Crown-of-throns starfish

Appearance

The body form of the crown-of-thorns starfish is fundamentally the same as that of a typical starfish, with a central disk and radiating arms. Its special traits, however, include being disc-shaped, multiple-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 multiple-armed, it has lost the five-fold symmetry typical of starfish, although it begins its life cycle with this symmetry.

Adult crown-of-thorns starfish normally range in size from 25 to 35 cm . They have up to 21 arms. 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."Crown of Thorns Starfish ." Crown of Thorns Starfish Videos, Photos and Facts. Web. 5 November 2014. . They are usually of subdued colours, pale brown to grey-green, but they may be garish with bright warning colours in some parts of their wide range.

The long, sharp spines on the sides of the starfish's arms and upper 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. 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. When the starfish is removed from the water, the body surface ruptures and the body fluid leaks out, so the body collapses and flattens. The spines bend over and flatten, as well. They recover their shape when reimmersed, if they are still alive.

Naming

''Acanthaster planci'' has a long history in the scientific literature with great confusion in the generic and species names from the outset, with a long list of complex synonyms. 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'', based on an illustration by Plancus and Gualtieri , when he introduced his system of binomial nomenclature. No type specimens are known; the specimen described by Plancus and Gualtieri is no longer extant.

Subsequent generic names used for the crown-of-thorns starfish included ''Stellonia'', ''Echinaster'' and ''Echinites'', before settling on ''Acanthaster'' . Species names included ''echintes'', ''solaris'', ''mauritensis'', ''ellisii'', and ''ellisii pseudoplanci'' . Most of these names arose from confusion in the historical literature, but ''Acanthaster ellisii'' came to be used for the distinctive starfish in the eastern Pacific Gulf of California.

The eastern Pacific ''Acanthaster'' is very distinctive with its rather 'plump' body, large disk to total diameter ratio and short, blunt spines. It gives the impression of living in a habitat where having sharp defenses against predators has little value, although it lives on coral reef and feeds on coral.

Distribution

''A. planci'' has a very wide Indo-Pacific distribution. It is perhaps most common in Australia, but can occur at tropical and subtropical latitudes from the Red Sea and the east African coast across the Indian Ocean, and across the Pacific Ocean to the west coast of Central America. It occurs where coral reefs or hard coral communities occur in this region.

Large populations of crown-of-thorns starfish 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 Indo-Pacific region to French Polynesia. There were at least two substantiated repeated 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; however, at densities below 100/ha there may be feeding by A. planci that exceeds the growth of ''coral'' and there is net loss of coral.

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 outstanding 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 modeling the population outbreaks on the GBR as a means to understand the phenomenon, e.g.

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 able to give definitive but unsubstantiated 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' .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
⤷  decline in predator populations through habitat destruction
⤷  warmer sea temperatures enhance larvae development

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 juvenile 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. 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 regenerating 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. This seems to imply that there is apparently a dense population outbreak when there has already been a more diffuse population outbreak that has been dense enough to comprehensively prey on large areas of hard coral.

Female crown-of-thorns starfish are very fecund. Based on the eggs in ovaries, 200, 300 and 400 mm diameter females potentially spawn approximately 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% 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 effect on the survival of crown-of-thorns larvae. However, abundance and species of the particular component of phytoplankton 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 have been further conformations of these connections, however research by Olson , Kaufmann , and Byrne suggests terrestrial runoff has little or no impact on larval survival. The conflicting data describing the negligible role of terrestrial agricultural runoff has been described as "an inconvenient study".

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 of 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. Manual removals have been successful, but are relatively labour-intensive. 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 three times. Therefore, it is for this reason and not rumours that they might be able to regenerate that dismembering is not recommended.

An even more labour-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.

A 2015 study by James Cook University showed that common household vinegar is also effective, as the acidity causes the starfish to disintegrate within days. Vinegar is also harmless to the environment, and is not restricted by regulations regarding animal products such as bile.

A new successful method of population control is by the injection of thiosulfate-citrate-bile salts-sucrose agar . Only one injection is needed, leading to the organism's death in 24 hours from a contagious disease marked by "discoloured and necrotic skin, ulcerations, loss of body turgor, accumulation of colourless mucus on many spines especially at their tip, and loss of spines. Blisters on the dorsal integument broke through the skin surface and resulted in large, open sores that exposed the internal organs."

