Read an Academic Passage

The Formation of Coral Reefs

Coral reefs are often called the "rainforests of the sea" due to their incredible biodiversity. These underwater structures are built by colonies of tiny animals called coral polyps. Each polyp secretes a hard exoskeleton made of calcium carbonate. Over thousands of years, the accumulated skeletons of countless generations of polyps build up to form the massive structures we know as reefs. Coral reefs are primarily found in warm, shallow, clear tropical waters, as they depend on sunlight to survive.

The survival of corals is dependent on a symbiotic relationship with a type of algae called zooxanthellae. These algae live within the tissues of the coral polyps. Through photosynthesis, the algae produce oxygen and essential nutrients that the corals use for energy and growth. In return, the coral provides the algae with a protected environment and the compounds necessary for photosynthesis. This relationship is also responsible for the vibrant colors of coral reefs; the algae impart these bright hues to their coral hosts.

Coral reefs are facing numerous threats, many of which are caused by human activities. Rising sea temperatures can cause "coral bleaching," a phenomenon where corals expel their symbiotic algae, turning white and becoming vulnerable to disease and death. Ocean acidification, a result of increased carbon dioxide in the atmosphere, makes it difficult for corals to build their calcium carbonate skeletons. Pollution from land and destructive fishing practices also damage these fragile ecosystems. Protecting coral reefs is a critical environmental priority, as they support about 25% of all marine species and provide livelihoods for millions of people.