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The Biology of Coral Reef Formation

Often described as the "rainforests of the sea," coral reefs are among the most diverse and productive ecosystems on Earth. These massive underwater structures are not rocks or plants but are built by vast colonies of tiny animals known as coral polyps. Each individual polyp, which is related to sea anemones and jellyfish, secretes a hard, protective exoskeleton made of calcium carbonate. Over countless generations, the skeletons of these polyps accumulate, fusing together to form the intricate and enormous frameworks that constitute a coral reef.

The survival and growth of most reef-building corals depend on a critical symbiotic relationship with single-celled algae called zooxanthellae, which live inside the coral's tissues. Through photosynthesis, these algae produce organic nutrients that provide the coral with up to 90 percent of its energy. In exchange, the coral polyp offers the algae a safe, sheltered environment and the chemical compounds needed for photosynthesis. This partnership is also responsible for the vibrant colors characteristic of healthy corals.

Coral reefs are delicate ecosystems that can only flourish under a specific set of environmental conditions. They require clear, warm, and shallow saltwater. The water must be clear to allow sufficient sunlight to penetrate for the zooxanthellae's photosynthesis. For this reason, the majority of the world's coral reefs are located in tropical and subtropical waters. Any significant or prolonged deviation from these optimal conditions, such as a rise in sea temperature or ocean acidity, can stress the corals, causing them to expel their symbiotic algae in a process known as coral bleaching.