Corals are invertebrate marine organisms in the class Anthozoa (phylum Cnidaria) that have skeletons of stonelike, horny, or leathery consistency. A coral's body consists of a polyp, which is a hollow cylindrical structure attached to a surface. The polyp has a mouth surrounded by tentacles that gather food and are armed with stinging structures called nematocysts that paralyze prey. Corals reproduce sexually and asexually. Most stony corals are yellowish, brownish, or olive, depending on the color of the algae living on them, but their skeletons are always white. Coral reefs are made of the coral’s skeleton, colonies of coral polyps, zooxanthellae, and other organisms. Coral reefs can take the form of fringing reefs , barrier reefs , atolls , or platform reefs .

Are corals plants or animals?

Corals are invertebrate marine organisms in the class Anthozoa. They are not plants, but rather a combination of both animal and plant organisms. The animal portion, called a polyp, is a simple creature similar to a miniature sea anemone. Algae cells live inside the cells of the polyp. This symbiotic relationship is mutually helpful because the polyp requires the algae to provide energy and recycle nutrients. Coral polyps have skeletons of stonelike, horny, or leathery consistency, and they build massive, intricate structures called coral reefs .

How do corals get their food?

Corals get their food in a few ways. As carnivores, they depend on a constant supply of zooplankton, which requires phytoplankton and plant nutrients. Corals have tentacles to capture food, moving it to their mouths to swallow. Some corals, like colonial corals, eat particulate material gathered in mucus moved to the mouth by cilia. Many reef corals also have a symbiotic relationship with zooxanthellae, dinoflagellate algae, living in their tissues. Zooxanthellae use photosynthesis to help the coral by removing carbon dioxide and aiding in the formation of the coral skeleton.

What is a coral reef and how is it formed?

A coral reef is a complex system made of living coral, algae, and other features. Coral polyps , similar to sea anemones, form colonies by dividing into daughter polyps, creating large structures held together by a rigid calcareous skeleton. These colonies build limestone by extracting calcium carbonate from seawater to create their skeletons. Reef-building corals , mainly stony corals or Scleractinia, thrive in sunlit waters, typically between the low-water mark and depths of 11 meters, with a preference for normal salinity and temperatures between 22 and 28 °C. Calcareous algae , mollusks, echinoderms, and protozoans also contribute to reef formation, with corals providing the main structural framework.

Why are coral reefs important for ocean life?

Coral reefs are vital to ocean life because they provide a complex habitat for a vast array of marine species. They are home to approximately 25 percent of all marine species and are second only to tropical rainforests in terms of biodiversity. Reef-building corals create a calcareous substrate that serves as a habitat for various organisms. These reefs support numerous species of fish, invertebrates, and other sea life. They also facilitate essential ecological functions, such as sediment stabilization by plants and algae, contributing to the overall health and diversity of marine ecosystems. However, coral reefs are fragile and face threats such as predatory species, harmful human activity, and sedimentation.

What are some threats to coral reefs, and how can we protect them?

Coral reefs face threats such as predatory species, coral bleaching , and destructive human activities. Overfishing, tourism, and harmful fishing practices damage the reefs. Excess sedimentation and pollution, including nutrient runoff, also harm these ecosystems. Rising ocean acidity inhibits the ability of corals to maintain their skeletons, while increased water temperatures from climate change can cause bleaching. To protect coral reefs, efforts include expanding protected marine areas, improving fisheries management, and reducing pollution. Addressing climate change to lower ocean temperatures and acidity is also crucial. Supervision by organizations such as the Great Barrier Reef Marine Park also helps to preserve reefs.

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Images & Videos

Illustration showing a cross section of a coral polyp with labeled parts including tentacles with nematocysts, mouth, zooxanthellae, living tissue linking polyps, gastrovascular cavity, skeleton, and limestone calice. An inset details a nematocyst within a tentacle.

