Coral is formed by a polyp living in a rigid calcium carbonate (calcite or aragonite) shell or skeleton which it secretes itself. Along with its close relatives the jelly-fish, sea anemones, eel grass, millepores and other members of the cnidarian branch, they have the simplest mode of organization of all multicellular animals. This polyp has a cylindrical body and its mouth is surrounded by a ring of tentacles. In fact, apart form a few small differences, it is simply a miniature anemone.

Coral species can be either solitary (a single polyp inhabiting a single shell) or colonial. In the latter case, the shells, each of which contains a single polyp, fuse together, forming a colony of several thousand individuals which can occupy an area of several square metres. In this underwater condominium, the occupants benefit from several ecological advantages as far as their protection, nutrition, reproduction, genetic stability and respiratory requirements are concerned.

Hermatypic coral species are those which form reefs containing the unicellular algae called zooxanthella , which can have densities of up to 1 million cells per cm². There are 12 families and 47 genera. A Formal Taxonomic listing of Zooxanthellate Corals is given at the following address: