- The official PNAS paper is online here
- Read about the crown-of-thorns starfish here
- Read about the Great Barrier Reef here
- Read about the Australian Institute of Marine Science (AIMS) here
Info sheet: crown-of-thorns starfish
The crown-of-thorns starfish (COTS) poses one of the biggest existing threats to Australia’s Great Barrier Reef.
COTS feed on live coral, preferring hard corals. They possess few natural predators, have a short lifespan, and an ability to produce huge number of larvae under the right conditions.
COTS have a wide Indo-Pacific distribution, from the Red Sea to the west coast of Central America. They are infamous in Australia because they undergo massive population explosions, in which waves of these marine pests move along the Great Barrier Reef (GBR) consuming masses of coral as they go.
Scientists are working on ways to control the COTS without harming other organisms on the Reef. The most recent solution focuses on the susceptibility of the pest to disease.
Adult COTS are disc-shaped and may have up to 21 arms, meaning that they do not generally display the five-fold symmetry characteristic of many starfish. They normally range in size from 25 to 45 cm and come in a variety of different colours, which vary from region to region, although dull grey-green colours are typical in Australian waters. COTS have excellent natural defences, their upper sides being covered in spines that contain a virulent toxin. The animal will rapidly curl itself into an impregnable ball if its vulnerable underbelly becomes exposed.
Although slow moving, the carnivorous COTS shows a clear preference for living coral, and will climb onto its prey before extruding its stomach out through its mouth to cover an area comparable to its own diameter. Coral tissue is then liquefied by digestive enzymes that are secreted from the stomach, allowing the COTS to absorb a nutrient-rich meal.
A single COTS can consume up to 10 m2 of living coral per year. Feeding rates vary with the changing temperature but a study in the central GBR observed large adults consuming as much as 478 cm2 of coral per day in summer. Although COTS prefer branching and table-shaped corals, they are able to eat virtually any coral on the reef, and will resort to feeding on soft corals if hard corals become scarce.
The larvae of COTS spend between 14-30 days as plankton before they settle and change into five-armed juvenile starfish. After only six months they change again into the adult form and begin consuming corals. Within two years they can be sexually mature. During these first two years of their existence, COTS adopt a cryptic lifestyle, only coming out to feed at night. Adults, however, will feed during the daytime. When COTS reach high densities they will move day and night as they compete for living coral.
Only a handful of other species have been recorded feeding on adult COTS, probably because the starfish presents a difficult prey target for many would-be predators on the reef. Among the reef fish there are reports of pufferfish, triggerfish and the humphead wrasse eating COTS but these large species are relatively rare on most coral reefs. The large triton sea-snail is known to be an effective predator of COTS but again this species is not usually abundant.
Juvenile COTS may be preyed on by some marine invertebrates including shrimp, crabs and polychaete worms; they may also be targets for small generalist-feeding reef fish. Data from the GBR has shown lower numbers of COTS on reefs in marine protected areas (MPAs). This may be explained by high disappearance rates of juvenile COTS on these reefs, which in turn may result from increased predator abundance as MPAs create conditions for more natural, balanced food-chains.
In ecological terms an outbreaks are defined as situations where the density of an organism exceeds the level that the available resources can sustain. The first outbreak of crown-of-thorns starfish (COTS) on the GBR was recorded at Green Island (near Cairns) in 1962. Since then there has been a severe outbreak about every 13 to 14 years, with the last major outbreak in the late 1990s. These outbreaks appear to begin in the northern Great Barrier Reef (GBR), and at their peak may cause upwards of 90% loss of coral cover on individual reefs as the wave of COTS outbreaks moves southward along the GBR.
Skeletal remains found in sediment on GBR reefs show that COTS have been part of the ecosystem for at least 8000 years, but the information is not sufficient to determine if there have always been outbreaks or whether only moderate densities of the starfish occurred in the past. What is clear is that this disturbance to the GBR has never historically been so severe as to undermine its basic integrity. One theory even holds that, under natural conditions, COTS outbreaks may have helped to regulate the community diversity of coral species by selectively removing the otherwise dominant Acropora corals, which are superior competitors in low-disturbance environments.
The disturbance regime has now shifted. The evidence suggests that climate change not only is exacerbating existing impacts like cyclones and coral bleaching, but also may be introducing new threats to coral reefs including increased susceptibility to disease and reduced calcification rates. The combined effect of these disturbances may result in such regular coral mortality and reduced growth that communities cannot fully recover.
