Corals, sea anemones and their relatives belong to a group of animals
called Cnidarians. Coral colonies are composed of individual polyps
that have six or more tentacles, each lined with stinging cells
(nematocysts). There are several different kinds of corals and
coral-like animals. Hard corals secrete limestone skeletons and are
responsible for the creation of tropical coral reefs. Related animals
- such as soft corals, gorgonians and sea pens - do not construct reefs,
but are nevertheless important to reef ecology.
Reef-building corals gain energy and nutrition from symbiotic algae that live within their tissues, and that may impart a distinctive color to their hosts. Most corals grow very slowly - often only inches a year - but over a period of thousands of years they can create enormous structures such as atolls and barrier reefs.