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expeditions/June-July
2006/ Day 25
Coral Predators of the Northwestern Hawaiian Islands
Patricia Greene, NOAA Teacher-at-Sea
When you think of the Northwestern Hawaiian Islands and predators, the first thing that comes to mind may
be the apex predators: tiger sharks, Galapagos sharks, and species of huge fish such as the jacks.
The crown of thorns (Acanthaster
planci) is a major predator of coral reefs. Hawaiian Islands National Wildlife Refuge, U.S. Fish and Wildlife Service, Dept. of Interior.
Photo: Claire Johnson/NOAA
However, while snorkeling the Kure Atoll, I was reminded that there is another group of predators here:
the corallivores. I observed a crown of thorns that appeared to be feeding on the coral and upon further
research I discovered and recognized a variety of Northwestern Hawaiian Islands creatures that I have seen
that also specialize in feeding on corals.
Corallivores (an animal that feeds on corals) may include fish, sea stars, or mollusks. Generally, two
types are recognized; obligate corallivores; those that feed only on corals and facultative corallivores;
which feed on corals, algae, sponges, and mollusks.
The crown of thorns (Acanthaster planci) has cryptic coloration and
toxin-filled spines. It prefers to feed on rice corals (Montipora),
lace corals (Pocillopora), and cauliflower corals (Acropora).
Ironically, the crown of thorn eggs and larvae are often fed on by the stony
corals. Other natural enemies of the crown of thorns is the harlequin shrimp
and the fireworm. The little harlequin shrimp does not kill the crown of thorns,
but merely creates a small, open wound. This is known as “facilitated predation.” The larvae
of the fireworm then enter the cavity, reproduce, and the offspring eat the crown
of thorns from the inside out; eventually causing death.
The crown of thorns feeds by inverting its stomach through its mouth, and then
digests the corals externally. Human attempts at controlling populations
of crown of thorns have been relatively
unsuccessful and causes of these population spikes or outbreaks have been a topic
of debate. Some believe they are natural occurrences and occur in cycles
while other scientists believe
they are due to human causes such as increased sedimentation and pollution.
The ornate butterflyfish (Chaetodon ornatissimus) is one type of butterflyfish that is also a coral predator. Hawaiian Islands National Wildlife Refuge, U.S. Fish and Wildlife Service, Dept. of Interior. Photo: Claire Johnson/NOAA
We have also observed a variety of butterflyfish on the reefs; all of which are coral predators.
The ornate butterflyfish (Chaetodon ornatissimus), the oval butterflyfish (Chaetodon
lunulatus), the fourspot butterflyfish (Chaetodon quadrimaculatus), and the multiband
butterflyfish (Chaetodon multicinctus), are all obligate corallivores. Other butterflyfish that
eat both corals and invertebrates include the threadfin butterflyfish (Chaetodon
auriga) and the teardrop butterflyfish (Chaetodon unimaculatus).
The shortbodied blenny (Exallias
brevis) is an obligate corallivore, which feeds on coral. Photo: Claire Johnson/NOAA
The shortbodied blenny (Exallias brevis) is an obligate corallivore. It prefers the lobe
(Porites lobata) and finger coral (Porites compressa). The spotted color of these
fish blends nicely with the colonies of coral. Removing tiny bites these fish have little impact
on the health of the corals. The coral colony is able to regenerate new polyps and fill in the bite marks.
We have also identified the spotted pufferfish (Arothron meleagris) hiding in the corals of
Kure Atoll’s lagoon. This unique creature has a beak-like mouth with sharp frontal teeth for removing
pieces of substrate and flat teeth in the back for grinding. They feed on a variety of organisms,
including the stony corals and calcareous algae. They have a unique adaptation that allows them to
lodge their bodies into a crevice or hole and then puff up so it is impossible for a predator to
dislodge them. Their tissue is relatively toxic to humans.
The blue-eye damselfish (Plectroglyphidodon johnstonianus) inhabits the Northwestern Hawaiian
Islands coral reefs. It feeds only on coral, preferring the lace, antler, cauliflower, finger and lobe
corals. These small fish are very territorial and will defend their nests, hiding in the corals that
also serve as food.
Most of the coral predators do not pose any major threats to the coral reefs. They are natural inhabitants
of the reefs and do little damage. The crown of thorns can cause mass devastation; during major outbreaks
at other Pacific Ocean locations the coral cover was reduced from 78% to 2%. In 1970, approximately 26,000
crown of thorns were destroyed off the southern coast of Moloka`i. However, during all of dives in the
Northwestern Hawaiian Islands we only observed two crown of thorns, which is good news for this remote
region.
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