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The Biology of Coral Reef Formation

Often called the "rainforests of the sea," coral reefs are among the most diverse ecosystems on Earth. These massive underwater structures are not rocks or plants, but are built by colonies of thousands of tiny animals known as coral polyps. Each individual polyp is a soft-bodied organism that secretes a hard, protective limestone skeleton of calcium carbonate. Over countless generations, the skeletons of these polyps accumulate, fusing together to form the vast and complex structures of a reef.

The growth and survival of reef-building corals are dependent on a crucial symbiotic relationship with microscopic algae called zooxanthellae. These algae live within the tissues of the coral polyps. Through photosynthesis, the zooxanthellae produce nutrients that provide the coral with up to 90% of its energy requirements. In return, the coral provides the algae with a protected environment and the chemical compounds needed for photosynthesis. This relationship is also responsible for the vibrant colors characteristic of healthy corals.

Because the symbiotic algae require sunlight for photosynthesis, coral reefs can only thrive in specific environmental conditions. They are generally found in clear, shallow, warm marine waters, primarily in tropical and subtropical regions. The water must be clear to allow sufficient sunlight penetration. Water temperature is also critical; most reef-building corals are confined to a narrow temperature range. Significant changes in these conditions, such as rising water temperatures, can stress the corals, causing them to expel their zooxanthellae in a phenomenon known as coral bleaching, which can lead to the death of the coral.