Acanthaster planci (crown-of-thorns starfish)
- Summary of Invasiveness
- Taxonomic Tree
- Distribution Table
- Habitat List
- Biology and Ecology
- Means of Movement and Dispersal
- Pathway Vectors
- Impact Summary
- Risk and Impact Factors
- Prevention and Control
- Principal Source
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Acanthaster planci
Preferred Common Name
- crown-of-thorns starfish
International Common Names
- English: coral-eating starfish; coral-feeding starfish; crown of thorns starfish; giant thorny starfish
Local Common Names
- Palau: rrusech
Summary of InvasivenessTop of page
Coral gardens from Micronesia and Polynesia provide valuable marine resources for local communities and environments for native marine species such as marine fish. In coral ecosystems already affected by coral bleaching, excess tourism and natural events such as storms and El Nino, the effects of the invasive crown-of-thorns starfish (Acanthaster planci) on native coral communities contributes to an already dire state of affairs. Acanthaster planci significantly threatens the viability of these fragile coral ecosystems, and damage to coral gardens by the starfish has been quite extensive in some reef systems. Outbreaks in the Pacific appear to be more massive and widespread than those elsewhere. This may reflect different patterns of outbreak between Pacific and Indian Ocean populations, which have recently been shown to form separate clades of an A. planci species complex. (Vogler et al. 2008; and see 'Description' section).
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Echinodermata
- Class: Stelleroidea
- Subclass: Asteroidea
- Order: Valvatida
- Family: Acanthasteridae
- Genus: Acanthaster
- Species: Acanthaster planci
DescriptionTop of page
Recent molecular analysis has shown that Acanthaster planci is in fact a species complex consisting of four distinct clades from the Red Sea, the Pacific, the Northern and the Southern Indian Ocean. Benzie (1999) had previously demonstrated the genetic differentiation between A. planci from the Pacific and the Indian Ocean, and this genetic grouping is reflected in the distribution of colour morphs: grey-green to red-brown in the Pacific Ocean, and blue to pale red in the Indian Ocean (Benzie, 1999). Colour combinations can vary from purplish-blue with red tipped spines to green with yellow-tipped spines (Moran, 1997). Those on the Great Barrier Reef are normally brown or reddish grey with red-tipped spines, while those in Thailand are a brilliant purple (Moran, 1997). Adult A. planci usually range in diameter from around 20 to 30cm (PERSGA/GEF 2003) although specimens of up to 60cm (and even 80cm) in total diameter have been collected (Chesher, 1969; Moran, 1997). The juvenile starfish begins with 5 arms and develops into an adult with an astounding 16 to 20 arms, all heavily armed with poisonous spines 4 to 5cm in length, which can inflict painful wounds (Moran, 1997; Birk, 1979). Arm values vary between localities with a range of 14 to 18cm given for the Great Barrier Reef (Moran 1997). Starfish are usually concealed during daylight hours, hiding in crevices (Brikeland and Lucas, 1990; Chesher, 1969). Groups of starfish often move as huge masses of 20 to 200 individuals, presenting a terrifying "front" which destroys the reef as it moves through (Chesher, 1969). Signs of starfish presence are obvious; the coral skeleton is left behind as the result of starfish feeding and stands out sharply as patches of pure white, which eventually become overgrown with algae (Chesher, 1969). In some cases, herbivorous sea urchins move in to feed on algae, creating a pattern against the white coral that resembles the holes of swiss cheese (Tsuda et al. 1970).
