Corals are small marine animals that stay in one place. They live in dense colonies in warm, shallow waters. Coral reefs are formed by the skeleton remains of many generations of stony corals. Massive reef structures are built over thousands of years by tiny coral polyps (coral animals). Tiny algae live in their tissues, as well as other organisms that produce calcium carbonate (the hard stuff in coral). Reef-building corals are generally found close to shore, where there is clear water where sunlight shines through. Reef-building corals, along with the algae, require warm ocean temperatures (20– 28°C), in waters close to the equator. These reefs are amongst the earth’s oldest living communities of plants and animals. They vary in shape, size, and color.