A
ACANTHASTER PLANCI
Ian Miller
Australian Institute of Marine Science, Townsville,
QLD, Australia
Acanthaster planci (Class Asteroidea; Order Spinulosida;
“crown-of-thorns sea star or starfish”) is a large (up to
70 cm), mobile, multi-armed (7–23) sea star covered in
sharp, toxic spines. It feeds almost exclusively on hard
corals and is found on coral reefs throughout the IndoPacific. No other reef sea stars remotely resemble its
appearance, nor possess comparable life-history traits as
a predator on corals.
Crown-of-thorns are prone to population outbreaks,
with aggregations of thousands or more adults per hectare
not uncommon (Figure 1). Such populations often
advance in fronts through coral habitat, leaving formerly
luxuriant coral areas dead in their wake. The sea star
has a number of life history traits that predisposes
these destructive population outbreaks: absence of any
equivalent coral predator (little competition for food);
a large stomach (that is pushed out through the mouth
to digest coral tissue externally); a high fecundity
(a mature female can produce some 50 million eggs);
planktonic larvae (that can feed in the water column and
disperse over long distances); rapid growth ($10 cm.y
À1
,
that is faster than any other coral reef sea star); large
size and toxic spiny armature (that provide protection
from potential predators); multi-armed morphology and
tube feet (allowing them to climb and feed in nearly any
position). Repeated population outbreaks have decimated
hard corals throughout the Indo-Pacific over the last
50 years.
Outbreaks were first observed in the 1960s. The
geographical extent (two oceans) and impact (an ecosystem changed from one dominated by hard corals to one
dominated by algae), shocked scientists. A key management issue was whether human activity had somehow
precipitated the population outbreaks. Two main hypotheses have been developed that implicate anthropogenic
factors. The first is “the predator removal hypothesis”
(Endean, 1969), which holds that overfishing (in particular sweetlips (Family Lethrinidae), some wrasses (Family Labridae) and some triggerfish (Family Balistidae))
and collecting of predators of the sea star (notably
a large Gastropod mollusc the giant triton Charonia
tritonis), allow crown-of-thorns to build up in numbers
on a reef. On reaching a critical abundance, their reproduction and larval dispersal leads to successful recruitment of larvae on reefs downstream in prevailing
currents. A cascade of outbreaks across tracts of neighboring reefs ensues. The second hypothesis (possibly
synergistic with the first) is “the nutrient enrichment
hypothesis” (Birkeland, 1982, Lucas, 1982). In this scenario, river runoff from human-modified catchments
enhances nutrients in coastal waters, resulting in an
increase in phytoplankton upon which the sea star larvae
feed. Because crown-of-thorns 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 a primary outbreak.
Today, despite repeated outbreaks and years of
research the exact events leading to the initiation of an
outbreak remain enigmatic. This is because the lifehistory of crown-of-thorns makes it difficult to disentangle the natural processes leading to an outbreak from
those forced by human activities. As a result crown-ofthorns remains a major management problem for coral
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011
ACANTHASTER PLANCI
Ian Miller
Australian Institute of Marine Science, Townsville,
QLD, Australia
Acanthaster planci (Class Asteroidea; Order Spinulosida;
“crown-of-thorns sea star or starfish”) is a large (up to
70 cm), mobile, multi-armed (7–23) sea star covered in
sharp, toxic spines. It feeds almost exclusively on hard
corals and is found on coral reefs throughout the IndoPacific. No other reef sea stars remotely resemble its
appearance, nor possess comparable life-history traits as
a predator on corals.
Crown-of-thorns are prone to population outbreaks,
with aggregations of thousands or more adults per hectare
not uncommon (Figure 1). Such populations often
advance in fronts through coral habitat, leaving formerly
luxuriant coral areas dead in their wake. The sea star
has a number of life history traits that predisposes
these destructive population outbreaks: absence of any
equivalent coral predator (little competition for food);
a large stomach (that is pushed out through the mouth
to digest coral tissue externally); a high fecundity
(a mature female can produce some 50 million eggs);
planktonic larvae (that can feed in the water column and
disperse over long distances); rapid growth ($10 cm.y
À1
,
that is faster than any other coral reef sea star); large
size and toxic spiny armature (that provide protection
from potential predators); multi-armed morphology and
tube feet (allowing them to climb and feed in nearly any
position). Repeated population outbreaks have decimated
hard corals throughout the Indo-Pacific over the last
50 years.
Outbreaks were first observed in the 1960s. The
geographical extent (two oceans) and impact (an ecosystem changed from one dominated by hard corals to one
dominated by algae), shocked scientists. A key management issue was whether human activity had somehow
precipitated the population outbreaks. Two main hypotheses have been developed that implicate anthropogenic
factors. The first is “the predator removal hypothesis”
(Endean, 1969), which holds that overfishing (in particular sweetlips (Family Lethrinidae), some wrasses (Family Labridae) and some triggerfish (Family Balistidae))
and collecting of predators of the sea star (notably
a large Gastropod mollusc the giant triton Charonia
tritonis), allow crown-of-thorns to build up in numbers
on a reef. On reaching a critical abundance, their reproduction and larval dispersal leads to successful recruitment of larvae on reefs downstream in prevailing
currents. A cascade of outbreaks across tracts of neighboring reefs ensues. The second hypothesis (possibly
synergistic with the first) is “the nutrient enrichment
hypothesis” (Birkeland, 1982, Lucas, 1982). In this scenario, river runoff from human-modified catchments
enhances nutrients in coastal waters, resulting in an
increase in phytoplankton upon which the sea star larvae
feed. Because crown-of-thorns 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 a primary outbreak.
Today, despite repeated outbreaks and years of
research the exact events leading to the initiation of an
outbreak remain enigmatic. This is because the lifehistory of crown-of-thorns makes it difficult to disentangle the natural processes leading to an outbreak from
those forced by human activities. As a result crown-ofthorns remains a major management problem for coral
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011