Corals are anthozoans, the largest class of organism within the phylum cnidaria. Stony corals make up the largest order of anthozoans, and are the group primarily responsible for laying the foundation of, and building up the reef structures. Stony corals are colonial organisms composed of hundreds to hundreds of thousands of individual, called polyps. The massive reef structures are formed when each stony coral pholyp secrets a skeleton of calcium carbonate. Although all corals secretes calcium carbonate, not all are reef builders.
Colonies of reef building corals exhibit a wide range of shapes but most can be classified within ten genral forms. They are 1. Branching corals 2. Digitate corals 3. Table corals 4. Elkhorn coral 5. Foliose corals 6. Encrusting corals 7. Submassive corals 8. Massive corals 9. Mushroom corals 10. Cup corals.
Coral Reefs grow optimally in water temperatures between 23 degree and 29 degree centigrade, but some can tolerate temperatures as high as 40 degree centigrade for a limited period of time. Most require very salty water ranging from 32 to 42 parts per thousand. The water must also be clear to permit high light penetration. Hence the number of species of corals on a reef declines rapidly in the deeper water.
View Source: http://www.coris.noaa.gov/about/what_are/