DistributionTop of page
The geographical range of Acanthaster planci extends from Mauritius in the western Indian Ocean, including the Red Sea, across the Pacific to the west coast of America (Sladen 1889, in Birkeland and Lucus 1990).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 10 Jan 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Madagascar||Present||Invasive||Also found at the Mascarene Basin (Appeltans et al. 2012)|
|Mauritius||Present||Invasive||Also recorded at Cargados Carajos Shoals, northeast of Mauritius (Appeltans et al. 2012)|
|British Indian Ocean Territory|
|Malaysia||Present||Present based on regional distribution.|
|Philippines||Present||An outbreak in 2009 affected the Tubbataha reef, Sulu Sea; a UNESCO World Heritage Site (Bos, 2010).|
|United States||Present||Present based on regional distribution.|
|-Lord Howe Island||Present|
|New Zealand||Present||Present based on regional distribution.|
|Northern Mariana Islands||Present||Invasive|
|Papua New Guinea||Present||Invasive|
|Indian Ocean - Western||Present||Invasive|
|Pacific - Western Central||Present||Invasive|
HabitatTop of page
The crown-of-thorns starfish (A. planci) is limited by the location of its food source, coral, from just below spring tide level to a depth limit of 65 metres (Chesher, 1969). Soft substrate is avoided by the crown-of-thorns starfish as it lacks a gripping surface for the tube feet to hold on to (Chesher, 1969). In areas of strong wave action, sand can provide a barrier to movement of the starfish between reef patches (Chesher, 1969). The starfish prefers to live in more sheltered areas, such as lagoons, and in deeper water along reef fronts (Moran, 1997). They generally avoid shallow water on the tops of reefs, where the water conditions are likely to be more turbulent (Moran, 1997). When the weather is calm the potential range of the starfish increases and the starfish may cross sand patches and may feed in shallow water areas (Chesher, 1969; Moran, 1997).
Habitat ListTop of page
|Marine||Present, no further details||Harmful (pest or invasive)|
Biology and EcologyTop of page
Acanthaster planci larvae feed on phytoplankton (Birkeland, 1982) and dissolved organic matter (Hoegh-Guldberg, 1994). Once they have developed into juvenile starfish they feed on encrusting algae (Moran, 1997). Adult Acanthaster planci feed primarily on coral, hence one of its names (coral-feeding starfish). The starfish feeds on polyps of corals by everting its stomach and secreting enzymes (Birk, 1979). Other animals feed on coral but none so efficiently as Acanthaster planci (Chesher, 1969), which is aptly referred to as a "corallivore" and spends on average about 45% of its time feeding (De'ath and Moran, 1998). A single starfish of Acanthaster planci can graze ten square metres a year of coral (Vicente, 1999). Measurement of feeding rates of Acanthaster planci have shown that feeding rates in summer are about twice that in winter, but are significantly depressed following the summer spawning season (Keesing and Lucas, 1992). In the laboratory, specimens have eaten molluscs and echinoderms, however scleractinian corals are their primary prey (Chesher, 1969). Scleractinia is an order of coral known as stony or hard corals which is made up of 18 families. Preferred species in the Western Pacific include Montipora spp., Acropora spp. and other members in the Acroporidae and Pocilloporidae families (Colgan, 1987; Quinn and Kojis, 2003). Acropora gemmifera, A. nasuta, A. loripes, Seriatopora hystrix, Pocillopora damicornis and Stylophora pistillata are preferred species too, however, they are protected by mutualistic crustaceans (see notes) (Colgan 1987; Glynn, 1976, 1980, 1983, in Colgan, 1987; Pratchett, 2001). In French Polynesia, Acanthaster planci show a feeding preference for all growth-forms of Acropora as well as the genus Montipora and Pocillopora (Faure, 1989).
Sexes are separate and females release huge amounts of gametes directly into the sea (Benzie, 1999). An individual female Acanthaster planci can produce up to 60 million eggs per year (Conand, 1985, in Babcock and Mundy, 1992). If conditions are favourable and there is an abundant larval survival, the high reproductive potential of even a few adult A. planci may allow the production of a massive settlement of juveniles (Birkeland, 1982). According to data derived from one location in the Great Barrier Reef, Australia, major spawning occurred in December 1991, with smaller spawning events following in January (Babcock and Mundy, 1992). Over two-thirds of the population aggregate to participate in this spawning event, which usually occurs in the morning or afternoon and may be driven by pheromones released into currents (Babcock and Mundy, 1992). A. planci often spawns in a characteristic arched posture, usually on top of elevated rocks or corals at elevations of 30m to reefs flats (Babcock and Mundy, 1992). Migration to shallow water is commonly associated with A. planci spawning (Babcock et al. 1994). Babcock and Mundy (1992) recorded 47% fertilisation rates between animals separated by 32m and 23% for animals separated by over 60m. Fertilisation rates achieved are two orders of magnitude greater than those recorded for other marine organisms, due to the large amounts of gametes produced (Babcock and Mundy, 1992).
