Acanthaster planci, commonly known as the crown-of-thorns starfish, is one of the largest starfish in the world. Yet, what makes A. planci unique is not just the fact that it is large, but the fact that it has an interesting diet. In particular, A. planci enjoys feeding on the coral reefs that dot the Pacific and Indian Oceans. This in and of itself wouldn’t be very interesting, except for the fact that A. planci is an incredibly efficient predator. In the late 1960’s up to ninety percent of the corals in affected areas off the coast of Guam were completely destroyed by A. planci predation (Colgan 1987). Some adult crown-of-thorns starfish can consume up to one metre squared per month of living corals (Chesher 1969)! A. planci also has a few interesting evolutionary adaptations, including venomous spines and a feeding method that includes everting its stomach through its mouth to digest prey (Ault et al 2011). Fascinatingly, A. planci has a very strange method of sexual reproduction that is based on spawning. Finally, A. planci has been the subject of intensive study over the last few decades, yet we still have no real idea as to what causes major infestations of A. planci, and we have barely any idea of how to control them. In this paper, we plan to spread some light on the life cycle, reproduction, feeding habits, predators, potential human causes, and human impacts of this obscure starfish.

The appearance of the crown-of-thorns starfish is characterized by a purplish-blue colour with multiple arms and venomous thorn-like spines about 4-5 cm in length (Lotha 2007). These spines are venomous due to the toxic compounds known as saponins which are released when the spine punctures its targets skin. The toxins cause a number of symptoms in humans such as intense pain, nausea, vomiting, and extreme swelling in the punctured area (Moran 1988). However, although saponin is toxic, the amount released from a spine is not always enough to cause the symptoms seen in humans and it is still unknown what other compound in the spine works in conjunction with the saponin to cause the effect (Ault et al 2011). Further analysis has yet to be done to determine the other compound(s) that work in conjunction with the saponin to produce their painful effect. Due to the detrimental effects of the toxins the spines are most likely a survival mechanism developed in order to ward off prey.

There are few organisms that feed on the starfish due to the danger of the the spines. Consequently, the majority of A. planci predation happens during the larval and juvenile stages before the spines fully develop (Moran 1988). However, some organisms have overcome the problem posed by the spines by different mechanisms of feeding. One of the starfishes predators is a shrimp about ten centimeters in length known as Hymenocera picta (Moran 1988). H. picta feeds on A.planci by flipping the starfish on its back and feeding on the less protected tissues underneath. This leads to the question of how evolution will accommodate for this shortfall in the future? Will evolution employ natural selection for starfish that have spines extending underneath A. planci’s bodies to aid survival and protection? This, however, is a question that cannot be fully answered as natural selection is unpredictable and the effects of it can only be seen in the future. However, the crown-of-thorns starfish already have a few mechanisms in place in order to escape one of their most common predator, Charonia tritonis (Chesher 1969). C.tritonis feeds on the crown-of-thorns starfish by using its foot to restrain the starfish and insert its proboscis (feeding organ) into the A. Planci in order to feed on its organs (Moran 1988). Yet, A. planci are often able to escape C.tritonis by leaving behind the part of body with which it was held (Moran 1988). This evidence is corroborated by the fact that an unusual number of crown-of-thorns starfish are found lacking one or more limbs (Ault et al 2011). This is an incredibly interesting survival mechanism, as the crown-of-thorns starfish is able to survive without a limb and function normally. Interestingly, the starfish will also not regenerate these lost parts over a long enough period of time as it is perfectly capable of surviving without its limb (Chesher 1969). A. planci reproduces in a similar fashion to many other organisms that do not copulate. This method is still a form of sexual reproduction known as spawning (Ault et al 2011). Spawning is a mating technique that involves both male and females releasing their gametes, sperm and ovum, respectfully in the water to be fertilized (Lotha 2007). This is not a very efficient method of reproduction as many of the gametes do not become fertilized (Gladstone 1992). To compensate for this, William Gladstone observed that the A. planci tends to gather together in groups to spawn, perhaps increasing the chance of fertilization (Gladstone 1992). The spawning of the crown-of-thorns starfish seem to be controlled by a number of factors including the time of day and time of year as well as the number of starfish present (Gladstone 1992). The starfish, although they can spawn throughout the entire year, have the highest rate of spawning occurring during December and January in the Great Barrier Reef, as that is when the water is the warmest (Babcock et al 1994). The time of the year is also important as it affects larval survival. In other times of the year there are more chances of cyclones and floods which would be detrimental to the survival of the larvae (Gladstone 1992). Spawning also is observed to occur during low tide as it gives the gametes a better chance of coming in contact with one another to become fertilized as there is a smaller gamete to water ratio (Babcock et al 1994). An interesting behaviour of the A. planci is that the amount of time it spends spawning is different between the sexes (Babcock et al 1994). Males tend to spawn for much longer times (over 40 minutes) and release extremely large amounts of sperm compared to females (who spend roughly 20 minutes) which release relatively few ovum compared to the males (Babcock et al 1994). The females also begin spawning later than the males do, perhaps to ensure that enough sperm are in the water to increase chances of fertilization. Another interesting behaviour of A. planci is that once a single starfish begins spawning it seems to attract other starfish which accumulate and spawn successively as well due to a release of pheromones in the gametes (Gladstone 1992). However, this behaviour can be wasteful as in many cases many males spawn together without any females present, leading to a massive waste of sperm (Babcock et al 1994). Fertilization rates are largely affected by the distances between spawning females and males and whether they are up or downstream (Babcock et al 1994). Babcock and his colleagues found that when a female A. planci is eight meters upstream from a spawning male the fertilization rate was only 0.7% compared to the 29.5% rate when eight meters downstream. When both sexes are spawning adjacent to one another in a large group the spawning rate increased immensely to 90.3% (Babcock et al 1994). When A.planci spawns many of their normal behaviours change (Beach et al 1975). A. Planci normally stays in covered areas, such as hidden within the coral during the day, and usually only come out in the open at night to avoid being exposed to the light. However, during the spawning season A.planci is seen on many occasions out during the day exhibiting a spawning behaviour (Beach et al 1975). This behaviour consists of all the A. planci’s arms rapidly moving with the starfish at the water surface partially inverted (Beach et al 1975). In some cases A. planci spawn at the top of a coral head with their bodies hunched over the coral top (Gladstone 1992).

