Coral Reefs  Coral reefs in tropical waters form wave barriers.
All reefs existing today have been built up during the last
6,000 years. Fringe reefs grow around the shores of continents
or islands. Barrier reefs grow several miles offshore. A coral
atoll is a broken ring of coral islands with water in the
middle. The forward edge of the reefs consist of massive coral.
All reef building corals contain symbiotic plant cells.  Coral
reefs are found in shallow seas. They are built by tiny animals
called polyps. Each polyp builds a small stone cup or a stony
skeleton around itself to live in. When the polyp dies, it
leaves its cup behind. New polyps build on top and gradually the
reef grows. The reef is made up of layers of living coral
animals attached to a wall of many millions of dead skeletons.
The polyps eat mostly at night when their tiny arms come out of
its stony skeleton. The arms catch plankton and feed them into
its mouth.  Corals get their colors from the algae that lives
inside the coral polyps. With