Learn how non-mobile male and female corals reproduce by releasing gametes that form planulae

Learn how the sweeping motions of coral cilia draw nutrients inward and expel wastes away from the coral

Unique behaviors of gorgonians and trumpetfish

At a Glance

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Quick Facts

Key People:: Christian Gottfried Ehrenberg; Otto Heinrich Schindewolf; William Lonsdale

Related Topics:: blue coral; black coral; stony coral; horny coral; thorny coral

On the Web:: National Center for Biotechnology Information - Coral Toxicity (Oct. 24, 2026)

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Nov. 8, 2026 •History

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Scientists detect ‘pregnant’ coral using ultrasound device • Nov. 6, 2026, 12:47 PM ET (Popular Science)

Better together? Coral fusion could throw bleached reefs a lifeline • Oct. 15, 2026, 3:20 PM ET (Phys.org)

Top Questions

What are corals?
Corals are invertebrate marine organisms in the class Anthozoa (phylum Cnidaria) that have skeletons of stonelike, horny, or leathery consistency. A coral's body consists of a polyp, which is a hollow cylindrical structure attached to a surface. The polyp has a mouth surrounded by tentacles that gather food and are armed with stinging structures called nematocysts that paralyze prey.

Corals reproduce sexually and asexually. Most stony corals are yellowish, brownish, or olive, depending on the color of the algae living on them, but their skeletons are always white. Coral reefs are made of the coral’s skeleton, colonies of coral polyps, zooxanthellae, and other organisms. Coral reefs can take the form of fringing reefs, barrier reefs, atolls, or platform reefs.
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Are corals plants or animals?
Corals are invertebrate marine organisms in the class Anthozoa. They are not plants, but rather a combination of both animal and plant organisms. The animal portion, called a polyp, is a simple creature similar to a miniature sea anemone. Algae cells live inside the cells of the polyp. This symbiotic relationship is mutually helpful because the polyp requires the algae to provide energy and recycle nutrients.

Coral polyps have skeletons of stonelike, horny, or leathery consistency, and they build massive, intricate structures called coral reefs.
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How do corals get their food?
Corals get their food in a few ways. As carnivores, they depend on a constant supply of zooplankton, which requires phytoplankton and plant nutrients. Corals have tentacles to capture food, moving it to their mouths to swallow. Some corals, like colonial corals, eat particulate material gathered in mucus moved to the mouth by cilia.

Many reef corals also have a symbiotic relationship with zooxanthellae, dinoflagellate algae, living in their tissues. Zooxanthellae use photosynthesis to help the coral by removing carbon dioxide and aiding in the formation of the coral skeleton.
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What is a coral reef and how is it formed?
A coral reef is a complex system made of living coral, algae, and other features. Coral polyps, similar to sea anemones, form colonies by dividing into daughter polyps, creating large structures held together by a rigid calcareous skeleton. These colonies build limestone by extracting calcium carbonate from seawater to create their skeletons.

Reef-building corals, mainly stony corals or Scleractinia, thrive in sunlit waters, typically between the low-water mark and depths of 11 meters, with a preference for normal salinity and temperatures between 22 and 28 °C. Calcareous algae, mollusks, echinoderms, and protozoans also contribute to reef formation, with corals providing the main structural framework.
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1. coral reef: Corals and other reef-building organisms
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Why are coral reefs important for ocean life?
Coral reefs are vital to ocean life because they provide a complex habitat for a vast array of marine species. They are home to approximately 25 percent of all marine species and are second only to tropical rainforests in terms of biodiversity. Reef-building corals create a calcareous substrate that serves as a habitat for various organisms.

These reefs support numerous species of fish, invertebrates, and other sea life. They also facilitate essential ecological functions, such as sediment stabilization by plants and algae, contributing to the overall health and diversity of marine ecosystems. However, coral reefs are fragile and face threats such as predatory species, harmful human activity, and sedimentation.
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What are some threats to coral reefs, and how can we protect them?
Coral reefs face threats such as predatory species, coral bleaching, and destructive human activities. Overfishing, tourism, and harmful fishing practices damage the reefs. Excess sedimentation and pollution, including nutrient runoff, also harm these ecosystems. Rising ocean acidity inhibits the ability of corals to maintain their skeletons, while increased water temperatures from climate change can cause bleaching.