After the gametes (eggs and sperms) and hormones (which stimulate other individuals to release gametes) of A. planci are shed into the seawater the gametes have a short amount of time to become fertilised before they become unviable (Madl, 1998). After fertilisation, the zygote develops into a larva. After drifting around for two to three weeks, the 0.5mm small larva starts to morph and eventually settles and attaches itself to the sea floor where it completes its metamorphosis (Madl, 1998). Larval life may last longer than three weeks if conditions are unfavourable (Birkeland and Lucus, 1990, in Benzie, 1999). Various substrates, particularly crustose coralline algae with bacterial surface films, induce Acanthaster's planktonic larvae to settle and metamorphose (Johnson and Cartwright, 1996). One group of scientists found that thyroxine accelerates development in Acanthaster through larval stages (Johnson and Cartwright, 1996). After settlement, the larva metamorphoses into a juvenile starfish, a process which takes about two days (Moran, 1997). Initially the juvenile starfish has only five rudimentary arms, but additional arms develop rapidly as the starfish begins to feed on encrusting algae (Moran, 1997). At the end of six months, the starfish is about 1cm in size and begins to feed on corals (Moran, 1997). Individuals are able to reproduce after two years (Lucas, 1973, in Babcock and Mundy, 1992). Being a rapid grazer of coral polyps, it takes only three to four years for the crown-of-thorns starfish to reach a reasonable size of 30-35cm (Madl, 1998). After three to four years, it is thought to go into a senile phase where growth declines dramatically and reproduction is low (Moran, 1997). It is not known how long the starfish live, although they have been kept in aquaria for as long as eight years (Moran, 1997).
An interesting example of mutualism has been described between the sessile branching pocilloporid corals, which obviously have a limited behavioural capacity to fend off enemies, and crustacean species. The crab Trapezia ferruginea and the shrimp Alpheus lottini live on the coral as symbionts and are protected by coral mucus from predators. In return, they protect corals from enemy attacks, including predation by the crown-of-thorns starfish, Acanthaster planci (Glynn, 1976, in Hay et al. 2004). Species the starfish would readily feed on if it weren't for the presence of these mutualistic crustaceans include: Acropora gemmifera, A. nasuta, A. loripes, Seriatopora hystrix, Pocillopora damicornis and Stylophora pistillata (Pratchett, 2001).
Means of Movement and DispersalTop of page
Natural dispersal (local): Tagged specimens of A. planci move as far as 250m per week but feeding starfish move slower (Chesher 1969). Strong surges and high wave action can slow colonisation of new reef sites (Chesher 1969).
Water currents:Acanthaster larvae are carried by currents to widely-scattered areas, as has been the case in the Ryukyus (Japan) (Yamaguchi, 1986, in Nakamura 1986).
Pathway VectorsTop of page
Impact SummaryTop of page
ImpactTop of page
Predation of corals by Acanthaster planci, storm damage, coral diseases and temperature-related stresses were the most commonly recorded natural impacts to coral reefs. The impact of crown-of-thorns starfish on natural coral assemblages can be severe and long-lasting. In some reefs 90% of live coral cover is lost.
Since the 1960’s outbreaks of the crown-of-thorns starfish Acanthaster planci have been recorded throughout the Indo-Pacific region. From the time these outbreaks were first recorded, it has been recognised that they pose a threat to the viability of coral reef habitats and the creatures that depend on them. The impact of outbreaks of the crown-of-thorns starfish on natural coral assemblages can be severe and long-lasting. On some reefs up to 90% of live coral cover has been lost, as was the case in areas of Saipan (Tsuda et al. 1970), the Marshall Islands (Pinca et al. Undated) and Guam (Chesher 1969). The impact of outbreaks can be profound. For example the branching corals of Iriomote Island (Ryukyu Islands, Japan) were completely decimated by A. planci and replaced by flat plains of rubble, significantly lower in fish diversity (Sano 2000). Similarly coral gardens at Tanguisson Reef (Guam) were devastated following the 1960s outbreak of the crown-of-thorns starfish and the composition of coral communities shifted from preferred prey species such as Montipora and Acropora to non-preferred prey species such as Porites, Millepora and Leptastrea (Colgan 1987). A study by Cameron, Endean and De Vantier (1991) found that coral structure was significantly different between A. planci affected and non-affected sites. Members of the Poritidae family were predominant on outbreak-affected reefs, while members of the Faviidae family were predominant on non-outbreak reefs. In addition few large (old) colonies occurred on the outbreak reefs, whereas such large corals were common on unaffected reefs. On the Great Barrier Reef and elsewhere significantly higher abundances of turf algae are found to occur on reefs affected by starfish predation in comparison with live coral (Hart and Klumpp 1996). Fortunately, secondary invasion by competitively strong groups of macro-benthos, such as soft corals or macro-algae, appears not to be a limiting factor when it comes to coral recovery (Fabricius 1996).
