Coral reef is a type of underwater environment shaped by limestone formations. Reef limestone consists largely of a framework of skeletons from marine animals called corals. Reefs also contain the remains of other limestone-producing organisms, such as algae and shellfish, as well as sand and other particles. A coral reef is a complex, biologically rich ecosystem—that is, a community of living things and their environment. Many of the world’s most colorful animals live in coral reefs. Reefs also provide important benefits to people and to the larger environment.
Coral animals produce formations that may resemble branching trees, large domes, small irregular crusts, or tiny organ pipes. They can glow with rich colors, including beautiful shades of green, orange, purple, tan, and yellow. Coral limestone resists damage from waves, and it can grow hundreds of yards or meters thick. Overall, coral reefs range widely in area. Small patch reefs measure only a few square yards or meters, while some interconnected reef systems span hundreds of square miles or kilometers.
Kinds of coral reefs.
There are three basic types of coral reefs: (1) fringing reefs, (2) barrier reefs, and (3) atolls. Each type may consist of distinct areas, such as reef flats that extend from the shore to the reef crest, and reef slopes that slope downward to the ocean floor.
Loading the player...Coral reef near Fiji
Fringing reefs lie close to shore. They usually are younger, and thus smaller, than barrier reefs or atoll reefs.
Barrier reefs also follow the contours of the shore, but they occur farther from the coast than do fringing reefs. Barrier reefs form a barrier between a body of water near the shore, called a lagoon, and the open sea. Barrier reefs may consist of one fairly continuous reef, or they may be made up of a series of reefs separated by open water channels. Some barrier reefs grow to a considerable size. Australia’s Great Barrier Reef ranks as the largest group of coral reefs in the world, measuring about 1,400 miles (2,300 kilometers) long.
Atolls are ring-shaped reefs that often create coral islands in the open sea. An atoll forms as an old water-bound volcano slowly sinks into the sea, or the sea level rises around it. The reef grows upward from the surface of the volcano’s rim and creates a large ring that surrounds a lagoon. One or more channels connect the lagoon to the open sea. Many coral islands of the South Pacific Ocean are atolls.
Where coral reefs exist.
Coral reefs lie mainly in shallow tropical or subtropical seas. Most reef-forming corals cannot live in water colder than 61 to 68 °F (16 to 20 °C). Reefs require enough sunlight to support photosynthesis in their algae and plants. Photosynthesis is the process in which organisms use energy from sunlight to make food.
Coral reefs occur throughout warm waters of the Pacific Ocean, in coastal waters of Southeast Asia, in the Indian Ocean as far north as Sri Lanka, around Madagascar on the southeast African coast, and in the Red Sea. They also form along the tropical eastern coast of Brazil, in the Caribbean Sea, and along the coasts of Florida and Bermuda. Though coral reefs do not develop in colder seas, certain corals live as far north as the Arctic Circle.
How coral reefs form.
Most reef-building coral animals, also known as stony corals, live together in colonies. A single member of the colony, called a polyp, has a cylinder-shaped body. Its mouth lies at the top of the body, and tiny tentacles surround the mouth. Most polyps grow less than 1 inch (2.5 centimeters) in diameter. The polyps remove calcium carbonate from ocean water and use this material to form their cup-shaped outer skeletons. Coral skeletons accumulate over time in layers to help create a reef. Living corals inhabit the surface layer, while lower layers contain the skeletons of dead coral colonies.
Algae and other organisms also help build coral reefs. Some algae may deposit limestone that cements fragments of coral and other sediment into spaces in the reef framework. This limestone serves as a glue that holds the reef together. In some reefs, coralline algae produce a large portion of the reef structure. The shells or skeletons of clams, snails, sea urchins, and other creatures also provide building materials for reefs.
Coral reefs generally grow less than about 5 inches (13 centimeters) per year and can continue to grow over long periods. The Great Barrier Reef took millions of years to reach its present size.
Life on coral reefs.
Tens of thousands of living species inhabit reef areas, including at least 800 species of stony corals. Except for rain forests, coral reefs contain more living species than any other type of ecosystem. Indeed, coral reefs often are called the “rain forests of the oceans.”
