A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Often called “rainforests of the sea”, shallow coral reefs form some of Earth’s most diverse ecosystems. 

The corals have a symbiotic, or mutually beneficial, relationship with the zooxanthellae. These algae live inside the coral polyp’s body where they photosynthesize to produce energy for themselves and the polyps. The polyps, in turn, provide a home and carbon dioxide for the algae. Additionally, the zooxanthellae provide the coral with their lively colours.

The process of formation of coral reefs: 

Coral reefs are large underwater structures composed of the skeletons of colonial marine invertebrates called coral. Each individual coral is referred to as a polyp. Coral polyps live on the calcium carbonate exoskeletons of their ancestors, adding their own exoskeleton to the existing coral structure.

Reefs form when corals grow in shallow water close to the shore of continents or smaller islands. As the corals grow and expand, reefs take on one of three major characteristic structures —fringing, barrier or atoll. 

• Fringing reefs are the most commonly seen reef and grow near coastlines.
• Barrier reefs differ from fringing reefs in that they are separated from the coastlines by deeper, wider lagoons. 
• The rings of coral that makeup atolls create protected lagoons in the middle of the oceans, typically around islands that have sunk back down into the ocean.

The largest of these coral reef systems is the Great Barrier Reef in Australia. It plays an important role in maintaining the coastal ecology of the world in the following manner.

• It provides food, shelter and breeding area to at least 400 individual species of coral.
• It is the primary habitat for more than thousands of different species of fish, mollusks, sea snakes, sea turtles, whales, dolphins, birds and more. Their destruction can lead to the extinction of thousands of species of marine life. 
• They reduce the damage in case of storms, hurricanes and tsunamis by absorbing wave energy and contribute to environmental protection through the reduction of coastal erosion.
• They protect ecosystems located between the reefs and coasts, such as seagrass and lagoon for example, and human settlements located by the sea.
• Reefs also protect the highly productive wetlands along the coast.

Threat to Coral reefs: Coral reefs are fragile because they are sensitive to water conditions. They are under threat from excess nutrients (nitrogen and phosphorus), rising temperatures, oceanic acidification, overfishing, sunscreen use, and harmful land-use practices, including runoff and seeps. Many of these threats can stress corals, leading to coral bleaching and possible death of these delicate ecosystems. 

Coral reefs deliver ecosystem services for tourism, fisheries and shoreline protection. They are also are a source of food and new medicines. The need of the hour is to protect coral reefs by reducing and eventually eliminating dumping materials and chemicals, reduce fishing and monitor the water quality of run-off directed toward the reef. Healthy reefs lead to healthy oceans, and healthy oceans are vital to all life on Earth.