Coral reefs are among the most biologically rich ecosystems on earth. About 4,000 species of fish and 800 species of reef-building corals have been described to date. Coral reefs are the most complex, species-rich and productive ocean ecosystems. Reefs cover only a small percentage of the ocean's area but they provide home to one-third of all the fish in the ocean and thousands of other animals. Coral reefs have often been described as the Rainforests of the Sea because they have many different species that call them home.