Reefs usually occur in waters that are low in nutrients (nourishing substances). But reef inhabitants overcome this problem by efficiently recycling nutrients. The waste products of many reef organisms become food for other organisms. Thus nutrients cycle through organisms again and again, decreasing the need for new nutrient sources.
Vast numbers of colorful fish inhabit coral reefs. Many have special adaptations for reef environments. Parrotfish possess beaklike front teeth that they use to scrape off algae, their main food, from the hard corals. Moray eels have long, thin bodies that enable them to hide in limestone cracks, from which they catch passing prey.
Reefs also provide homes for reptiles. Sea turtles have paddlelike flippers for swimming. Sea snakes rank among the world’s most venomous animals. In addition, coral reefs house a huge variety of invertebrates (animals without backbones), including clams, crabs, lobsters, octopuses, sea anemones, sea stars, and sponges.
Unusual plants called sea grasses grow in many reef areas. They resemble true grasses but are not closely related to them. Unlike most plants, sea grasses can survive completely underwater. But they need to receive enough sunlight for photosynthesis to occur. Sea grasses provide food for a variety of reef animals.
Numerous microscopic organisms also play important roles in coral reefs. One common group, the bryozoans, encrust reefs and help cement their structures.
Many reef dwellers have special relationships with one another. For example, the tissues of most reef corals contain tiny zooxanthellae, a group of single-celled algae. Coral polyps consume certain waste products released by the algae, and the algae use as nutrients some of the polyps’ waste. Zooxanthellae also speed the process by which the corals produce their limestone skeletons. Such symbiotic relationships play an important role in reef life. In symbiosis, different organisms live together in a manner that benefits one or more of them.
Many other reef creatures have symbiotic relationships with one another. For instance, the tentacles of sea anemones provide refuge for clownfish. The clownfish, in turn, may lure prey toward the anemone or feed it scraps of food. Colorful, cigar-shaped fish called wrasses feed on parasites from the bodies of larger fish, and thus help keep the larger fish clean.
Coral reefs and the environment.
Coral reefs provide a number of vital benefits to the world’s environment. They contribute to biodiversity, or the planet’s variety of life forms, by providing homes for many different animals and plants. They also protect coastlines from erosion by strong ocean waves and can even produce the sand that makes up beaches.
Some scientists believe that coral reefs help regulate the planet’s temperature. The processes by which many reef creatures produce their shells causes them to absorb carbon dioxide, a gas that can raise the earth’s temperature if too much of it enters the atmosphere. Thus these reef creatures may help keep the planet from becoming too warm. However, coral reefs also produce carbon dioxide, so scientists continue to study how coral reefs affect global climate change.
Coral reefs and people.
Reefs aid people in numerous ways. Many societies depend on reef animals for food. Coral reefs also benefit the economies of some nations by drawing large numbers of tourists. In medicine, doctors use coral limestone to replace parts of human bones, and medical researchers believe chemicals from certain reef organisms may help them discover new medicines.
Despite the benefits coral reefs provide, human activities have led to the destruction of many reef environments. The fishing industry poses numerous threats to reefs. Fishing methods using dynamite, cyanide, or bleach can destroy coral life. Overfishing may upset reef stability by endangering certain reef species. Ships passing too close to reefs can hit and damage them.
The widespread burning of fossil fuels also damages reefs. Such pollution produces large amounts of carbon dioxide in the atmosphere. Excess carbon dioxide contributes to global warming, and thus to a rise in ocean temperatures. High water temperatures can cause corals to bleach and die. Bleaching occurs when corals become stressed and their zooxanthellae die or are expelled from the polyps.
Many other human activities harm coral reefs. Deforestation (cutting down forests) causes soil to wash into the sea and block out sunlight that corals need to survive. Agricultural fertilizers seep into oceans and promote the overgrowth of algae that smother polyps. The development of seaside homes and hotels may cause pollution that poisons corals. Some sunscreens (substances worn to protect the skin from the damaging effects of sunlight) also harm coral reefs. Ingredients found in sunscreen—oxybenzone and octinoxate—cause damage to larval (young) corals and may increase susceptibility to bleaching. In Hawaii, a law banning the use of sunscreen with these chemicals went into effect in 2021.
Proper reef management can help protect coral reefs from some of the most harmful human activities. Moreover, many governments and conservation organizations are working to preserve these important ecosystems.
