Read an Academic Passage

The Formation of Coral Reefs

Coral reefs are often called the "rainforests of the sea" due to their extraordinary biodiversity. These underwater structures are built by colonies of tiny animals called coral polyps. A single coral polyp is a soft-bodied organism related to sea anemones and jellyfish. To protect their soft bodies, they secrete a hard outer skeleton of calcium carbonate. Over thousands of years, the skeletons of countless polyps fuse together to form the massive structures we know as coral reefs, which can be found in warm, clear, shallow tropical waters.

The growth and survival of coral reefs depend on a delicate symbiotic relationship with microscopic algae called zooxanthellae. These algae live within the tissues of the coral polyps. The algae perform photosynthesis, providing the coral with up to 90 percent of its energy needs, and also contribute to the coral's vibrant colors. In return, the coral provides the algae with a protected environment and the compounds necessary for photosynthesis. This mutual dependence is essential for the health of the entire reef ecosystem.

There are three main types of coral reefs: fringing reefs, which grow close to the shore; barrier reefs, which are separated from the mainland by a lagoon; and atolls, which are ring-shaped reefs that enclose a lagoon. The Great Barrier Reef in Australia is the world's largest barrier reef system, so vast that it can be seen from space. These ecosystems are vital, providing habitats for a quarter of all marine species and protecting coastlines from storms, but they are increasingly threatened by environmental stressors.