Acanthaster planci, commonly known as the crown-of-thorns starfish, is a large multi-armed starfish (or seastar) that usually preys upon hard,or stony, coral polyps (Scleractinia). The crown-of-thorns receives its name from poisonous thorn-like spines that cover its upper surface. It is the second largest sea star in the world. Only the sunflower seastar (Pycnopodia helianthoides) is larger.
A. plancí has a very wide Indo-Pacific distribution. It occurs at tropical and subtropical latitudes from the Red Sea and the east African coast across the Pacific Ocean, across the Indian Ocean to the west coast of Central America. It occurs where there are coral reefs or hard coral communities in this region.
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. 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). They have up to 21 arms They are usually of subdued colours, pale brown to grey-green, but they may be more brightly coloured in some parts of their wide distribution.
The long 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). Despite the battery of sharp spines on the aboral surface and blunt spines on the oral surface, the crown-of-thorns starfish's general body surface is membranous and soft. Field collection without damaging them requires careful handling with something like a blunt rod. When the starfish are removed from the water, the body surface ruptures and the body fluid leaks out so that the body collapses and flattens. The spines bend over and flatten, and the starfish becomes a sorry sight. They recover their shape when re-immersed if not left out of water to die. Handling the starfish in an aquarium system during experiments was done carefully by hand under the oral surface
The family Acanthasteridae is monogeneric whose position within the Asteroides is unsettled. It is generally recognised as a distinctly isolated taxon. Recently Blake concluded from comparative morphology studies of Acanthaster planci that it has strong similarities with various members of the Oreasteridae. He transferred Acanthasteridae from the Spinulosida to the Valvatida and assigned it a position near to the Oreasteridae, from which it appears to be derived He attributed Acanthaster morphology as possibly evolving in association with its locomotion over irregular coral surfances in higher energy environments. There is a complication, however, in that Acanthaster in not a monospecific genus and any consideration of the genus must also take into account another species, Acanthaster brevispinus, which lives in a completely different environment. A. brevispinus lives on soft-substrates, perhaps buried in the substrate at times like other soft substrate-inhabiting starfish, at moderate depths where presumably the surface is regular and there is little wave action.
Genus and species
The crown-of-thorns sea star has generally been considered as a single widespread species: Acanthaster 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. Differences between these putative species in behaviour, diet, or habitat may be important for the design of appropriate reef conservation strategies.
Acanthaster planci has a long history in the scientific literature with great confusion in the generic and species names from the outset. There is a long list of complex synonymy As a very distinctive starfish, it is not surprising that it was first described in 1705. Rhumphius used the name Stella marina quindecium radiotorum. Later, Linnaeus described it as Asterias planci, baed on an illustrataion by Plancus and Gualtieri (1743), when he introduced his system of binomial nomenclature. Subsequent generic names used for the crown-of-thorns starfish included Stellonia, Echinaster and Echinites before settling on Acanthaster (Gervais 1841). Species names included echintes, solaris, mauritensis, elisii and elisii pseudoplanci (with subspecies). Most of these names arose from confusion in the historical literature, but Acanthaster elissii came to be used for the distinctive starfish in the eastern Pacific Gulf of California.
Ecology and biology
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. 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 immediately causes a sharp stinging pain that can last for several hours, persistent bleeding due to the haemolytic effect of saponins, and nausea and tissue swelling that may persist for a week or more. These are personal experiences and observations of colleagues researching the starfish over a number of years. The spines, which are brittle, may also break off and become embedded in the tissue where they must be removed in a surgery.
It seems that saponins occur through the life-cycle of the crown-of-thorns starfish. There are saponins in the eggs that are similar to those in the adult tissues and presumably these carry over to the larvae Mouthing behaviour of predators to juvenile starfish suggests that they contain saponins.
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 fits closely to the surface of the coral, even the complex surfaces of branching corals. It then extrudes its stomach out through its mouth over the surface 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 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. In a study of feeding rates on two coral reefs in the Central Great Barrier Reef region, it was found that large starfish (40 cm and greater diameter) killed approximately 161 cm²/day in winter and 357–478 cm²/day in summer. Smaller starfish 20–39 cm killed 155 and 234 cm²/day in the equivalent seasons. The area killed by the large starfish is equivalent to about 10 square metres (110 sq ft) from these observations. Differences in feeding, and locomotion, rates between summer and winter reflect the fact that the crown-of-thorns like all marine invertebrates is a poikilotherm whose body temperature and metabolic rate are directly affected by the temperature of the surrounding water.
