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Giant
triton strikes back
|
Feature
by
Kellie Lobb
January 17, 2003
After
several days diving in the Swain sector of the Great
Barrier Reef, seeing dark, dreary corals in places where
I had expected to find colourful underwater gardens, I
was pleased to observe one of nature’s own fighting
back against the perpetrators of devastation. |
To
view a 3mb Quick-time movie
click on the movie panel
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Fascinated,
I watched as a giant triton fed on an upturned Crown-of-thorns
starfish (COTS). The giant triton, historically used as a
signal horn by mariners, was fully engaged in devouring its
meal and had the spiny body of the starfish half inside its
shell.
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The
triton is immune to the toxin of COTS, which causes painful
stings to human beings that last for hours, as well as skin
lesions that don’t heal for months. While I knew the scene
unfolding before me was only a small counter-attack, it was
comforting to observe some natural biological controls at
work.
The
Crown-of-thorns starfish is one of the major threats to the
health of the Great Barrier Reef, along with declining water
quality in near-shore areas, coral bleaching due to global
warming, and the effects of fishing.
"Outbreaks
of Crown-of-thorns remain the single most important factor
affecting coral cover on the Great Barrier Reef," said
AIMS scientist Ian Miller, a member of the Long Term
Monitoring Team.
COTS
systematically travel across a reef in search of its favourite
coral food. When it finds it, the starfish extrudes its
stomach out through its mouth and releases digestive juices
onto the coral polyps below. The tissues of the polyps then
break down, enabling the starfish to consume coral material.
The coral skeleton is left white and susceptible to invasion
by algae, worms, boring molluscs or reef-settling organisms.
When
large numbers of COTS persist in a confined area and they
consume coral tissue faster than the corals can grow,
scientists refer to the population density as an outbreak.
The
Australian Institute of Marine Science recently confirmed
active outbreaks of COTS on four of seven outer reefs surveyed
by its Long Term Monitoring Team in the Swain sector of the
Great Barrier Reef, off the coast from Yeppoon in central
Queensland.
The
affected reefs were Gannet Cay, Horseshoe Reef, Chinaman Reef
and Turner Cay. In each case, the outbreaks had been underway
for several years and had resulted in declines in reef-wide
live coral cover. Small numbers of COTS, below outbreak
levels, were also observed on East Cay.
AIMS
divers observed up to 350 starfish in 200m strips at Horseshoe
Reef. Population surveys of COTS restricted to only the 15km
perimeter of Horseshoe Reef revealed a population of 1,176
starfish.
In
total, reef-wide coral cover on surveyed Swain reefs has
dropped from 75-100% (recorded when COTS were first observed)
to just 0-5% cover in 2002.
"There
have been COTS outbreaks on Swain reefs for the past three to
13 years, yet on some reefs it now looks like numbers are
declining simply because the coral coverage is now virtually
nil," said another scientist on the AIMS Long Term
Monitoring Team, Angus Thompson.
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Adult crown-of-thorns starfish
Close-up of the spines
Adult
COTS feeding
Adult
COTS feeding
COTS
feeding scars |
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Revelations
of continued declines in reef-wide coral cover on some reefs
have renewed calls for increased research into the ecology and
population dynamics of crown-of-thorns.
"Data
from the AIMS Long Term Monitoring Program shows that the
Great Barrier Reef is in a continued state of flux, with coral
cover declining in the face of impacts such as COTS, cyclones,
freshwater incursions and disease, followed by longer periods
of recovery," said Ian Miller.
Of
concern is the potential for repeated COTS outbreaks to result
in degraded reef structures underpinning marine communities.
"Coupled with other threats to the Great Barrier Reef,
the potential for long-term decline is enhanced," Mr
Miller said.
Crown-of-thorns
starfish have developed a set of unique physical and
biological features that have led to great success and
survival in the reef environment. These characteristics make
it difficult for scientists to pinpoint external factors that
may influence an outbreak.
COTS and more COTS
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Stomach of an
adult COTS
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Some
of their natural advantages include:
- A
high reproductive capability (fecundity), with an average
female producing 12 to 24 million eggs (a single large
individual can produce up to 60 million eggs);
- A
larval dispersal phase that enables the starfish to migrate
a long way from their mother reef using prevailing water
currents;
- Rapid
growth, with COTS reaching reproductive maturity within two
years of settling on a reef;
- No
equivalent coral predator, which means there is no
competition for food;
- A
large stomach that allows them to feed on an abundant
high-energy food source;
- A
preference for feeding on relatively fast growing Acropora
sp. corals; and
- Relative
longevity, probably more than eight years, allowing
successive breeding seasons over several years.
Their
reproductive capability is so great that even a 1% increase in
the survival rate of young starfish means as many as 240,000
more adult starfish from a single female.
There
are several theories about the environmental factors that may
boost the survival chances of COTS larvae. Enhanced runoff of
nutrients from fertilised farmlands, washing into coastal waters
during tropical downpours, may help larvae to survive before
they settle on a reef. The extra nutrients are a potential food
source, enabling more larvae to get to their next growth stage.
Fishing
of natural predators such as fish and shellfish may also be a
factor. With fewer starfish predators on the reef, the survival
of COTS larvae is enhanced after they have settled on particular
reefs. This allows adult starfish to build up in numbers.
Scientists
have observed north-to-south movements of Crown-of-thorns from
the Cairns/Lizard Island sector, down to reefs off Innisfail and
Townsville and finally to the Whitsundays. The East Australian
Current appears to act as a giant transport system.
But on
outlying reefs, there have also been independent outbreaks of
COTS, quite separate from the obvious southward drift of
starfish. Outbreaks in the Swain reefs, for example, appear to
be restricted to reefs only in that outer region, with no net
movement of larvae to other parts of the Great Barrier Reef.
"Whether
the pattern of outbreaks we see on the Great Barrier Reef is due
to natural or human factors remains to be answered. Given the
level of usage by extractive industries, such as fisheries, and
the changes to land management practices over the last 200
years, the Barrier Reef is clearly not the environment it once
was. With this in mind, it is critical to continue long-term
monitoring studies to detect chronic declines in the system, and
the role Crown-of-thorns starfish play in this process,"
said Ian Miller.
After my
own trip to the Swain sector with the AIMS Long Term Monitoring
Team, I couldn’t agree more. These animals can rob reefs of
their spectacular colour. It’s time we stole the march on
them.
For more information
contact
Theresa Millard, AIMS
Science Communication,
Phone: 07 47534250, 47534444,
Mobile: 0409596271
Email: t.millard@aims.gov.au
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