviour, diet, or habitat may be important for the design of appropriate reef conservation strategies.
External links
- An overview of the crown-of-thorns starfish as observed on the Great Barrier Reef
- Current status of crown-of-thorns starfish on Australia's Great Barrier Reef
- History of outbreaks of crown-of-thorns starfish on Australia's Great Barrier Reef since 1986 (Animation)
- Recent information on crown-of-thorns starfish on Australia's Great Barrier Reef
- Controlling crown-of-thorns starfish
- Crown-of-thorn outbreak simulation
- Microdocs: Crown-of-thorns
- Photographs and text, Australia 1971