Riches of the reef.
Coral polyps are small marine animals that create limestone shells. These build up into massive structures called coral reefs, but only in waters that are warmer than 25 c' and less than 10 meters deep. They are among the most productive ecosystems on Earth, achieving 3000 times the photosynthetic productivity of the surrounding waters. The reef owes its wealth to a special relationship between corals and plants. Inside each polyp there are tens of thousands of single-celled plants called zooxanthellae, which supply the coral with additional energy through photosynthesis. They also recycle limited nutrients. Corals catch zooplankton and other prey, and the waste products are used by the zooxanthellae. The Odum brothers discovered this relationship when they calculated that the zooplankton in the surrounding sea could not provide enough energy and nutrients for the coral reef to survive. It was the zooxanthellae that provided the missing figures. Energy and nutrients are exchanged very efficiently in a coral reef and, as in the tropical rain-forest, most of the nutrients are locked up in living organisms