Once fertilization is complete the egg forms into an embryo and eventually a larvae which is considered in a platonis stage as the starfish does not move, it simply floats and drifts in the ocean (Moran 1988). Naturally, it would be impossible for such a small organism that floats and drifts around to feed on its normal food source so at this stage of its life it only feeds on phytoplankton (Moran 1988). The larvae goes through several developmental stages as a larvae including, gastrula, bipinnaria and brachiaria (Ault et al 2011). While in the Brachiaria stage the larvae develops a structure known as a primordium which cues the organism to search for a place to settle to metamorphosize (Moran 1988). The primordium is used to help the larvae remain on a surface, such as a rock, while it undergoes a two day long metamorphosis into a juvenile A.planci. While in the juvenile stage the crown-of-thorns starfish have only five arms and are still too small (less than one centimeter) to feed on corals so they feed on algae (Ault et al 2011). Once the starfish reaches a size of about one centimeter tall is becomes large enough to be able to feed on coral. As A. planci feeds on the corals they begin to grow rapidly, until over the course of 2 years they grow to adult size, which is about twenty centimeters long (Moran 1988). A.planci, like humans, have a prime age for reproduction, which for A. planci is between the ages of two and three years old. Once they are three or four years old their growth and reproduction rates decrease dramatically (Moran 1988). The starfish however can live as long as 8 years in this senile stage. It is interesting to note how little time A. planci are able to reproduce effectively, out of their twelve to fifteen year lifespan they are only truly successfully reproductive for a couple years.

A. planci has evolved an interesting niche. In particular it primarily feeds on living coral reef. While it is far from the only species that feeds on corals, the alarming outbreaks of A. planci have left the scientific community bewildered. Whilst A. planci had been known to exist for quite some time, they were relatively rare up until the 1960’s (Chesher 1969). What is truly alarming is that the outbreaks of A. planci, something that is poorly understood, have the ability to devastate coral reefs. Before and after studies of an A. planci outbreak have given us some shocking observations. One study of an outbreak in French Polynesia was particularly interesting. Outbreaks tend to begin in the deep water understructure of dead corals and other rubble that make up the base of the coral reef (Kayal et al 2012). These areas offer plenty of shelter for the young starfish to feed and grow, particularly in their most vulnerable stage of development. As the density of sea stars increase during the early phases of an infestation, they begin to deplete the coral supply in the area of the immediate outbreak and begin to move outwards in a migratory wavefront (Kayal et al 2012). This migratory feeding has the capacity to devastate corals, and once the corals are destroyed, biodiversity in the ecosystem suffers as ancillary species leave due to lack of food or shelter (Sano et al 1984). Some studies have noted an over 90% decline in the coral cover in Guam, the Great Barrier Reef and in Japan (Leray et al 2012). Clearly, such a major impact on coral reef health and biodiversity is a serious concern.

