Eastern Pacific Coral Reef Ecosystem
293
Acanthaster planci has been observed preying on corals in several
eastern Pacific mainland coral communities, ranging from the Gulf of California to Panama, including most oceanic island localities. Acanthaster is
potentially the most important corallivore in the eastern Pacific, due to its
large size and high rate of consumption (about 0.5 m 2 of coral tissue per
month). This corallivore is especially interesting ecologically because of its
preferential feeding behavior; it tends to prey on massive, encrusting and
nodular coral species, often avoiding branching corals such as Pocillopora
spp. Acanthaster does, however, prey on small colonies of Pocillopora and
their broken branches, which harbor few or no crustacean guards. The sea
star generally avoids intact colonies or continuous stands of Pocillopora.
The coral's nematocysts and crustacean guard defenses are usually sufficient to thwart Acanthaster's feeding. If a foraging sea star attempts to
mount a Pocillopora colony, the crustacean guards will quickly snap and
pinch (Alpheus) or pinch, jerk and clip tube feet and spines (Trapezia) in
order to discourage the attack. Some examples of community-scale effects
of Acanthaster's selective feeding behavior are: (1) the presence of smaller
and younger colonies of preferred coral prey, (2) high relative abundances
of non-preferred prey, and (3) the survival of preferred prey species surrounded by protective stands of pocilloporid corals. Unlike many IndoPacific reef areas that experience large outbreaks of Acanthaster, with tens
of thousands of sea stars stripping reefs of most of their coral cover,
eastern Pacific sea star populations have not been observed to exceed
about 25-30 ind. ha- 1 • The population sizes of adult sea stars seem to be
limited by shrimp (Hymenocera picta Dana) and worm [Pherecardia striata (Kinberg)] predators. These invertebrates are abundant on Panamanian
pocilloporid reefs, and numerous attacks on sea stars have been witnessed.
In a typical attack scenario, a pair of shrimp will cut through the aboral
dermis of Acanthaster and then begin to remove and eat such internal
organs as the hepatic caeca and gonads. The shrimp ride along with the sea
star, feeding on its soft parts for several days. Eventually, polychaete worm
scavengers are attracted to the wounded sea star and will invade its internal body cavities through the incisions made by the shrimp. As many as
15 to 20, 5- to 10-cm-long worms will continue feeding on Acanthaster's
soft parts until no internal organs remain and the sea star dies.
Other echinoderms feeding on live corals include two other sea stars
and two species of sea urchins. The dub-spined sea urchin, Eucidaris, is
perhaps the most persistent corallivore, often observed grazing on pocilloporid corals in the Galapagos Islands. This sea urchin abrades both the soft
tissues and skeletons of corals, and figures prominently in bioerosion. At
least eight fish species, representative of five families, feed on live corals,
with their feeding strategies ranging from removing mainly live tissue and
293
Acanthaster planci has been observed preying on corals in several
eastern Pacific mainland coral communities, ranging from the Gulf of California to Panama, including most oceanic island localities. Acanthaster is
potentially the most important corallivore in the eastern Pacific, due to its
large size and high rate of consumption (about 0.5 m 2 of coral tissue per
month). This corallivore is especially interesting ecologically because of its
preferential feeding behavior; it tends to prey on massive, encrusting and
nodular coral species, often avoiding branching corals such as Pocillopora
spp. Acanthaster does, however, prey on small colonies of Pocillopora and
their broken branches, which harbor few or no crustacean guards. The sea
star generally avoids intact colonies or continuous stands of Pocillopora.
The coral's nematocysts and crustacean guard defenses are usually sufficient to thwart Acanthaster's feeding. If a foraging sea star attempts to
mount a Pocillopora colony, the crustacean guards will quickly snap and
pinch (Alpheus) or pinch, jerk and clip tube feet and spines (Trapezia) in
order to discourage the attack. Some examples of community-scale effects
of Acanthaster's selective feeding behavior are: (1) the presence of smaller
and younger colonies of preferred coral prey, (2) high relative abundances
of non-preferred prey, and (3) the survival of preferred prey species surrounded by protective stands of pocilloporid corals. Unlike many IndoPacific reef areas that experience large outbreaks of Acanthaster, with tens
of thousands of sea stars stripping reefs of most of their coral cover,
eastern Pacific sea star populations have not been observed to exceed
about 25-30 ind. ha- 1 • The population sizes of adult sea stars seem to be
limited by shrimp (Hymenocera picta Dana) and worm [Pherecardia striata (Kinberg)] predators. These invertebrates are abundant on Panamanian
pocilloporid reefs, and numerous attacks on sea stars have been witnessed.
In a typical attack scenario, a pair of shrimp will cut through the aboral
dermis of Acanthaster and then begin to remove and eat such internal
organs as the hepatic caeca and gonads. The shrimp ride along with the sea
star, feeding on its soft parts for several days. Eventually, polychaete worm
scavengers are attracted to the wounded sea star and will invade its internal body cavities through the incisions made by the shrimp. As many as
15 to 20, 5- to 10-cm-long worms will continue feeding on Acanthaster's
soft parts until no internal organs remain and the sea star dies.
Other echinoderms feeding on live corals include two other sea stars
and two species of sea urchins. The dub-spined sea urchin, Eucidaris, is
perhaps the most persistent corallivore, often observed grazing on pocilloporid corals in the Galapagos Islands. This sea urchin abrades both the soft
tissues and skeletons of corals, and figures prominently in bioerosion. At
least eight fish species, representative of five families, feed on live corals,
with their feeding strategies ranging from removing mainly live tissue and