Corals live in very nutrient poor waters and have certain zones of tolerance to water temperature, salinity, UV radiation, opacity, and nutrient quantities.
Scleractinian corals build skeletons of calcium carbonate sequestered from the water. When the coral polyp dies, this skeleton remains incorporated in the reef framework.
Scleractinian corals are in the Phylum Cnidaria, and they receive their nutrient and energy resources in two ways. They use the traditional cnidarian strategy of capturing tiny planktonic organisms with their nematocyst capped tentacles, as well as having a obligate symbiotic relationship with a single cell algae known as zooxanthellae. Zooxanthellae are autorophic microalgaes belonging to various taxa in the Phylum Dinoflagellata.