Actualité scientifique
Scientific news
N° 420
December 2012
Actualidad cientifica
On the trail
of the coral-killing starfish
Acanthaster planci is the
principle natural enemy
of reef-building corals.
Outbreaks of this coralfeeding starfish occur
periodically, due to
reasons that remain
unclear. It decimates
entire reefs in the space
© IRD/E. Folcher
of just a few years, as
has been the case in
French Polynesia since
2004. A new study
The Acanthaster planci starfish feeds on coral.
conducted by IRD
researchers and their
partners1 describes this
population explosion
around Moorea, the
“sister island of Tahiti”2.
The rate of living coral
Good to know
In the past 40 years, coral reefs have had to cope with increasingly frequent and intense natural stressors (coral
bleaching, cyclones and outbreaks of Acanthaster planci), in addition to human disturbances (pollution, coastal
development, overfishing, etc.). Worldwide, 75% of coral reefs are under threat and approximately 20% have
already been irreparably destroyed.
Acanthaster planci is a coral-feeding starfish, meaning that it preys on coral polyps, the animal part of the reef.
This species is very fertile: each female lays 4 to 60 million eggs per season. The larvae survival rate is generally
low. From time to time, however, the delicate balance is upset...
cover in ocean depths
and lagoons alike has
dropped from 50%
(healthy reef) to under
5% in 2009. The
ecosystem will need at
least a decade to be
restored to its original
state.
The crown-of-thorns starfish (Acanthaster planci)
is the greatest natural threat to coral populations.
Outbreaks of the species occur periodically in the
Indo-Pacific ocean and lead to the devastation of
entire reefs, as IRD researchers and their partners1
observed in French Polynesia.
The starfish has spread from
island to island
The archipelago has been suffering from a new
population explosion of the predatory starfish
since 2004. It is one of the most intense and
devastating outbreaks ever recorded. The
outbreak of Acanthaster began in a very specific
location in the Austral and Leeward Islands, then
in 2006, the starfish colony spread to Tahiti and
Moorea2. Thanks to a dozen stations around the
island of Moorea, scientists were able to make
spatio-temporal observations of the dynamics of
the infestation of coral populations. Thus, in a new
study published in PLoS One, they described the
spread of the coral reef invasion.
FOR MORE INFORMATION
Ocean depths and lagoons alike
The starfish first settled in the deeper areas along
the outer slopes of the reef, around 20 to 30 metres
below the ocean surface. It then rose to a depth of
approximately 6 metres, and even colonised
certain parts of the lagoon. The damage was
gradually observed: from 47% of living coral cover
at one of the stations in 2006, for example, this
rate dropped to 21% in 2007, 6% in 2008 and 2%
in 2009: a disastrous state of affairs that disrupts
the structure and functioning of all reef communities (including other coral-feeding species, such as
butterflyfish, etc.).
The causes remain unclear
What are the reasons behind outbreaks of Acanthaster planci? In Australia, where the pest is also
rife, invasions occur after years with high rainfall.
Rainfall leads to the excess release of nutrients
Contacts
Mehdi Adjeroud, researcher at IRD
[email protected]
Tel: (687) 26 07 41
from human activities and the proliferation of algae
on which echinoderm* larvae feed. In Polynesia,
however, anthropic pressure seems too low and
localised3 to explain such an outbreak of the starfish. The current lack of data on the subject means
the phenomenon remains a mystery.
Mohsen Kayal, researcher at IRD
[email protected]
Tel: (687) 70 78 50
Address
IRD
101 Promenade Roger Laroque
Anse Vata
BP A5
98848 Nouméa cedex
Unité Biocomplexité des écosystèmes
coralliens de l’Indo-Pacifique - CoRéUs 2
Since the causes of outbreaks remain unclear,
there is limited ability to fight against Acanthaster
planci in order to protect economic activities
around the coral barrier, such as tourism and
diving. Researchers are currently studying
processes to «recruit» new corals, in other words
to repopulate the reef and make it more resilient.
Without a new widespread disturbance, a coral
ecosystem would need 10 to 30 years to be
restored to its original state.
References
Kayal Mohsen , Vercelloni J.,
Lison de Loma T., Bosserelle P.,
Chancerelle Y., Geoffroy S., Stievenart
C., Michonneau F., Penin L., Planes S.,
Adjeroud Mehdi. Predator crownof-thorns starfish (Acanthaster planci)
outbreak, mass mortality of corals,
and cascading effects on reef fish and
benthic communities. Plos One, 2012,
7(10), p. e47363. ISSN 1932-6203
fdi:010057281
By Gaëlle Courcoux, DIC
1. CORAL - CRIOBE Labex in Moorea, Universities of Brisbane in Australia, of Florida in the United States, of Réunion and the Australian Institute of Marine Science.
2. Tahiti and Moorea are called the «Windward Islands» as opposed to the «Leeward Islands» (Bora-Bora, Raiatea, Maupiti). Together,
they form the Society Islands.
Keywords
Acanthaster planci, Polynesia, coral,
starfish
3. With 250,000 inhabitants across 3,400 km² of the total land mass, French Polynesia has a low population density.
* DID YOU KNOW?
Like sea urchins, starfish are echinoderms, which in Greek means «spiny skin». They have neither
head nor tail, and no right of left! Their bodies can be divided into five sections, based on a central axis
of symmetry. They have no brain, nervous system or blood: seawater flows through the organism to
ensure the animal’s breathing, nutrition and evacuation and to activate its thousands of tiny feet,
equipped with suckers.
Coordination
Gaëlle Courcoux
Information and Culture Department
Tel: +33 (0)4 91 99 94 90
Fax: +33 (0)4 91 99 92 28
fi[email protected]
www.ird.fr/la-mediatheque
Indigo, IRD Photo Library
Daina RECHNER
Tel: +33 (0)4 91 99 94 81
[email protected]
IRD photographs on this topic, free for media
reproduction without additional permission:
www.indigo.ird.fr
An outbreak of starfish causes the living coral reef cover to drop from 50% to under 5%, as in the centre of Polynesia. The ecosystem will need at least a
decade to recover.
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© IRD/DIC, Novembre 2012 - Conception et réalisation graphique : L. CORSINI
© IRD/E. Folcher
© OEIL / M. Juncker
© IRD/E. Folcher
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