One question that has generated a good deal of debate in the scientific community is the degree to which human activities may be driving outbreaks of COTS. Certainly this is a species that is naturally predisposed to population explosions, having highly fecund females that are capable of reproducing at a relatively early age. Large females can potentially spawn 50 million eggs each, and there have been multiple observations of COTS gathering together high on a reef to spawn, a strategy that is common among marine invertebrates to maximise the chances of successful fertilisation.
The periodic nature of outbreaks has led some marine biologists to argue that changes in the physical or biological environment must release a key bottleneck at the larval stage of development, where the population is most abundant and constrained. COTS larvae feed on a particular component of plankton in the ocean, which itself undergoes a population boom when increased nutrients are available. These conditions arise following periods of intense rainfall that wash inorganic nutrients, such as the nitrogenous compounds used in farming, into the ocean. Field data and population models suggest that river floods and regional differences in plankton availability are strongly related to patterns of COTS outbreaks on the GBR.
While this theory may account for an increase in the frequency of COTS outbreaks, it does not explain all outbreaks. It appears that some outbreaks occur on remote coral reefs in the northern sector of the GBR and the Swains which are relatively isolated from terrestrial inputs such as river floods. Moreover, due to the short larval life of the starfish, it is difficult to directly link major periods of rainfall with major larval population increases. One study conducted at Lizard Island in the northern GBR during the build-up of the last major outbreak found that high densities were a result of several successive cohorts of COTS, rather than any one sudden or substantial increase in rates of recruitment.
What is the ultimate fate of COTS outbreaks and how might they be controlled. Left uncheck the outbreaking populations ultimately starve to death as they consume all available food sources.
Echinoderms—such as starfishes, sea urchins, sea cucumbers—are highly susceptible to disease, and despite their formidable defences, COTS are not exempt. One theory suggests that disease spreads rapidly through a dense population of COTS, as the proximity of individuals to each other speeds up the transmission process, ultimately putting a swift end to the outbreak.
Attempts to directly control COTS populations have so far been largely unsuccessful. Cutting up the starfish is not effective, as COTS are able to withstand severe damage and will eventually regrow missing arms. In the past the recommended control method was for trained divers to inject sodium bisulfate (dry acid) into the starfish, as this chemical does not harm the surrounding reef and oceanic ecosystems. However, this process is expensive and inefficient. Some tourism operators in the Cairns region reported spending up to $300,000 per year in COTS control using this method. During active outbreaks, between 200 and 500 COTS must be exterminated each day to keep sites free from them.
The latest direction in COTS control is based on the species’ susceptibility to disease. Scientists have been experimenting with injecting thiosulfate-citrate-bile-sucrose agar (TCBS) into a COTS individual, as this chemical has been shown to induce disease in the creature. In 2011, a research team published the first report of successful induction of transmissible disease in COTS using TCBS, and documented no introduction of new pathogens into the marine environment. If this process can safely speed up the predisposition of COTS populations to disease, it may potentially have an important role to play in limiting outbreaks of this voracious starfish.
About the Great Barrier Reef
The Great Barrier Reef (GBR) ecosystem is unrivalled in size and ecological importance in the tropical marine environment. Tourism and recreational activities on the Reef make a substantial contribution to Australia’s economy, not to mention its wider social and environmental benefits.
Threats to the GBR are many and varied. They include rising ocean temperatures, extreme weather events, overfishing and pests. Addressing these threats and ensuring sustainable use are critical to the Reef’s survival and the ongoing viability of the industries that rely on it.
The Australian Institute of Marine Science (AIMS) is passionate about better understanding this natural wonder and its many facets.
Here are 10 key issues facing the GBR and adjacent regions today.
Bleaching and rising ocean temperature
This recent analysis from the AIMS Long Term Monitoring data indicates that 10% of coral decline on the GBR is attributable to coral bleaching.
It is difficult to determine the precise contribution of all the different threats potentially affecting the health and mortality of reef organisms, largely because most observations occur after the fact, and because many stressors have compounding effects—a bleached coral may become more susceptible to coral disease, for instance. Mass bleaching events are notorious because of their relatively rapid and dramatic effect on coral reefs. The GBR has experienced eight such events since 1979, with none recorded before this. Coral mortality tends to be higher on inner-shore reefs, possibly because the corals there are often already stressed by freshwater run-off from the nearby coast.
