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Info sheet: Crown-of-thorns starfish
Summary
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.
Profile
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.
Feeding
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 cm 2 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.
Early years
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.
Predators
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.
Outbreaks
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.
Control
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.