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Description: Stony corals are like small anemones, but
are usually colonial and secrete limestone cups into which the
small polyps can retract. Most stony corals live in colonies.
Soft corals consist of thick masses of flesh, toughened by the
particles of limestone imbedded in them, and bear delicate polyps.
They are all colonial and lend a variety to the reef in shape,
pattern and color.
Range and Habitat: Reef building corals are found in shallow
tropical seas between 30 degrees north and 30 degrees south latitude.
Living coral are confined to the top layer of the reef on top
of skeletons of coral from years past. Deep water corals are not
the reef building variety. Cold deep waters of the Norwegian fjords
also support coral.
Habits and Adaptations: Reef building corals have mutualistic
algae (zooxanthellae) living in their tissues. The microscopic
zooxanthellae are very important to the corals: their photosynthesis
and fixation of carbon dioxide furnish food molecules for their
hosts, they recycle phosphorus and nitrogenous waste compounds
that otherwise would be lost, and they enhance the ability of
the coral to deposit calcium carbonate. Reef forming corals develop
in three basic formations: fringing reefs (close to land), barrier
reefs (several miles from land), and atolls (horseshoe-shaped).
Diet: Coral feed primarily at night. They catch tiny animals,
absorb dissolved organic matter and obtain food from the zooxanthellae
that live in their body.
Breeding and Maturation: New individuals arise asexually
by budding from the coral itself. Each polyp can function as male
or female, although not at the same time. Sperm is released into
the water and find their way into the body of another polyp containing
eggs and fertilization takes place. The fertilized eggs grow into
planula which are released into the water. Upon reaching the bottom
they search for a site on which to settle. They can also reproduce
through fission, where the coral divides in half and becomes two
separate coral bodies.
Miscellaneous: Corals have a natural defense against sedimentation,
for they secrete a mucus which entangles dirt particles falling
on the reef's surface. Waving cilia carry the particles to the
edge and dump them off. But if siltation is too heavy, the coral
becomes coated with a film of sediment that it cannot remove.
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