FaunaHub

Coral (class Anthozoa)

Marine invertebrateCnidarianReef builder

A healthy table coral colony (Acropora cytherea) on a reef in the Maldives.

Reef-building coral (Acropora cytherea), Maldives.

Image: Christian Jensen, CC BY 2.0, via Wikimedia Commons.

Overview

Corals are animals — a point that often surprises people. They are cnidarians (class Anthozoa), related to sea anemones and jellyfish, and each coral is built from many tiny individual animals called polyps. Reef-building (“hard”) corals secrete a stony calcium-carbonate skeleton, and over time countless colonies build the vast structures we call coral reefs.

Most reef-building corals live in partnership with microscopic algae (zooxanthellae) inside their tissues: the algae photosynthesise and supply much of the coral's energy, which is why these corals thrive in clear, sunlit, warm waters.

Conservation note: coral reefs are among the most threatened ecosystems on Earth, affected by warming seas (which cause coral bleaching), pollution, and other pressures. Many corals are of serious conservation concern; verify specifics at authoritative sources.

Habitat & Range

Reef-building corals live mainly in warm, clear, shallow tropical and subtropical seas where sunlight reaches their algal partners. Other corals, including many soft corals and deep-water (cold-water) corals, live without light-dependent algae and can occur in deeper, colder waters far from the tropics.

Diet

Corals feed in two ways. Their polyps capture tiny drifting animals (plankton) with stinging tentacles, especially at night. In addition, reef-building corals receive much of their energy from the photosynthetic algae living in their tissues — a partnership that makes them highly efficient in nutrient-poor tropical waters.

Behavior

A coral colony grows as its polyps divide and add to the shared skeleton. Many corals reproduce in spectacular synchronised mass-spawning events, releasing eggs and sperm into the water on particular nights. When stressed — most importantly by unusually warm water — corals can expel their algae and turn white, a process called bleaching; bleached corals are not dead but are weakened and can die if stress continues.

Human Interaction & Conservation

Coral reefs support an enormous share of marine biodiversity and provide people with fisheries, coastal protection, tourism, and more. They are also highly vulnerable: rising sea temperatures, pollution, destructive fishing, and other pressures have damaged reefs worldwide. Conservation, reef restoration, and reducing these pressures are major global priorities. Consult authoritative sources for current status.

More photos of the coral

A bleached, white Acropora coral colony.

A bleached Acropora colony — corals expel their algae under stress.

Image: Vardhanjp, CC BY-SA 4.0, via Wikimedia Commons.

Frequently Asked Questions — Coral

Is coral a plant, a rock, or an animal?
Coral is an animal. Each coral is a colony of tiny animals called polyps, which are cnidarians related to sea anemones and jellyfish. Reef-building corals also make a stony skeleton (which can look like rock) and host photosynthetic algae (which can make them seem plant-like), but the coral itself is an animal.
What is coral bleaching?
Bleaching happens when corals are stressed — most importantly by unusually warm water — and expel the microscopic algae living in their tissues, losing both colour and a key energy source. Bleached corals turn white and are weakened; they can recover if conditions improve, but may die if the stress continues.
How do coral reefs form?
Reef-building corals secrete a hard calcium-carbonate skeleton. As polyps grow and divide, colonies expand, and over very long periods the accumulated skeletons of countless colonies build the large structures we call coral reefs.
Why are coral reefs so important?
Although they cover a small fraction of the ocean floor, coral reefs support a large proportion of marine species and provide people with food, coastal protection from waves, tourism income, and more. This combination of biodiversity and human value is why their decline is such a serious concern.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.