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The Formation of Coral Reefs

Coral reefs are often called the "rainforests of the sea" due to their incredible biodiversity, yet they are built by tiny, simple animals known as coral polyps. These polyps, which are related to jellyfish, live in large colonies and secrete skeletons of calcium carbonate. Over thousands of years, the accumulation of these skeletons builds up the massive, complex structures we recognize as coral reefs. The polyps themselves are mostly transparent, and their vibrant colors come from microscopic algae called zooxanthellae that live within their tissues.

The relationship between coral polyps and zooxanthellae is a classic example of a symbiotic partnership. The algae find a protected environment inside the polyps and use sunlight to perform photosynthesis, producing oxygen and energy-rich compounds. In return, the coral polyps receive up to 90 percent of their nutrients from the algae. This mutual dependence is vital for the reef's survival, as it allows corals to thrive in the nutrient-poor tropical waters where most reefs are found. This relationship is also fragile; if water temperatures rise too high, corals will expel the algae, leading to a phenomenon known as coral bleaching.

Geographically, there are three main types of coral reefs. Fringing reefs grow directly from the shore, forming a border along the coastline. Barrier reefs, like the Great Barrier Reef in Australia, are separated from the mainland by a wide, deep lagoon. The third type, atolls, are ring-shaped reefs that enclose a central lagoon. Atolls typically form on the tops of submerged volcanic islands, growing upward as the island slowly sinks beneath the sea. Each of these reef types supports a unique and complex ecosystem, providing a critical habitat for thousands of marine species.