The relatively recent advent of bleaching events on the GBR is attributed primarily to rising sea-surface temperatures, thought to be a result of man-made climate change. Sea surface temperatures are expected to rise between 1 °C and 3 °C by 2100, depending on the extent to which the global community can curb its carbon emissions. From a marine biology perspective, there is still a great deal to understand about the different susceptibility of various species and shapes of corals, and the factors contributing to regional differences in bleaching risk.
In November 2011, the Commonwealth Government released a draft Coral Sea Commonwealth Marine Reserve covering 989,842 sq km.
The proposed Coral Sea Marine Reserve includes four different zones which provide varying levels of protection from threats. As one of the healthiest areas of ocean remaining on Earth, the Coral Sea is internationally recognised for its rich biodiversity and important heritage values. It is under increasing threat from fishing as stocks elsewhere become depleted and enterprises look further afield for new sources. The Coral Sea is already important from an economic perspective, being an important destination for recreational and charter fishing, snorkelling, scuba diving, kayaking, whale watching, and cruising.
Some conservation and marine research organisations have so far expressed concern that the planned marine reserve does not provide sufficient levels of protection for large portions of the Coral Sea, including important breeding and feeding sites. Our understanding of the structure and function of the reef communities in the remote Coral Sea is much less developed than it is for the GBR and there will be an ongoing need to provide baseline data and targeted research on key processes in this unique area in order to ensure that it can be effectively managed for multiple competing uses in the future.
The coral-eating crown-of-thorns starfish (COTS) is one of the main threats to coral cover on the GBR.
Although the attention now given to climate change and, increasingly, ocean acidification, has taken the focus somewhat away from COTS, populations of this outbreaking species continue to oscillate on the GBR. AIMS surveys COTS populations using manta tows (towing an observer with a manta board behind a small boat). Documenting the density of COTS individuals is important as these coral-eating starfish undergo periodic population explosions, COTS outbreaks, which can lead to massive reductions in coral cover. In 2011, only 2% of reefs surveyed as part of the Long-Term Monitoring program were subject to COTS outbreaks, but this trend fluctuates greatly and there are indications that anew outbreak may be starting in the north. During previous outbreak episodes in 1987, 2000 and 2004, more than 15% of reefs surveyed had outbreaking populations.
There is an ongoing debate about the causes of COTS outbreaks. Certainly COTS are native to the Indo-Pacific, and sedimentary evidence suggests they have been part of the GBR ecosystem for at least 8,000 years, having had a significant impact on the environment during that time. Their life-history predisposes COTS to rapid population pulses that are ultimately self-defeating as food sources are decimated. Human influences causing or increasing the intensity these outbreaks has been suggested. Mechanisms for this include the ‘terrestrial run-off hypothesis’ and the ‘predator-removal hypothesis’. Ongoing monitoring is important in testing these alternative views, but it is clear that once outbreaks occur it is very difficult to intervene. Current research is also now focussing on the development of more effective ways to reduce outbreaking populations.
Extreme weather events
In the GBR, 48 per cent of coral decline recorded between 1985 and the present has been attributed to tropical cyclones.
While coral communities take more than 10 years to recover fully from major disturbances, the average time between cyclones for a given reef on the GBR is seven years. Category 4-5 cyclones in 2006, 2008 and 2011 caused catastrophic damage to coral reefs off the coast of Queensland, especially Cyclone Hamish in March 2008, whose path tracked along the coastline, causing more coral mortality than all previous bleaching events combined. The severe physical damage from the cyclone greatly reduced the structural complexity of reefs in the cyclone’s path leading to considerable knock-on effects for fishes and other marine organisms, whose populations are structured by the complexity of the habitat. Cyclones also appear to herald periods of increased rainfall, which can have secondary effects on the survival of coral reefs (see also ‘Water quality’).
Consensus is forming about the relationship of increases in sea-surface temperature to cyclones in the Indo-Pacific region. Models show that warmer conditions will increase the intensity of tropical cyclones, although their frequency may remain unchanged. An important research focus is the genetic connectivity of reefs on the GBR, as interconnectivity has important implications for the speed of recovery of reefs and their associated marine communities following major disturbances. Observations from the Caribbean have shown that for stressed coral reef ecosystems—such as overfished reefs with poor water quality—a powerful cyclone can be sufficient to create a ‘phase-shift’ to an alternative, degraded stable state.
Microbes and pathogens
Some 28,000 types of viruses are thought to be associated with corals, but it would be simplistic to suggest that they are all harmful.
