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.
Corals have very specific requirements for light, temperature, water clarity, salinity and oxygen.
Coral growth is relatively slow, especially in areas where sediments are regularly disturbed because silted substrates prevent larval settlement. If light penetration is decreased these is a reduction in photosynthesis by the zooxanthellae algae. Most corals, therefore, are restricted to depths of less than 30 meters. Their lack of mobility makes them vulnerable to environmental disturbances, such as oil spills, through smothering and oxygen depletion. Coral reefs in Hawaii are fairly well developed but display low species diversity because of Hawaii's extreme isolation and more northerly latitude.