The worlds Coral reefs have formed through the mineral deposits of billions of small invertebrate animals, called coral polyps. All of these polyps survive via colonies, which are made up of one to many thousands of individuals. They are able to construct homes made from calcium carbonate, similar to the bones in our body. They can retract themselves into their little homes when attacked by predators. Over millions of years, these animals have been able to create vast underwater mountains which we call coral reefs.

Coral polyps are similar to anemones, having tentacles, a mouth and an internal body sac called a gastrovascular cavity which is used to digest food. These polyps are sessile, meaning they cannot move; they catch prey with their tentacles, which may be floating on the water currents or swimming towards certain death. After having caught a prey, their cnidocytes release tiny poisonous harpoons. These powerful stinging cells will paralyze the prey, after which they ingest it.

Students of corals have gradually come to understand the symbiotic relationship between a stoney coral with its mineralized skeleton and fleshy polyps and the millions of zooxanthellae cells living in its flesh. The sun feeds the zooxanthellae and the zoo's. feed the coral which were able to build carbonate skeletons and huge reefs.

Just how corals manage to feed themselves has been a controversial debate among aquarium reef hobbyists for many years. Some experts still argue that corals with zooxanthellae need only appropriate lighting to do very well in captive aquarium conditions. Although aquarium enthusiasts still argue about the various ways corals accumulate their daily energy, marine biologist have reached a consensus about what they actually consume. These can be divided into 3 main catagories; (1) Photosynthetic products that are formed by symbiotic algae;(2) food such as plankton and drifting detritus and (3) The oceans dissolved nutrients.

For anyone who's goal it is to keep and grow healthy corals in a thriving reef environment a basic knowledge of coral nutrition is essential.

References from WikiPedia:

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps are like tiny sea anemones, to which they are closely related. Unlike sea anemones, coral polyps secrete hard carbonate exoskeletons which support and protect their bodies. Reefs grow best in warm, shallow, clear, sunny and agitated waters.

Often called “rainforests of the sea”, coral reefs form some of the most diverse ecosystems on Earth. They occupy less than 0.1% of the world's ocean surface, about half the area of France, yet they provide a home for 25% of all marine species,^[1] including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians.^[2] Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but deep water and cold water corals also exist on smaller scales in other areas.

Coral reefs deliver ecosystem services to tourism, fisheries and shoreline protection. The annual global economic value of coral reefs has been estimated at $US375 billion. However, coral reefs are fragile ecosystems, partly because they are very sensitive to water temperature. They are under threat from climate change, ocean acidification, blast fishing, cyanide fishing for aquarium fish, overuse of reef resources, and harmful land-use practices, including urban and agricultural runoff and water pollution, which can harm reefs by encouraging excess algae growth.