The changes in coral composition mentioned in the last paragraph may be long-lasting, as is the case in the western Pacific Islands of Rota, Saipan and Tinian where non-preferred coral prey such as Poritidae dominate and preferred prey Acroporidae and Pocilloporidae are kept low by A. planci (Quinn and Kojis 2003). Alternatively, such changes may be temporary. Data taken from reefs in both Guam and Japan suggests that coral reefs may recover (in terms of species richness, density and fish assemblages) from starfish damage in as little as 10 to 20 years (Colgan 1987; Sano 2000). Colgan (1987) commented that the rapid recovery of a coral community from natural disturbance by A. planci demonstrates that some reef ecosystems have a greater resilience than once estimated. On the other hand, evidence to the contrary exists which is both alarming and disheartening. A study by Seymour and Bradbury (1999) showed that the average reef recovery time on the Great Barrier Reef (Australia) is lengthening over time, and it is harder for reefs to recover from recent outbreaks of A. planci than it has been in the past. The authors believe this indicates that key features of reef community structure have been damaged over time. Lourey Ryan and Miller (2000) found that for coral cover in areas of the Great Barrier Reef damaged by A. planci to increase by 30%, it would take an estimated 5 years to 1000+ years. This highlights the variability of rate recovery times between reefs and raises the possibility that not all reefs will recover from sustained outbreaks of A. planci (Lourey, Ryan and Miller 2000).
The possible implications of ongoing outbreaks of A. planci are alarming. However what makes the scenario even more alarming is that, throughout its range, coral reefs are coming under increasing pressure from human impacts. A recent report (Wilkinson 2004) predicts that 24% of the world’s reefs are under imminent risk of collapse through human pressures; and a further 26% are under longer term threat of collapse. Human impacts exacerbate the effects of natural disturbance such as A. planci outbreaks by contributing to coral mortality and reducing a reef’s ability to recover. For example, large-scale silt depositions contribute to reef degradation in the Indian Ocean, where disease, predation and stress are listed as the key factors in causing coral mortality (Ravindran, Raghukumar and Raghukumar 1999). Similarly, the degradation of coral reefs in Oman was found to be due to both natural and human causes, with damage from fishery activities being recorded as the most common human impact (Al-Jufaili 2005). Other human impacts to coral ecosystems in the area include coastal construction, recreational activities, oil pollution and eutrophication (causing nuisance algal growth) (Al-Jufaili 2005). Predation of corals by A. planci, storm damage, coral diseases and temperature-related stresses were the most commonly recorded natural impacts to coral reefs in the country (Al-Jufaili 2005). In the Cocos Island (off the coast of Costa Rica) coral reefs, already significantly damaged by the 1982-83 El Nino event, became further degraded by high densities of the corallivores A. planci and Arothron meleagris (the Guinea fowl pufferfish) and the bio-eroding Diadema mexicanum (the Mexican Pacific sea urchin) (Guzman and Cortes 1992). Surveys from 1987 found that in some areas of the reef live coral cover was as low as 3% and scientists believe the coral reefs of the Cocos Islands will need a time-frame in the order of centuries to recover their original reef-framework and thickness (Guzman and Cortes 1992). While human impacts on coral reefs can be mitigated relatively easily by reducing human impacts on the reef ecosystem, it is difficult to predict outbreaks of A. planci starfish and harder still to manage their populations once they have reached significant proportions.
