Coral

Coral facts at a glance

What is a group of corals called?

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What corals are

Although they look like colourful rocks or plants, corals are animals, closely related to sea anemones and jellyfish. A single coral is actually a colony of tiny individuals called polyps, each only a few millimetres across, joined by shared living tissue. Reef-building species secrete a hard skeleton of calcium carbonate beneath their bodies, and generation after generation builds on the remains of the last. Over centuries these skeletons accumulate into the massive limestone framework of a coral reef.

Polyps and how they live

Each polyp is a soft, sac-like body topped by a ring of stinging tentacles surrounding a central mouth. Corals are fixed permanently in place and do not move about; they are sessile animals anchored to their skeleton. At night many polyps extend their tentacles to capture drifting plankton and tiny particles from the water using stinging cells. By day the polyps often retract, relying on their algal partners for energy while the colony rests.

Symbiosis with zooxanthellae

Reef-building corals owe their success to a partnership with microscopic algae called zooxanthellae that live inside the coral's tissues. Through photosynthesis these algae produce sugars that supply most of the coral's energy, while the coral gives them shelter and nutrients in return. The algae also lend corals much of their brilliant colour. This symbiosis is why reef corals thrive in the clear, sunlit, nutrient-poor waters of the tropics, where sunlight can reach them.

Reefs as ecosystems

Coral reefs are among the richest habitats on Earth, often called the rainforests of the sea. Though they cover a tiny fraction of the ocean floor, reefs shelter roughly a quarter of all marine species, providing food and refuge for fish, invertebrates and larger animals. The three-dimensional structure of the reef creates countless nooks and crannies for creatures to live in. Reefs also protect coastlines by absorbing the force of waves and storms.

Coral bleaching and threats

When the water grows too warm, corals expel the zooxanthellae living in their tissues, turning ghostly white in a process called bleaching. Without their algae the corals lose their main food supply, and if stress continues they can starve and die. Rising sea temperatures, ocean acidification, pollution and destructive fishing have all put reefs under severe pressure worldwide. Some coral species are now threatened, though corals as a whole are broadly assessed as Least Concern.

Growth and lifespan

Corals grow slowly, with many reef-building species adding only a centimetre or two of skeleton each year. Because colonies keep building over long periods, individual coral colonies can live for decades to centuries, and whole reefs are thousands of years old. Corals reproduce both by budding new polyps and through mass spawning events, when whole reefs release eggs and sperm into the water on the same night. New larvae drift away to settle and start fresh colonies elsewhere.

Did you know? Coral facts

• Corals are small marine animals in the class Anthozoa, related to sea anemones and jellyfish, that often live in vast colonies of individual polyps.
• Coral is an animal, not a plant. Each coral is made of many tiny animals called polyps, related to sea anemones and jellyfish, though many corals also host algae in their tissue.
• Reef-building corals secrete hard skeletons of calcium carbonate beneath their polyps. As generations of coral grow and stack up over thousands of years, these skeletons form massive reefs.
• Coral bleaching happens when stressed corals, often from water that is too warm, expel the algae living in their tissue and turn white. If the stress continues, the coral can starve and die.
• Corals get much of their food from algae living in their tissue, which photosynthesize and share sugars. They also catch plankton and tiny animals using their stinging tentacles, mostly at night.
• Conservation: Varies by species.

Diet & feeding

Coral feeds primarily as a filter feeder plus algae symbionts. Each coral is built from many small animals called polyps, soft cup-shaped bodies ringed with stinging tentacles. Reef-building corals secrete hard skeletons of calcium carbonate beneath them, and as generations stack up over thousands of years they form re

Adaptations

• Each coral is built from many small animals called polyps, soft cup-shaped bodies ringed with stinging tentacles. Reef-building corals secrete hard skeletons of calcium carbonate beneath them, and as generations stack up over thousands of years they form reefs. The Great Barrier Reef, made by countless coral colonies, is so large it can be seen from space.
• Many corals host tiny algae called zooxanthellae inside their tissue, which photosynthesize and supply much of the coral's food and its bright color. In return the coral gives the algae shelter and nutrients, a partnership that lets reefs thrive in clear, sunlit, nutrient-poor tropical waters. Corals also catch plankton and tiny animals using their stinging tentacles, especially at night.

Behaviour & ecology

• Each coral is built from many small animals called polyps, soft cup-shaped bodies ringed with stinging tentacles. Reef-building corals secrete hard skeletons of calcium carbonate beneath them, and as generations stack up over thousands of years they form reefs. The Great Barrier Reef, made by countless coral colonies, is so large it can be seen from space.
• Many corals host tiny algae called zooxanthellae inside their tissue, which photosynthesize and supply much of the coral's food and its bright color. In return the coral gives the algae shelter and nutrients, a partnership that lets reefs thrive in clear, sunlit, nutrient-poor tropical waters. Corals also catch plankton and tiny animals using their stinging tentacles, especially at night.
• Corals can reproduce both by budding, where polyps clone themselves to expand a colony, and by spawning, releasing eggs and sperm into the water. On many reefs, mass spawning events see corals release their eggs and sperm together on the same few nights of the year. Growth is slow, with many corals adding only a few centimeters or less per year.

Communication

• Coral uses scent, posture, and vocal signals to mark territory and coordinate social behaviour.
• Communication intensity often peaks during breeding seasons and territorial disputes.

Habitat & range

Warm, shallow tropical seas worldwide

Ecological role

Coral plays a recognised ecological role in warm, shallow tropical seas worldwide.

Conservation status of the Coral

Sources & references

This guide is compiled and reviewed against established zoological and conservation references. Key sources for the Coral:

• IUCN Red List of Threatened Species

  Conservation status (Least Concern) reflects the IUCN Red List category for Anthozoa.

• IUCN Red List — Anthozoa.

  Conservation status (Least Concern) and population trends.

• Wikipedia — Coral.

  General taxonomy and overview (cross-check primary sources).

• Global Animal Guide editorial standards

  How we research, source, review, and update every guide for accuracy.