The Centre for Marine Microbiology and Genetics was opened at AIMS in October 2008 and is an important part of AIMS’ overarching research strategy. Researchers working in this area are learning more about the crucial roles played by viruses in the evolution and ecology of aquatic systems. Because some viruses can potentially enable marine organisms to adapt to environmental stresses, such as those associated with climate change, this research is both cutting-edge and highly relevant to the GBR today.
Marine microbes are also a potential source of revenue for Australia. As custodian of the Bioresources Library, AIMS facilitates access to a collection of over 20,000 marine organisms from around Australia’s marine territory, which may lead to the discovery of products of commercial potential. The global revival of biodiscovery as a source of new medicines and other useful products means that Australia may receive long-term economic benefits from its rich marine resources.
Ocean acidification is in its infancy as a topic of research; a lot is still unknown about its likely impact on our oceans and the organisms that inhabit them.
Ocean acidification is a new topic of research with scientific publications dating back only to the mid-2000s. However there has been an exponential increase in research since then. The majority of studies have been laboratory based and have increased CO2 levels artificially in order to examine the effects of ocean acidification on organisms such as fish and corals. The research suggests that adverse effects, including the erosion of the skeletons of carbonate-based organisms like corals, become pronounced when certain threshold levels of CO2 are reached.
AIMS scientists have studied reefs around natural carbon seeps in Papua New Guinea and found that the coral communities in these relatively acidic environments had low species diversity and structural complexity. With predicted increases in the atmospheric concentration of CO2 in the coming decades, the oceans are likely to become more acidic, and the Papua New Guinea study may presage the future of coral reefs on the GBR. At present, however, there is little evidence to show that ocean acidification is taking a toll on rates of coral calcification. Moreover, the pattern is more complex than it may appear; two research papers published in late 2011 showed that ocean acidification does not act uniformly across complex ecosystems. This is evidently an area in which greater research and monitoring is required, coupled with the curbing of carbon emissions on a global scale.
Tourism and fishing
The GBR is a $5 billion asset for our economy and supports more than 60,000 jobs in Queensland alone.
Tourism makes up the overwhelming majority of GBR based revenue, with an estimated 820 operators and 1,500 vessels involved in activities on the GBR. Recreational fishing is popular and contributes to revenue from tourism, as well as recreational (non-tourist) revenue, where at the last estimate in 2007, it contributed $281 million to the economy in combination with other activities such as diving and boating. Commercial fishing on the GBR accounts for a smaller proportion of revenue, being estimated at $165 million in 2007. The largest contributions came from the prawn and coral trout fisheries.
Sustainable use of the GBR is critical to ensuring the long-term viability of these activities. A review of zoning patterns in 2004 increased the coverage of no-take zones from 4.6% to 33.3%. The rationale was that this creates substantial areas where GBR flora and fauna can persist without direct human exploitation while still allowing plenty of access to the reef for recreational and commercial activities. In turn, this led to research exploring the effectiveness of the zoning, particularly with regard to the connectivity of marine populations. Studies also explored the conservation of habitats that act as genetic sources for other areas on the GBR, and the overspill effect of no-take zones on adjacent fishing zones.
Terrestrial run-off has and continues to have a disproportionate effect on the inner-shelf of the GBR.
Influxes of sediment, nutrients and freshwater all pose increased costs on the complex ecosystem of the GBR, including reductions in water clarity, the introduction of novel pathogens and physical impediments to ecological processes. Striking a healthy balance between the demands of industry, agriculture and coastal development and the quality of the coastal waters off Queensland is critical to the success of all. Corals reduced diversity and may face stiff competition from turf-algae and macro-algae.
Research at AIMS has considered the impact of terrestrial run-off on the viability of reef, seagrass, algae and mangrove communities and their resident species. Continued co-operation between scientists, management agencies, government and industry will allow the setting of standards to maintain the health of the GBR and taking appropriate action to meet these standards and improve the health of the inner Great Barrier Reef.
The Australian Institute of Marine Science (AIMS) is a Commonwealth Government organisation and a leader in tropical marine science.
The Institute is consistently ranked among the top one per cent of specialist research institutions internationally and is known for its unique capacity to investigate topics from broad-scale ecology to microbiology.
AIMS is committed to the protection and sustainable use of Australia’s marine resources. Its research programs support the management of tropical marine environments around the world, with a primary focus on the Great Barrier Reef World Heritage Area, the pristine Ningaloo Marine Park in Western Australia and northwest Australia.
For more information, see http://www.aims.gov.au