Coral reefs are one of the most diverse and biologically productive ecosystems on earth.
Their high productivity is the result of efficient biological recycling and a high retention of nutrients in an otherwise nutrient-poor environment. These reefs provide habitats for a wide variety of marine organisms, such as fish, sea turtles, marine mammals, crustaceans and invertebrates. A critical aspect of reef-building corals is their symbiotic relationship with the unicellular algae, zooxanthellae. The algae live, conduct photosynthesis, and process the coral's waste products all within the cells of their host. The entire biological productivity of the coral reef ecosystem rests on this symbiotic relationship.