KOROR STATE GOVERNMENT MARINE TOUR GUIDE ... - C3

3.6 Threats to Coral Reefs
3.6.1 Predators on Corals
The COT is an unusually large starfish and can grow to more than 80cm in diameter. It
has up to 21 arms, covered in long venomous spines.
Breeding
Recent research by David Idip Jr., a Researcher at the PICRC, has revealed two
spawning times for COTs, March-June and September, which seem to be associated
with a decrease in water temperature.
They release eggs and sperm into the water. When the eggs are fertilized, they become
larvae which spend 2-3 months drifting in the ocean currents. The juveniles settle onto
the reef when they are about 1-2mm across. They live among rocks and rubble on the
reef and are almost invisible until they are about 6 months old.
A COT can breed for 5-7 years and each female can produce up to 250 million eggs in a
single spawning season. The starfish gather in the shallow water to spawn which
increases the chance of fertilizing the eggs. So a small population of COTS could
potentially produce a very large number of offspring.
Feeding
Young COTS eat encrusting (coralline) algae which is common among rocks and rubble
on the reef. At about 6 months of age, they start to eat coral and begin to grow more
rapidly. Over the next two
years, the starfish can
grow from 1cm to 25cm in
diameter. When there are
few COTS, they tend to be
hidden under corals during
the day and move into the
open to feed at night. They
feed mainly on coral
species that they prefer,
and may not eat the entire
coral colony.
Pat Colin
As a result, the reef can recover quite rapidly from low levels of coral feeding. Some
reefs seem to support small populations of crown-of thorns starfish for many years, with
only a small reduction in coral cover.
However, when starfish occur in large numbers, there is intense competition for food and
most corals will be eaten, including the less-favored species. The starfish aggregate and
stay in the open, feeding night and day. During a severe outbreak, there can be several
COTs per square meter and they can kill most of the living coral in an area of reef,
reducing coral cover from the usual 25-40% to less than 1%.
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