The sudden outbreaks of A. planci have also had human impacts. The characteristic destruction of coral reefs threatens to harm the coral reef tourism industry, specifically along the Great Barrier Reef. While it remains unlikely that A. planci could destroy the whole Great Barrier Reef, certain areas that are major tourist attractions could be potentially devastated by an A. planci outbreak (Ault et al 2011). There is also concern that human activity may be a contributing cause for the periodic outbreaks. For example, one study examined the possibility that terrestrial runoff during the typhoon season after a record breaking drought could provide enough of an algal bloom that could sustain enough A. planci larvae to cause an outbreak (Birkeland 1982). Another theory put forward is that human activities, such as dredging and blasting can create new predator free zones that are perfect for A. planci larvae to grow in peace (Chesher 1969). However, the true definitive cause of A. planci outbreaks is simply unknown, and is one of the areas of potential study for this starfish in the near future. Another interesting question is what happens to the starfish after an outbreak? Well no one really knows. It has never been observed large numbers of the starfish dying on coral reefs during the later stages of an infestation (Moran 1988). So where do they go? Another interesting field of study that has yet to be explored.

With the massive increases in the population of A. planci, we are left wondering how to control this predatory starfish. What has happened to the natural predators of A. planci, who should have kept the populations in check in a normal system? One theory put forward was that the triton shell C. tritonis would normally be able to keep the A. planci population in check. However, human shell collectors prize C. tritonis for its shell, causing a major decline in their population (Chesher 1969). Richard Chesher carried out his own controlled test of the predation of A. planci by C. tritonis, and concluded that even if the C. tritonis were present in sufficient numbers, they would be unlikely to actually stop an A. planci outbreak. C. tritonis was simply too inefficient in its predation, as sometimes C. tritonis wouldn’t even kill the starfish (Chesher 1969)! More recently there has been discussion of using large commercial fishes introduced as predators of A. planci. However this method has been found to be inefficient (and also risky due to the possibility of introducing an invasive species) due to the simple fact that the fish do not consume A. planci juveniles at anything approaching a useful rate, and will mostly avoid adult A. planci (Sweatman 1994). One tried and true method of stopping an A. planci outbreak, if somewhat labour intensive, is the individual culling of starfish. A. planci is not particularly quick, which can enable a diver to capture and inject the starfish with an amount of formaldehyde, which is toxic to the crown-of-thorns starfish (Chesher 1969). In conclusion, the starfish A. planci is a very unique organism. Though its origins are obscure, the recent outbreaks and infestations caused by the crown-of-thorns starfish have had devastating impacts on certain coral reef ecosystems in the Pacific Ocean. Through its unique defences in the form of large venomous spines, to its interesting method of reproduction, something that seems almost alien to a human being, the A. planci has proved to be a fascinating species. Its life cycle exhibits some unique characteristics, like its specialized growth and reproductive cycles, and even its predators seem to be strangely adapted to preying on this creature as in the case of C. tritonis and H. picta. Yet while fascinating, it is not a species without specific impacts that have to potential to be detrimental to many human communities. The destruction of coral reefs, if they continue or increase in pace, will have economic impacts on island societies that depend on the coral reefs for both an important source of dietary protein, but also a tourism industry that generates major profits for many communities (Ault et al 2011). It should be important to note that the scientific community has not been idle over the last half century. A wealth of studies have been published which have studied the causes and effects, some of which may be human driven, that have lead to us to a deeper understanding of what effects this strange creature has on coral reefs. Finally, we have begun to develop measures to halt the destruction of the coral reef ecosystems by the crown-of-thorns starfish, but there clearly remains much work to be done.

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