Destruction of living coral reef ecosystems is a potential economic disaster for small isles and atolls of Oceania. Most inhabitants of the region derive almost all their protein from marine resources. The destruction of reefs would result in the destruction of fisheries, as well as increasing land erosion along the coast (Chesher 1969). Crown-of-thorns starfish outbreaks have hindered traditional fishing in Samoa (Birkeland and Lucus 1990) and elsewhere and dying coral reefs have put livelihoods in jeopardy. Tour boat operations, diving expeditions, eco-tourism and other tourist attractions based on reef environments are all at risk of economic loss due damage caused by the crown-of-thorns starfish. Popular tourist destinations in the Great Barrier Reef have been significantly degraded by A. planci. On Grub Reef, for example, live coral remaining after an outbreak was so poor that tour boat operators had to cease tours to the site altogether (Birkeland and Lucus 1990). Such potential outcomes have prompted control teams in Palau to engage other stakeholders in control efforts, approaching dive shops and tour operators to "adopt a reef" (Quarterly Report 2002), a control-management strategy which should be encouraged on a wider scale.
Many factors impact on the size and extent of A. planci outbreaks. The exact causes of outbreaks have not been determined with any certainty and may in fact vary depending on local and regional factors. Currently there are considered to be two main hypotheses for the anthropogenic causes of outbreaks. The first of these is the predator removal hypothesis. According to this view decreased post settlement mortality by the removal of predators such as fish and the shellfish (i.e. the giant triton) allows adult starfish to persist and build up in numbers on a reef. For instance some studies have shown that predation is an important determinant of survival rates of juvenile starfish (Keesing et al. 1996). In Mauritius high A. planci numbers have been linked to low numbers of its main predator, the mollusc Charonia tritonis (Triton’s trumpet). In Egypt removal of fish in the families Lethrinidae, Balistidae and Tetraododontidae have been linked to outbreaks of the crown-of-thorns starfish (Ormond et al. 1990, in PERSGA/GEF 2003). The second anthropogenic theory is the nutrient enrichment hypothesis. This revolves around increased pre-settlement survivorship of larval crown-of-thorns starfish. In this scenario terrestrial runoff due to extreme rainfall events or eutrophication causes nutrient enrichment of coastal waters. The increase in nutrients results in an increase in phytoplankton, upon which the starfish larvae feed, thereby increases their survival in the water column. Because starfish produce such a vast quantity of eggs even a small increase in survivorship leads to larger settlement of larvae onto a reef, which in turn leads to an outbreak. This mechanism has been implicated in outbreaks in Micronesia and Polynesia (Birkeland 1982). Similarly, frequent A. planci outbreaks on the Great Barrier Reef have been linked to increased nutrient delivery from the land (Brodie et al. 2005). River-promoted eutrophication (algal growth) is a significant factor in the demise of fringing reefs in the inner Great Barrier Reef lagoon and recorded levels of nano plankton growth in some regions are sufficient to promote the survival of A. planci larvae and may be implicated to starfish outbreaks (Bell 1992).
Finally it is worth noting that A. planci is not only a physical danger to corals. The crown-of-thorns starfish (as its name may suggest) has an array of penetrating spines which can produce a painful wound, as well as redness, swelling, vomiting, numbness and paralysis in humans. In at least one case, A. planci triggered a very nasty inflammatory response in a patient, which resulted in local swelling of the hand and fingers. Even following effective drug treatment the movement of the fingers was still limited six months later (Adler Kaul and Jawad 2002).
Risk and Impact FactorsTop of page
- Ecosystem change/ habitat alteration
- Negatively impacts human health
- Negatively impacts livelihoods
- Negatively impacts tourism
- Reduced native biodiversity
- Threat to/ loss of native species
- Causes allergic responses
UsesTop of page
During Acanthaster planci outbreaks in Japan, the carcasses of starfish were used as fertiliser (M. Yamaguchi, pers. comm., in Birkeland and Lucus 1990). It has also been suggested that A. planci can be successfully used for animal feed and may have potential as a viable alternative to fish feed. (Luo Peng et al. 2011). Acanthaster planci is a significant coral predator and is known as a keystone species. It has the potential to alter coral ecosystems in significant and important ways. This makes it a useful indicator species and one which should be monitored when assessing the health of coral reef ecosystems (see Hill and Wilkinson 2004).
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
There is substantial research and information on both ecological and management-based aspects of the crown-of-thorns starfish (Acanthaster planci) and its control. A. planci has been recently discovered to be a species complex (see ‘Description’) but it is unclear whether this finding has implications for future management, in terms of a possible need for species-specific management (Vogler et al. 2008).
