Read an Academic Passage

The Formation of Coral Reefs

Coral reefs are among the most diverse ecosystems on Earth, often called the "rainforests of the sea." These underwater structures are built by colonies of tiny animals known as coral polyps. A coral polyp is a soft-bodied organism that attaches itself to the ocean floor and secretes a hard skeleton made of calcium carbonate. Over thousands of years, the accumulated skeletons of countless polyps build up to form the massive structures we recognize as coral reefs. They are predominantly found in warm, clear, shallow tropical and subtropical waters.

The survival of coral reefs is dependent on a crucial symbiotic relationship. Living within the tissues of the coral polyps are microscopic algae called zooxanthellae. The algae perform photosynthesis, providing the coral with essential nutrients and energy, which also gives the coral its vibrant color. In return, the coral polyp provides the algae with a protected environment and the compounds necessary for photosynthesis. This partnership is vital; without the algae, the coral cannot survive for long.

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 coral reef system, so vast that it can be seen from outer space. These ecosystems are not only beautiful but also vital, as they protect coastlines from erosion and support the livelihoods of millions of people.