To protect coral reefs, efforts include expanding protected marine areas, improving fisheries management, and reducing pollution. Addressing climate change to lower ocean temperatures and acidity is also crucial. Supervision by organizations such as the Great Barrier Reef Marine Park also helps to preserve reefs.
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coral, any of a variety of invertebrate marine organisms of the class Anthozoa (phylum Cnidaria) that are characterized by skeletons—external or internal—of a stonelike, horny, or leathery consistency. The term coral is also applied to the skeletons of those animals, particularly to those of the stonelike corals.

Stony corals (order Madreporaria or Scleractinia) number about 1,000 species; black corals and thorny corals (Antipatharia), about 100 species; horny corals, or gorgonians (Gorgonacea), about 1,200 species; and blue corals (Coenothecalia), one living species.

The body of a coral animal consists of a polyp—a hollow cylindrical structure attached at its lower end to some surface. At the free end is a mouth surrounded by tentacles. The tentacles, which gather food, are more or less extensible and are armed with specialized stinging structures, called nematocysts, that paralyze prey.

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Eggs and sperm, usually produced by separate individuals, develop as outgrowths in the gastrovascular cavity and are expelled through the mouth into the open water. Fertilization usually takes place in the water but sometimes occurs in the gastrovascular cavity. The larva, a ciliated form known as a planula, swims about for several days or as long as several weeks, then settles onto a solid surface and develops into a polyp. Reproduction also occurs by budding. The bud remains attached to the original polyp. A colony develops by the constant addition and growth of new buds. As new polyps develop, the old ones beneath die, but the skeletons remain.

sea fanSea fan (genus *Gorgonia*), a branching variety of coral, beside an array of other organisms in the Belize Barrier Reef.

Soft coral, horny coral, and blue coral are colonial in habit. Individual polyps have eight feathery tentacles and, in the gastrovascular cavity, eight septa, or partitions. Cilia (tiny hairlike projections) on six septa draw water into the cavity. Cilia on the other two septa expel water. The skeleton is internal. Soft corals, a widely distributed group, have internal skeletons consisting of separate calcareous (calcium-bearing) spicules (needlelike structures). Some species are platelike in form; others (e.g., dead men’s fingers, Alcyonium) have fingerlike projections. Horny corals such as sea fans are most numerous in shallow tropical waters. They are ribbonlike or branching in form, sometimes growing to a length of 3 metres (10 feet). They include the so-called precious coral (also called red, or rose, coral) used in jewelry. A common species of precious coral, Corallium rubrum, is found in the Mediterranean Sea. Blue coral, Heliopora coerula, occurs on reefs of stony coral in the Indian and Pacific oceans. It forms lumps up to 2 metres in diameter.

Stony corals, the most familiar and most widely distributed forms, are both colonial and solitary in habit. They, as well as black and thorny corals, have more than eight septa and simple rather than feathery tentacles. Stony, black, and thorny corals differ from the related sea anemone chiefly in having an external skeleton. Stony corals occur in all oceans from the tidal zone to depths of nearly 6,000 metres (about 20,000 feet). The polyps of colonial forms are 1 to 30 mm (0.04 to 1.2 inches) in diameter. Most living stony corals are yellowish, brownish, or olive, depending on the colour of the algae living on the coral. The skeletons, however, are always white. The largest solitary form, a species of Fungia, grows to a diameter of about 25 cm (10 inches).

The skeleton of stony coral is almost pure calcium carbonate and is deposited in a cup-shaped form with the polyp inside. The growth rate varies with age, food supply, water temperature, and species. Atolls and coral reefs are composed of stony coral. Such formations grow at an average rate of about 0.5 to 2.8 cm per year. Common types of stony coral include brain coral, mushroom coral, star coral, and staghorn coral, all named because of their appearance.

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Learn how climate change affects the coral reefsOverview of the effects of global warming on coral, including those off the coast of Tahiti.
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Black corals and thorny corals are whiplike, featherlike, or treelike in form or are shaped like a bottle brush. They occur in the Mediterranean Sea, in the West Indies, and off the coast of Panama.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn.

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