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The Biology of Coral Reef Formation

Coral reefs are often called the "rainforests of the sea" due to their incredible biodiversity, yet they are built by tiny animals known as coral polyps. These polyps, which are related to jellyfish, form colonies that can grow into massive underwater structures over thousands of years. The process begins when a polyp attaches itself to a hard surface on the seafloor. It then secretes calcium carbonate, creating a protective limestone skeleton. As the colony grows, with new polyps building on the skeletons of old ones, a reef is formed. Most reef-building corals thrive in warm, shallow, clear, and sunlit tropical waters.

The survival of most reef-building corals depends on a crucial symbiotic relationship with microscopic algae called zooxanthellae. These algae live inside the coral's tissues. Through photosynthesis, the zooxanthellae produce nutrients, providing the coral with up to 90 percent of its energy. In return, the coral polyp provides the algae with a protected environment and the compounds necessary for photosynthesis. This relationship is also responsible for the vibrant colors of many corals; the pigments of the algae are visible through the coral's transparent tissue.

There are three main types of coral reefs, each distinguished by its structure and relationship to land. Fringing reefs grow directly from the shore. Barrier reefs are separated from the mainland by a wide, deep lagoon; the Great Barrier Reef off the coast of Australia is the most famous example. Atolls are circular or oval reefs with a central lagoon, typically formed on the rim of a submerged volcano. All three types of reefs are highly sensitive to environmental changes, and their continued existence is threatened by various factors.