The starfish show preferences between the hard corals that they feed on. They tend to feed on branching corals and table-like corals, such as Acropora species, rather than on more rounded corals with less exposed surface area, such as Porites species Avoidance of Porites and some other corals may also be due to resident bivalve molluscs and polychaete worms in the surface of the coral which dicourage the starfish. Similarly, some symbionts, such as small crabs, living within the complex structures of branching corals may ward off the starfish as it seeks to spread its stomach over the coralsurface.
In reef areas of low densities of hard coral, reflecting the nature of the reef community or due to feeding by high density crown-of-thorns, the starfish may be found feeding on soft corals (Alcyonacea).
The starfish are cryptic in behavior during their first two years, emerging at night to feed. They usually remain so as adults when solitary. 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 early to midsummer, the starfish may gather together high on a reef and synchronously release gametes to achieve high levels of egg fertilisation This pattern of synchronised spawning is not at all unique, but it is very common amongst marine invertebrates that don't copulate. Solitary spawning gives no opportunity for fertisation of eggs and wastes gametes and there is evidence of a spawning pheromone that causes the starfish to aggregate and release gametes synchronously
- When the starfish are at high densities they may move day and night competing for living coral.
The elongated sharp spines covering nearly the entire upper surface of the crown-of-thorns serve as a mechanical defense against large predators. It also has a chemical defense. Saponins presumably serve as an irritant when the spines pierce a predator, in the same way as they do when they pierce the skin humans. Saponins have an unpleasant taste. A study to test the predation rate on juvenile Acanthaster by appropriate fish species found that the starfish were often mouthed, tasted and rejected. These defenses tend to make it an unattractive target for coral community predators. In spite of this, however, Acanthaster populations are typically composed of a proportion of individuals with regenerating arms.
A variety of about eleven species have been reported to prey occasionally on uninjured and healthy adult A. planci. All of these are generalist feeds and none of these, however, seems to specifically prefer the starfish a food source. This number, however, is probably lower as some of these presumed predators have not been witnessed reliably in the field. Some off those witnessed are:
- A species of puffer-fish and two trigger-fish have been observed to feed on crown-of-thorns starfish in the Red Sea, and, although they may have some effect on the A. planci population, there is no evidence of systematic predation
- The triton, a very large gastropod mollusc, is a known predator of Acanthaster in some parts of the starfish' range. The triton has been described as tearing the starfish 'to pieces with its file-like radula'.
- A small shrimp, the painted shrimp Hymenocera picta, a general predator of starfish, has been found to prey on Acanthaster planci at some locations. A polychaete worm, Pherecardia striata was observed to be feeding on the starfish together with the shrimp on an east Pacific coral reef About 0.6% of the starfish in the reef population were being attacked by both the shrimp and polychaete worm, killing the starfish in about a week. Glynn suggested that this resulted in a balance between mortality and recruitment in this population, leading to a relatively stable population of starfish.
- Since Pherecardia striata can only attack a damaged A. planci and cause its death, it may be regarded as an 'impatient scavenger' rather than a predator As distinct from predators, dead and mutilated adult A. planci attract a number of scavengers. Glynn lists two polychaete worms, a hermit crab, a sea urchin and seven species of small reef fish. Apparently they are able to tolerate the distasteful saponins for an easy meal.
- A large polyp-like creature of the genus Pseudocorynactis has been observed attacking, and then wholly ingesting a crown-of-thorns starfish of similar size.
Gametes and embryos
Metamorphosis and early juvenile development
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 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. 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 also "promote transmission" of some coral diseases.
Large populations of crown-of-thorns starfish (sometime emotively known as ‘plagues’) have been substantiated as occurring at twenty one locations of coral reefs during the 1960s to 1980s. These locations ranged from the Red Sea through the tropical Indoo-Pacific region to French Polynesia. There were at least two substantiated substantiated outbreaks at ten of these locations.
Values of starfish density from 140/ha to 1,000/ha have been considered in various reports to be outbreak populations, while starfish densities less than 100/ha have been considered to be low. From the surveys of many reef locations throughout the starfish's distribution large abundances of Acanthaster can be categorised as:
- Primary outbreaks where there are abrupt population increases of at least two magnitudes that cannot be explained by the presence of a previous outbreak.
- Secondary outbreaks that can plausibly be related to previous outbreaks through the reproduction of a previous cohort of the starfish. These may appear as recruits to reefs down current from an existing outbreak population.
- Chronic situations where there is a persistent moderate to high density population at a reef location where the coral is sparse due to persistent feeding by the starfish.
