Acanthaster planci, commonly known as the crown-of-thorns starfish, is a large multi-armed starfish that usually preys upon stonycoral polyps. The crown-of-thorns receives its name from venomous thorn-like spines that cover its upper surface. It is endemic to tropical coral reefs in the Red Sea, the Indian Ocean, and the Pacific Ocean.

The crown-of-thorns is the second largest sea star in the world. Only the sunflower seastar (Pycnopodia helianthoides) is larger.

Physical description

Crown-of-thorns starfish

The body form of the crown-of-thorns starfish is fundamentally the same as that of a typical starfish. Its special traits, however, include being disc-shaped, multi-armed, flexible, prehensile and heavily spined, and having a large ratio of stomach surface to body mass.^[1] Its prehensile ability arises from the two rows of numerous tube feet that extend to the tip of each arm. In being multi-armed it has lost the five-fold symmetry, pentamerism, typical of starfish although it begins with this symmetry in its life cycle.

Adult crown-of-thorns starfish normally range in size from 25 to 35 cm (9.8 to 14 in).^[2] They have up to 21 arms^[3] The sharp spines on the sides of the starfish's arms and upper (aboral) surface resemble thorns and create a crown-like shape, giving the creature its name. The spines are stiff and very sharp and readily pierce through soft surfaces (below).

Adults are usually of subdued colours, pale brown to grey-green, but they may be more brightly coloured in some parts of their wide distribution.^[1]

Toxins

Starfish are characterised by having saponins in their tissues and these are known as 'asterosaponins'. Starfish contain a mix of these saponins and there have been at least 15 chemical studies seeking to characterise the sapononis in the crown-of-thorns starfish.^[1] A. planci has no mechanism for injecting the toxin, but, as the spines perforate tissue of a predator or unwary person, tissue containing the saponins is lost into the wound. In humans this causes a sharp stinging pain that can last for hours, persistent bleeding due to the haemolytic effect of saponins as well as nausea, and tissue swelling (oedema) that may persist for a week or more. These are personal experiences and observations of colleagues researching the starfish over a number of years.^[1] The spines may also break off and become embedded in the tissue where they must be removed in a surgery.

Behavior

The adult crown-of-thorns is a carnivorous predator that usually preys on reef coral polyps. It climbs onto a section of living coral colony using the large number of tube feet on its oral surface and flexible body. It then extrudes its stomach out through its mouth over the surface onto the coral to virtually its own diameter. The stomach surface secretes digestive enzymes that allow the starfish to absorb nutrients from the liquefied coral tissue. This leaves a white scar of coral skeleton which is rapidly infested with filamentous algae^[4] so that the original attractive reef surface is replaced by a dull surface of algae. They are voracious predators. An individual starfish can consume up to 6 square metres (65 sq ft) of living coral reef per year.^[5].

The starfish are cryptic in behavior in their first two years, emerging at night to feed. They usually may remain so as adults when living solitarily. The only evidence of a hidden individual may be white feeding scars on adjacent coral. However, their behavior changes under two circumstances. During the breeding season, which is typically during summer, the starfish may gather together on top of a reef and synchronously release gametes to achieve high levels of egg fertilisation. The other circumstance is when the starfish are at high densities, that is ‘plague’ levels, and they move day and night competing for living coral. They consume large areas of coral at high densities, leaving large areas of dead coral skeletons. As these are infested with algae, the effect is to completely change the surface and appearance of a reef.

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 Triton (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.^[6]

Sea star larvae are planktonic, 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.^[7]

Decline in predator populations (through overharvesting, habitat destruction) has also been offered as an explanation for increasing outbreaks of crown-of-thorns starfish.

Ecological impact

Crown-of-thorns amongst coral reef, off the coast of Madagascar

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.^[8]

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 blooms 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.^[9] 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.^[10]

The crown-of-thorns starfish may "promote transmission" of some coral diseases.^[11]

Crown-of-thorns starfish in French Polynesia

Population control

Population numbers for the crown-of-thorns have been increasing since the 1970s.^{[citation needed]} However, historic records distribution patterns and numbers are hard to come by, as SCUBA 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.^[12] To control areas of high infestations, teams of divers have had kill rates of up to 120 per hour per diver.^[12] 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 analyses 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 preferable), with distinct distributions in the Red Sea, Pacific, Northern and Southern Indian Oceans.^[13]

Differences between these putative species in behaviour, diet, or habitat may be important for the design of appropriate reef conservation strategies.^[14]

See also

• Sea Turtle Association of Japan, Kuroshima Research Station

References