Coral Reef
Coral Reef, ridge or elevated part of a relatively shallow area of the seafloor, approaching the sea’s surface. It is formed by a rocklike accumulation of calcareous (calcium-containing) exoskeletons of coral animals, calcareous red algae, and mollusks. Built up layer by layer by living corals growing on top of the skeletons of past generations, coral reefs grow upward at rates of 1 to 20 cm (0.4 to 7.8 in) per year. Coral reefs are tropical, extending to about 30° north and south of the equator and forming only where surface waters are never cooler than 20° C (68° F). Coral reefs are ecosystems with well-defined structures that involve both photosynthetic algae and consumers. The outer layer of a reef consists of living polyps of coral. Within the coral animals live single-celled, round algae called zooxanthellae. Below and surrounding the polyps is a calcareous skeleton, both living and dead, that contains filamentous green algae. Other species of algae, both fleshy and calcareous,