An autonomous starfish-killing robot called COTSBot has been developed and as of September 2015 was close to being ready for trials on the Great Barrier Reef. The COTSbot, which has a neural net-aided vision system, is designed to seek out crown-of-thorns starfish and give them a lethal injection of bile salts. After it eradicates the bulk of the starfish in a given area, human divers can move in and remove the survivors. Field trials of the robot have begun in Moreton Bay in Brisbane to refine its navigation system, according to Queensland University of Technology researcher Matthew Dunbabin. There are no crown-of-thorns starfish in Moreton Bay, but when the navigation has been refined, the robot will be used on the reef.

Behavior

The adult crown-of-thorns is a corallivorous 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. An individual starfish can consume up to 6 square metres of living coral reef per year. In a study of feeding rates on two coral reefs in the central Great Barrier Reef region, large starfish killed about 61 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 m2 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. In tropical coral reefs, crown-of-thorns specimens reach mean locomotion rates of 35 cm/min, which explains how outbreaks can damage large reef areas in relatively short periods.

The starfish show preferences between the hard corals on which they feed. 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 discourage 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 coral surface.

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 .

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 do not copulate. Solitary spawning gives no opportunity for fertilisation of eggs and wastes gametes and evidence exists 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.

Reproduction

By Day 1 the embryo has hatched as a ciliated gastrula stage . By Day 2 the gut is complete and the larva is now known as a bipinnaria . It has ciliated bands along the body and uses these to swim and filter feed on microscopic particles, particularly unicellular green flagellates . The SEM photograph is a scanning electron micrograph, which clearly shows the complex ciliated bands of the bipinnaria larva. By Day 5 it is an early brachiolaria larva. The arms of the bipinnaria have further elongated, there are two stump-like projections in the anterior and structures are developing within the posterior of the larva. In the late brachiolaria larva the larval arms are elongate and there are three distinctive arms at the anterior with small structures on their inner surfaces . To this stage the larva has been virtually transparent, but the posterior section is now opaque with the initial development of a starfish. The late brachiolaria is 1-1.5 mm. It tends to sink to the bottom and test the substrate with its brachiolar arms, including flexing the anterior body to orient the brachiolar arms against the substrate.

This description and assessment of optimum rate of development is based on early studies in the laboratory under attempted optimum conditions. However, not unexpectedly, there are large differences in growth rate and survival under various environmental conditions .

Predators

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 of 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 11 species have been reported to prey occasionally on uninjured and healthy adults of ''A. planci''. All of these are generalist feeders and none of these, however, seems to specifically prefer the starfish as a food source. This number, however, is probably lower, as some of these presumed predators have not been witnessed reliably in the field. Some of those witnessed are:
⤷  A species of pufferfish and two triggerfish 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, no evidence exists of systematic predation. In the indo-pacific waters, white-spotted puffers, and Titan triggerfish have also been found to eat COTS
⤷  The Triton's trumpet, a very large gastropod mollusc, is a known predator of ''Acanthaster'' in some parts of the starfish's range. The triton has been described as tearing the starfish to pieces with its file-like radula.
⤷  The small painted shrimp ''Hymenocera picta'', a general predator of starfish, has been found to prey on ''A. 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 this resulted in a balance between mortality and recruitment in this population, leading to a relatively stable population of starfish.
⤷  Since ''P. 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 adults of ''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 cnidarian genus ''Pseudocorynactis'' was observed attacking, and then wholly ingesting a crown-of-thorns starfish of similar size. Continued studies revealed this polyp is able to completely ingest a crown-of-thorns specimen of up to 34 cm in diameter.

Uses

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
⤷  decline in predator populations through habitat destruction
⤷  warmer sea temperatures enhance larvae development

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 juvenile 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. 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 regenerating 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. This seems to imply that there is apparently a dense population outbreak when there has already been a more diffuse population outbreak that has been dense enough to comprehensively prey on large areas of hard coral.

Female crown-of-thorns starfish are very fecund. Based on the eggs in ovaries, 200, 300 and 400 mm diameter females potentially spawn approximately 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% 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 effect on the survival of crown-of-thorns larvae. However, abundance and species of the particular component of phytoplankton 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 have been further conformations of these connections, however research by Olson , Kaufmann , and Byrne suggests terrestrial runoff has little or no impact on larval survival. The conflicting data describing the negligible role of terrestrial agricultural runoff has been described as "an inconvenient study".

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.