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The Biology of Coral Reef Formation

Often called the "rainforests of the sea," coral reefs are among the most diverse and productive ecosystems on Earth. These massive underwater structures are not rock formations but are in fact built by millions of tiny animals called coral polyps. These organisms thrive in specific environmental conditions, requiring clear, warm, shallow, and sunlit saltwater to grow. The Great Barrier Reef off the coast of Australia, for example, is so large that it can be seen from space, a testament to the constructive power of these small creatures.

The biological process of reef-building is centered on the coral polyp's ability to form a hard skeleton. Polyps extract calcium carbonate from seawater and secrete it to create a protective, cup-like structure. Critically, most reef-building corals have a symbiotic relationship with microscopic algae called zooxanthellae. These algae live within the coral's tissues, providing the polyps with up to 90 percent of their food through photosynthesis. In return, the coral provides the algae with a protected environment and the compounds they need for photosynthesis. This relationship also gives corals their vibrant colors.

Reefs grow over immense timescales, often thousands of years. As individual polyps die, their hard skeletons remain, providing a foundation upon which new polyps build their own skeletons. Over generations, this slow accumulation of calcium carbonate creates the massive limestone structures we recognize as coral reefs. There are three main types of reefs: fringing reefs, which grow near the coastline; barrier reefs, which are separated from the mainland by a lagoon; and atolls, which are rings of coral that grow on top of submerged volcanic islands.