Corals

Corals are organisms living underwater from the class Antozoa and exist as small sea anemones–like polyps, typically in colonies of many identical individuals. The group includes the important reef builders that are found in tropical oceans, which secrete calcium carbonate to form a hard skeleton.
A coral "head", commonly perceived to be a single organism, is formed from thousands of individual but genetically identical polyps, each polyp only a few millimeters in diameter. Over thousands of generations, the polyps lay down a skeleton that is characteristic of their type. A head of coral grows by asexual reproduction of the individual polyps.
Although corals can catch planktons using the stinging cells on their tentacles, these animals obtain most of their nutrients from symbiotic unicellular algae called zooxanthellae. Consequently, most corals depend on sunlight and grow in clear and shallow water, typically at depths shallower than 60m. These corals can be major contributors to the physical structure of the coral reefs hat develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not have associated algae and can live in much deeper water, such as in the Atlantic, with the cold-water genus Lophelia surviving as deep as 3000 m. Examples of these corals can be found living on the Darwin Mounds located north-west of Cape Wrath, Scotland. Corals have also been found off the coast of Washington State and the Aleutian Islands in Alaska.
Adapted from: Wikipedia (search CORALS)