The Great Barrier Reef is the most outstounding coral reef system in the world because of its great length, number of individual reefs and species diversity. When high densities of Acanthaster which were causing heavy mortality of coral were first seen about Green Island, off Cairns, in 1960-65 there was considerable alarm. High density populations were subsequently found of a number of reefs to the south of Green Island, in the Central Great Barrier Reef region Some popular publications suggested that the whole Reef was in danger of dying: 'Requiem for the Reef' and 'Crown of Thorns: The Death of the Barrier Reef?'. They influenced and reflected some public alarm over the state and future of Great Barrier Reef.
There have been a number of studies modelling the population outbreaks on the Great Barrier Reef as a means to understand the phenomenon. These are examples
The Australian and Queensland governments funded research and set up advisory committees during the period of great anxiety about the nature of the starfish outbreaks on the GBR. They were regarded as not coming to terms with the unprecedented nature and magnitude of this problem and the two references above. Many scientists were criticised for not being unable to give definitive but unsubstnatiated answers. Others were more definitive in their answers  Scientists were criticised for their reticence and for disagreeing on the nature and causes of the outbreaks on the GBR, hence the publication 'Starfish Wars' (cf. 'Star Wars').
Causes of population outbreaks
There was serious discussion and some strongly held views about the causes of this phenomenon. Some hypotheses focused on changes in the survival of juvenile and adult starfish - the "predator removal hypothesis":
- over-collecting of tritons, a predator of the starfish 
- overfishing of predators of the starfish 
Many of the reports of fish preying on "Acanthaster" are single observations or presumed predation from the nature of the fish. For example the humphead wrasse may prey on the starfish amongst its more usual diet. Individual puffer fish and trigger-fish have been observed to feed crown-of-thorns starfish in the Red Sea, but there is no evidence that they are a significant factor in population control. A study, however, based on the stomach contents of large carnivorous fish that are potential predators of the starfish found no evidence of the starfish in the fish's guts. These carnivorous fish were caught commercially on the coral reefs on the Gulf of Oman and examined at local fish markets.
One problem with the concept of predators of large juvenle and adult starfish causing total mortality is that the starfish have good regenerative powers and they wouldn't keep still while being eaten. Also, they would need to be consumed completely or almost completely to die. In fact, 17-60% of starfish in various populations had missing or regenerating arms. Clearly the starfish experience various levels of sublethal predation. When the damage includes a major section of the disk together with arms, the number of arms regerating on the disk may be less than the number lost.
Another hypothesis is the "aggregation hypothesis", whereby large aggregations of A. planci appear as apparent outbreaks because they have consumed all the adjacent coral.
Female crown-of-thorns starfish are very fecund. Based on the eggs in ovaries, 200, 300 and 400 mm diameter females potentially spawn 4, 30 and 50 million eggs, respectively Lucas adopted a different approach, focusing on the survival of the larvae arising from the eggs The rationale for this approach was that small changes in the survival of larvae and developmental stages would result in very large changes in the adult population. Considering two hypothetical situations.
Twenty million eggs, from a female spawning and having a survival rate of about 0.00000001% throughout development would replace two adult starfish in a low-density population where the larvae recruit. If, however, the survival rate increases to 0.1% (one in a thousand) throughout development from one spawning of 20 million eggs this would result in 20,000 adult starfish where the larvae have recruited. Since the larvae are the most abundant stages of development it is likely that changes in survival will be most importance during this phase of development.
Temperature and salinity have little affect on the survival of crown-of-thorns larvae However, abundance and species of the particular component of phytoplankton (unicellular flagellates) on which the larvae feed has a profound effect on survival and rate of growth. The abundance of phytoplankton cells is especially important As autotrophs, phytoplankton abundance is strongly influenced by the concentration of inorganic nutrients, such as nitrogenous compounds.
Birkeland had observed a correlation between the abundance of crown-of-thorns on reefs adjacent to land masses. These occurred on mainland islands as distinct from coral atolls about three years after heavy rainfall that followed a period of drought  He suggested that runoff from such heavy rainfall may stimulate phytoplankton blooms of sufficient size to produce enough food for the larvae of A. planci through input of nutrients.
Combining Birkeland observations with the influence of inorganic nutrients on survival of the starfish larvae in experimental studies, gave support for a mechanism for starfish outbreaks:
- increased terrestrial runoff → increased nutrients denser phytoplankton↑→ better larval survival increased starfish populations
There is also a flow-on effect in that where there are large starfish populations producing large numbers of larvae, there is likely to be heavy recruitment on reefs downstream to which the larvae are carried and then settle.
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 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. 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.
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