Coral, a hard material formed in the sea by colonies of tiny animals called coral polyps. These polyps are related to the sea anemones and jellyfish. Most kinds of coral polyps extract calcium from seawater and convert it into limestone shelters; these polyps are called hard, or true, corals. Other kinds of coral polyps convert calcium or protein into internal skeletons; these polyps are called soft corals.

A piece of coral is usually made up of the former shelters of many coral polyps. As one polyp dies, another builds its shelter on top of the first. In this manner, a large structure is eventually built up. Corals such as staghorn, elkhorn, brain, and lettuce-leaf coral are named for the characteristic shape of the structures they form.

Coral polyps live mainly in warm, shallow seawater. They eat tiny organisms that they capture with tentacles. Certain kinds of algae live in the tissues and on the shelters of coral polyps. Through photosynthesis, the algae produce various substances that the coral polyps use as food. Coral polyps reproduce both asexually (by budding) and sexually.

A coral reef is a structure made up of many colonies of coral polyps. Most species of reef-building coral polyps live only in waters in which the temperature never falls below 64° F. (18° C.). They are found primarily in the South Pacific, in the Indian Ocean, and in the Caribbean and adjacent waters. A barrier reef is a coral reef that lies near a mainland or island, from which it is separated by a channel. The Great Barrier Reef off northeastern Australia is a coral formation more than 1,200 miles (1,900 km) long. A fringing reef is a shelf of coral extending outward from the shore with no separating channel. An atoll is a circular reef with a lagoon in the center. Certain reefs commonly called coral reefs are actually composed chiefly of lime-secreting algae.

Coral reefs are among the most colorful and interesting places in the sea. The living coral animal may be red, blue, yellow, or other colors. Many fish and other animals, most of them brilliantly colored, take advantage of the shelter afforded by the coral. Coral reefs are popular sites for activities such as fishing and scuba diving. Coral reefs protect coastlines from waves that may cause erosion. Certain kinds of coral are made into jewelry; the coral most often used for this purpose is the precious, or red, coral, found in the Mediterranean Sea and the Sea of Japan.

How Are Many Corals Named?

Many corals have been named for the objects they look like. Lettuce-leaf coral looks like the makings of a salad. Boulder coral looks like huge rocks. Staghorn coral resembles the antlers of a deer. Star coral has tiny star shapes covering it. The sea pen probably doesn’t look like a pen you might use. But it does look like a quill, or feather pen, that people used long ago.

Many coral reefs are being destroyed because of a combination of factors that include human activities such as the pollution of the ocean and natural occurrences such as unusual changes in water temperature. These factors sometimes cause a condition called bleaching, in which the algae on the corals and within the polyps die, causing the corals and polyps to become white; without the food produced by the algae, the polyps also die.

What Is a Coral Reef Community?

Many animals and plants live in and around a coral reef. A coral reef is home to coral polyps and other stingers. It is home to fish and crabs. It is also home to algae (AL jee) and seaweed. Algae are tiny, plantlike things. All these plants and animals form a coral reef community.

Plants and animals in a community need one another. Here is something to think about: Coral reefs grow in warm, shallow waters, where sunlight can filter down. A coral reef is a good place for plants to grow. Fish that eat plants come to the reef. Fish that eat those fish also come to the reef. They live in the reef and lay their eggs here. The eggs and fish that hatch from them provide food for the coral polyps.

Are Stingers in Danger?

Changes to the world’s oceans do threaten some stingers. Perhaps the stingers most in danger are those in coral reefs. Most healthy coral polyps have algae living inside them. Like plants, algae store energy from sunlight. Coral polyps need this energy in order to survive.

In many coral reefs, the algae are dying. If the algae die, the coral polyps may die, too. Scientists think that pollution, disease, or water that is too warm may be the cause.

Activities like boating and diving also hurt a coral reef. Boaters may run into a reef or drop their anchors on the coral. Divers may kick or brush up against the delicate polyps. This type of damage can take years to heal.

Scientists are working to save the coral. Some reefs are being set aside as parks. And research is being done to help stop the coral from dying.

Corals belong to the class Anthozoa of the phylum Cnidaria. Hard corals belong to the subclass Zoantharia. Soft corals, including the precious coral (Corallium rubrum), belong to the subclass Alcyonaria, sometimes called Octocorallia.