Coral Reefs and Atoll Formation

Coral reefs are built not by a single organism but by colonies of tiny animals, scleractinian, or stony, coral polyps, each secreting a cup-shaped skeleton of calcium carbonate around its soft body and living in a tight symbiosis with microscopic photosynthetic algae called zooxanthellae, which live within the coral's tissue, supply it with the bulk of its energy through photosynthesis, and give reefs their vivid color. Because the zooxanthellae need sunlight, reef-building corals generally cannot grow in water much deeper than about 50 to 70 meters, and they thrive best in warm, clear, nutrient-poor tropical waters typically between 23 and 29 degrees Celsius, which is why coral reefs are essentially confined to a band running roughly between the Tropic of Cancer and the Tropic of Capricorn. As successive generations of polyps die, their limestone skeletons accumulate into a rigid, wave-resistant structure that can grow upward at rates of a few millimeters to several centimeters per year depending on the species and conditions, slowly building the massive geological formations, fringing reefs hugging a shoreline, barrier reefs separated from land by a lagoon, and atolls, that make coral among the very few organisms capable of constructing landforms visible from space.