Coral reefs are the largest biologically constructed structures on earth. They cover about 250,000 sq km
Coral animals (mostly scleractinian corals) are the building blocks of reefs, depositing a calcium carbonate skeleton as they grow.
Together with the shells of molluscs and skeletal remains of other animals and plants, calcareous algae cements the matrix together
Much of the success of a reef is due to symbiotic associations - corals have zooxanthellae (algae) within their tissues that provide the coral with most of its food, remove waste products, and help lay down their skeleton. Sponges and giant clams have similar associations with single-celled organisms, and small crustaceans, molluscs, worms live on their outer surfaces and within cavities
In addition to the zooxanthellae, algae and phytoplankton are the reef's primary producers, converting sunlight and nutrients into organic compounds. These are fed on by herbivores, which in turn are consumed by higher trophic levels.