It is important to first determine whether the starfish population is at normal levels or can be considered to be outbreaking. "Normal" densities of A. planci are considered to be in the range of between 1 and 15 per hectare, depending on the amount of coral cover available. It is not recommended that the starfish be controlled when occurring at this level, as damage to corals can be sustained with no obvious long-term damage when the starfish occur at low densities (Lassig, 1995). The difference between "normal" and outbreaking populations should be obvious, often involving up to a 10-fold increase in starfish numbers. The policy of the Great Barrier Reef Marine Park Authority is that control of A. planci should be limited to small-scale measures in areas important to tourism or science, unless it can be proven that the outbreak was caused or exacerbated by human activity (Lassig, 1995).
The Great Barrier Reef Marine Protection Authority recommend that any control programmes be carried out at a realistic scale (up to 2-4 ha), have adequate funding, and be initiated as soon as possible after the outbreak has been detected. Control programmes are rarely one-off operations, with starfish often moving into the cleared areas, hence it is necessary for there to be a long-term commitment to the programme (Lassig, 1995).
Control programs in Micronesia and the Great Barrier Reef have involved the killing of up to 350 000 individual starfish, but in many cases it is believed that the population of A. planci would have decreased in size anyway without a control program (Birkeland and Lucus 1990). Approximately 15 million starfish have been killed in control programs throughout the last 15 years in the Indo-Pacific region (Moran, 1997). By far the largest of these programs was undertaken in the Ryukyu Islands (Japan) where almost 13 million starfish were removed from the reefs in that region (Moran 1997). Despite this intensive Japanese effort (costing approximately A$6 million) the control program was regarded to have been unsuccessful in either eradicating the crown-of-thorns starfish or preventing further coral mortality. One of the greatest obstacles to any control project for A. planci has been the delay to initiate control before significant damage to coral ecosystems has occurred (Birkeland and Lucus 1990). Arranging volunteers, bad weather and funding availability can all delay control commencement (Birkeland and Lucus 1990). Research into accurate predictions of when and where A. planci outbreaks will occur are essential to improve future control and increase the chances of success (Birkeland and Lucus 1990). For example, a hydrodynamics study found that due to the interaction of tidal, gravity and wind flows, some areas within the reef retained higher numbers of larvae than others. Surveillance of these locations, which may be correlated with initial recruitment of A. planci, could provide an 'early warning' strategy for monitoring and controlling future outbreaks of this starfish on reefs (Black and Moran 1991).
In general there is substantial research and information on both ecological and management-based aspects of the crown-of-thorns starfish and its control. For example, since late 1985 the Australian Federal Government has provided about $A2.5 million for research; the Australian Institute of Marine Science and the Great Barrier Reef Marine Park Authority have also supported the effort (Moran 1997). As of 1997, about 70 scientists throughout Australia were collaborating on about 58 different projects. Topics of research include: larval dispersal, developing monoclonal antibodies for A. planci to identify them from other starfish larvae in samples, determining coral reef and fish recovery rates after outbreaks, use of satellite photography to investigate the effects of starfish outbreaks, developing mathematical models to understand starfish outbreaks, determining the efficacy of starfish control, investigating potential biological control options, undertaking a risk analysis of the starfish and investigating starfish predation (Moran 1997). Recently, an Australian team demonstrated the first successful induction of a transmissible disease in A. planci using injection of thiosulfate-citrate-bile-sucrose agar (TCBS) culture medium into the starfish; this technique has potential to be used in a biocontrol programme. (Rivera-Posada et al. 2011)
Research is not limited to Australia, however. Recently, scientists from Japan have identified a feeding attractant for this starfish derived from the viscera of the sea urchin (Toxopneustes pileolus). Arrachidonic acid and alpha-linolenic acid (both unsaturated fatty acids) are found to have biological activity and it is anticipated that these attractants could be used to control A. planci (Teruya et al. 2001).
