What Are Coral Reefs?
Table coral (Acropora cytherea) is common throughout the tropical Pacific, but in Hawaii its distribution is limited to French Frigate Shoals and neighboring atolls northwest of the main islands.
Credit: Greg McFall/NOAA

Coral reefs are large underwater structures composed of the skeletons of coral, which are marine invertebrate animals. The coral species that build coral reefs are known as hermatypic or"hard" corals because they extract calcium carbonate from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies.

Each individual coral is referred to as a polyp. New coral polyps live on the calcium carbonate exoskeletons of their ancestors, adding their own exoskeleton to the existing coral structure. As the centuries pass, the coral reef slowly grows, one tiny exoskeleton at a time, until they become massive features of the submarine environment.

Corals are found all over the world's oceans, from the Aleutian Islands off the coast of Alaska to warm tropical waters of the Caribbean Sea. The biggest coral reefs are found in the clear, shallow ocean waters of the tropics and subtropics where they grow quickly. The largest of these coral reef systems — the Great Barrier Reef of Australia — is more than 1,500 miles (2,400 km) in length.

There are hundreds of different species of coral, according to the Coral Reef Alliance (CORAL), a nonprofit environmental group. Coral have a dazzling array of shapes and colors, from round, folded brain corals that resemble a human brain to tall, elegant sea whips and sea fans that look like intricate, vibrantly colored trees or plants. [Photos: Stunning New Coral Species of Polynesia]

Corals belong to the phylum Cnidaria (pronounced ni-DAR-ee-uh), a group that includes jellyfish, anemones, Portuguese man o' war and other marine animals. Though each individual animal is referred to as a polyp, corals are often described as a colony of thousands of polyps.

Corals feed in two different ways: Some species are able to catch small sea life like fish and plankton by using the stinging tentacles on the outer edges of their bodies. Most corals, however, have a symbiotic (mutually rewarding) relationship with algae called zooxanthellae (pronounced zo-zan-THEL-ee), according to the U.S. Environmental Protection Agency (EPA).

These algae live inside the coral polyp's body and produce food for themselves and the polyp through photosynthesis. The polyps, in turn, provide a home and carbon dioxide for the algae. Additionally, the zooxanthellae provide the coral with their lively colors — most coral polyp bodies are clear and colorless.

Some coral species, such as brain coral, are hermaphrodites, producing both eggs and sperm at the same time. They reproduce in a mass coral spawning event that, in some species, occurs only once per year on a particular night.

Other species, such as elkhorn coral, are gonochoric, creating colonies composed of all males or all females. Among these coral colonies, all the polyps of one particular colony will produce only sperm; for reproduction, they rely on a neighboring colony that produces only eggs.

Images of corals Pictures from the Back reef of Ofu (a national park in American Samoa).
Images of corals Pictures from the Back reef of Ofu (a national park in American Samoa).
Credit: Dan Barshis

Most of the substantial coral reefs found today are between 5,000 and 10,000 years old, according to CORAL. These are generally found in warm, clear, shallow waters where there's plenty of sunlight to nurture the algae that the coral rely on for food.

Coral reefs cover less than 1 percent of the ocean floor — all of them combined would equal an area of about 110,000 square miles (285,000 square kilometers), roughly the size of the state of Nevada. Nonetheless, they are among the most productive and diverse ecosystems on Earth.

About 25 percent of all known marine species rely on coral reefs for food, shelter and breeding. Sometimes referred to as "the rainforests of the sea" for their biodiversity, coral reefs are the primary habitat for more than 4,000 species of fish, 700 species of coral, and thousands of other plants and animals, according to CORAL.

And their lives are in imminent peril, according to experts.

Coral reefs are critical marine habitat on which many ocean species depend; additionally, they provide an estimated $30 billion annually in direct economic benefit to people worldwide though food, fisheries and tourism, according to the Hopkins Marine Station of Stanford University.

But coral reefs are imperiled by several threats. The increasing acidification of the ocean — caused when oceans absorb immense amounts of carbon dioxide (CO2) released into the atmosphere through the burning of fossil fuels — inhibits coral's ability to produce the calcium carbonate exoskeletons they rely on for shelter.

Water pollution, too, is wreaking havoc on coral reefs. Agricultural pesticides and fertilizers, oil and gasoline, sewage discharge and sediment loads from eroded landscapes damage the complex relationships that exist between the plants, coral and other animals that are part of the reef ecosystem.

As the temperatures of the world's oceans increase due to global warming, coral polyps expel the zooxanthellae they depend on for food. Once they are gone, the coral also loses its brilliant color, and all that can be seen is the white exoskeleton; this is referred to as coral bleaching. Coral colonies subject to bleaching usually die off, according to CORAL.

And fishing practices such as cyanide fishing (using cyanide to make fish easier to catch), "blast fishing" with explosives, and overfishing with trawlers can destroy a thousand-year-old coral reef in a few minutes.

"Overfishing, ocean acidification and pollution are pushing coral reefs into oblivion," said Roger Bradbury, an ecologist at Australian National University in Canberra, in a New York Times opinion article. "Each of those forces alone is fully capable of causing the global collapse of coral reefs; together, they assure it."

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