Coral reefs are made from masses of lime of carbonate built up from the sea floor by the accumulation over thousands of years of the skeletons of minute animals called polyps. Algae called zooxanthellae live in the tissues of the polyps in a mutually beneficial way (symbiosis). Eventually, the coral reef breaks the surface of the water.
Reef building corals grow best in shallow, sunlit water up to a depth of 12 metres though they can still construct reefs from the sea floor up to 40 metres deep.
Reef building corals prefer sea water of normal salinity (between 30 and 40 parts per thousand), with temperatures ranging from a maximum to 28 degrees in summer to a minimum of 15 degrees in winter.
A second group of corals exist that grows in thickets and develop on banks rather than on reefs. They tend to be found on the outer, deeper and colder parts of the continental shelves and platforms. These organisms can live in minimum winter temperatures ranging from 4o to 15o C and at depths of about 60 to 200 metres. There are no such corals in Mauritius but they can be found along the eastern Atlantic shelf edge from Norway to Cape Verde islands for example.
A third group of corals can be found in even colder seas, where temperatures range form 2o to 6o C. They consist of small, solitary corals and are to be found on the abyssal floors of the oceans and on continental shelves around the Falkland Islands, Patagonia and Antarctica.
source: Institute For Environmental and Legal Studies