Coral are members of phylum Cnidaria. The coral animals secrete a calcium carbonate structure for protection and support inside wihich live polyps reside. Corals live in a mutual relationship with algae called Zooxanthellae, which photosynthesize and produce nutrients to benefits themselves and the coral they live on. They contribute to the color of the coral reefs. The zooxanthellae give corals a brown or golden-brown appearance, as well as other pigments such as red, blue, green which the coral themselves produce. When corals get "bleached" and become white, this condition can kill corals. This can occur if water temperature is too high or pollutants are present in the water. Corals also capture their own food called zooplankton with their tentacles and can absorb nitrogen and phosphorus from the water. Coral reefs are important for maintaining biodiversity in tropical seas as many fish, crustaceans, mollusks, worms, etc. depend on the reef for their habitat and for finding food.
Coral reefs are large underwater structures made up of thousands of coral lifeforms. They have a wide range of colours.Their wide range of colors are caused by a number of factors, including life that inhabits them and environmental conditions. Coral can cover the whole spectrum of visible colors and their coloring can indicate whether the coral populating a reef are healthy or unhealthy.