CORAL REEF

CORAL REEF

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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.

Corals are related to sea anemones, and they all share the same simple structure, the polyp. The polyp is like a tin can open at just one end: the open end has a mouth surrounded by a ring of tentacles. The tentacles have stinging cells, called nematocysts, that allow the coral polyp to capture small organisms that swim too close. Inside the body of the polyp are digestive and reproductive tissues. Corals differ from sea anemones in their production of a mineral skeleton.

Shallow water corals that live in warm water often have another source of food, the zooxanthellae  (pronounced zo-o-zan-THELL-ee). These single-celled algae photosynthesize and pass some of the food they make from the sun’s energy to their hosts, and in exchange the coral animal gives nutrients to the algae. It is this relationship that allows shallow water corals to grow fast enough to build the enormous structures we call reefs.


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

Scientists have explored only about 20 percent of the ocean's floor, according to the National Oceanic and Atmospheric Administration (NOAA). As such, ocean explorers continue to discover previously unknown coral reefs that have likely existed for hundreds of years.

The lives of coral

There are hundreds of different species of coral, according to CORAL. Coral have a dazzling array of shapes and colors, from round, folded brain corals (named for their resemblance to a human brain) to tall, elegant sea whips and sea fans that look like intricate, vibrantly colored trees or plants.

Corals belong to the phylum cnidaria (pronounced ni-DAR-ee-uh), a group that includes jellyfish, anemones, Portuguese man o' war and several other gelatinous and stinging marine invertebrates.

Corals feed by one of two ways. Some species catch small marine life, like fish and plankton, by using the stinging tentacles on the outer edges of their bodies. Most corals, however, depend on algae called zooxanthellae to provide energy via photosynthesis.

The corals have a symbiotic, or mutually beneficial, relationship with the zooxanthellae, according to the U.S. Environmental Protection Agency (EPA). 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 colors — most coral polyp bodies are clear and colorless without zooxanthellae.

Some coral species, such as brain coral, are hermaphrodites, which means they produce eggs and sperm at the same time. Sexual reproduction occurs during a mass coral spawning event that, for some species, happens only once a year.

Other species, such as elkhorn coral, are gonochoric, which means they create colonies composed of either all males or all females. Within each coral colony all the polyps will produce only eggs or only sperm. For successful reproduction, the colony must rely on a neighboring colony that produces the other reproductive cell.

The world of coral reefs

Most of the substantial coral reefs found today are between 5,000 and 10,000 years old, according to CORAL. They are most often found in warm, clear, shallow water 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 the reefs combined would equal an area of about 110,000 square miles (285,000 square km), only about 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.

Coral reefs are typically divided into four categories, according to CORAL: fringing reefs, barrier reefs, patch reefs and atolls. 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. Patch reefs typically grow between fringing and barrier reefs on the island platform or continental shelf. The rings of coral that make up atolls create protected lagoons in the middle of the oceans, typically around islands that have sunk back down into the ocean.