The best general management policy according to the Palau Conservation Society (1999) is not to interfere with outbreaks unless the areas are small and of special value such as a tourist or heritage site. Visual searches for starfish infestations using speed boats are a useful tool for locating A. planci aggregations (Crown of thorns starfish clean-up 2002). Hand-harvesting is the only widespread method used to date and is only feasible for protecting small areas - large scale control is not known (Birkeland and Lucus 1990). An estimated 1% of starfish will regenerate from fragments, which means cutting the starfish and dumping them back on the reef is a viable control option (Birkeland and Lucus 1990). This method is better than using chemicals, which foul the reef and are potentially harmful to humans (Birkeland and Lucus 1990). Alternatively the starfish may be removed from the reef and disposed of by dumping on land; however, this removes nutrients from the reef ecosystem and increases soil salinity at dumping sites (Birkeland and Lucus 1990). Chemical treatment of starfish with lethal compounds eliminates the disposal problem and is more efficient, with 100+ starfish injected per person per hour (Birkeland and Lucus 1990). The following toxins have been used successfully for killing the crown-of-thorns starfish: sodium bisulphate (Palau Conservation Society 1999), 25ml concentrated formalin, 15ml concentrated ammonia, 10ml 16% hydrochloric acid (HCl) and 10ml saturated copper sulphate (CuSO4) (Birkeland and Lucus 1990). Copper sulfate is the cheapest option, however, it is known to be toxic to some marine animals and there is concern about its unwanted effects on the coral-reef ecosystem (Birkeland and Lucus 1990). Harriott et al. (2003) and Lassig (1995) recommend sodium bisulfate as the safest and most effective toxin, although it is extremely costly.
Control teams may consist of bounty collectors, volunteers and/or employed control teams. As the spines of this starfish are poisonous care should be taken when reef walking (Moran 1997). Bounties are usually at the rate of $US.10 to $.20 per starfish, while the costs for employing a control team is significantly higher (Birkeland and Lucus 1990). For example, the bounty system was used in the Ryukyus, Japan, and cost 0.18 per starfish, whereas a control team used at Shikoku, Japan, cost $US16.29 per starfish (Birkeland and Lucus 1990). Bounty systems have also been used with success in American Samoa and Palau, but have failed in Guam, Saipan, Tinian and Rota - probably due to the inaccessibility of the areas of starfish infestation (Birkeland and Lucus 1990). Starfish need to be accessible to snorkellers and waders and there needs to be easy access to the site by car if the bounty system is to work (Birkeland and Lucus 1990). Sites that need to be accessed by scuba diving or other specialised means will not be suitable (Birkeland and Lucus 1990). Another limitation of the bounty method is that bounty hunters tend to focus on making a profit rather than controlling the starfish so they move from one dense aggregation to another leaving behind them a thin dispersal of starfish released from the competitive pressures of other starfish (Birkeland and Lucus 1990). A possible solution to this would be to combine the efforts of both bounty hunters with a control team whose job it would be to scour the area afterwards picking up the individual starfish which the bounty hunters had missed. An alternative option would be to educate the hunters on the importance of collecting, if possible, every last individual from a site in order to reduce future damage to coral colonies. Thirdly, the initial bounty price could be increased for the few remaining individuals at a site to encourage bounty hunters to collect them.
The effect of increased run-off from human terrestrial systems into rivers and then into the sea and associated reef ecosystems has been implicated with eutrophication (algal growth) (Brodie et al. 2005; Bell 1992). This could well be a principal causative factor of crown-of-thorns starfish outbreaks (Bell 1992). Therefore, in areas of the Great Barrier Reef where elevated levels of nutrients and algal growth exist, special precautions need to be exercised in the control of sewage effluents and run-off in the vicinity of coral reefs (Bell 1992). Such management policies should be applied elsewhere in the world where fragile coral systems exist.
Lastly, the removal by humans of large predators from marine systems through fishing activity may directly or indirectly (through trophic cascades) contribute towards A. planci outbreaks, though the causal link is unsubstantiated. Nevertheless, Sweatman (2008) showed that no-take zones in the Great Barrier Reef were less frequently affected by outbreaks of crown-of-thorns starfish than were unprotected zones. This suggests a role for marine protected areas (MPAs) in preventing A. planci outbreaks.
Harriott et al. (2003) discuss three current theories as to what causes A. planci outbreaks. They are: (1) Population fluctuations are a natural phenomenon, (2) Removal of natural predators of the starfish has allowed populations to expand, and (3) Human factors such as increased nutrient run-off (mentioned above) have caused an increase in the planktonic food available to the starfish larvae, hence increased numbers of adult starfish. Fraser et al. (2000) provide a guide to best management practices in the control of A. planci.
BibliographyTop of page
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ReferencesTop of page
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