# Coral (*Anthozoa*) > Corals are tiny colonial animals of the class Anthozoa that build the vast limestone structures we call reefs. Each coral is a colony of soft-bodied polyps, only millimetres wide, that secrete a hard calcium-carbonate skeleton and live fixed in place in warm, shallow tropical seas. Reef-building corals depend on symbiotic algae called zooxanthellae for most of their food and colour, and colonies can survive for decades to centuries. - **Category:** Invertebrate - **Conservation status (IUCN):** Least Concern - **Range:** Warm, shallow tropical seas worldwide, with the greatest diversity in the Indo-Pacific. - **Canonical page:** https://globalanimalguide.org/animals/coral/ - **Last reviewed:** 2026-07-16 ## Scientific classification - **Kingdom:** Animalia - **Phylum:** Cnidaria - **Class:** Anthozoa ## Facts at a glance | Attribute | Value | | --- | --- | | Scientific name | Anthozoa (class) | | Diet | Filter feeder plus algae symbionts | | Habitat | Warm, shallow tropical seas worldwide | | Lifespan | Colonies can live decades to centuries | | Size | Polyps mm-wide; colonies and reefs vast | | Top speed | None; corals are fixed in place | | Conservation status | Varies by species | ## 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. ## Frequently asked questions ### Is coral a plant or an animal? Coral is an animal, belonging to the class Anthozoa alongside sea anemones and jellyfish. A coral is a colony of tiny animals called polyps, which is why it is easily mistaken for a plant or a rock. ### Do corals move? No. Corals are sessile animals, fixed permanently in place on their hard skeleton, so they do not move about. Only their tiny polyps move, extending and retracting stinging tentacles to feed. ### What are zooxanthellae and why do corals need them? Zooxanthellae are microscopic algae that live inside coral tissues. Through photosynthesis they produce sugars that supply most of the coral's energy, and they also give corals much of their colour, which is why reef corals thrive in sunlit tropical waters. ### What is coral bleaching? Coral bleaching happens when stressed corals, usually from water that is too warm, expel the zooxanthellae living in their tissues and turn white. Without their algae the corals lose their main food supply and can starve and die if the stress continues. ### How do coral reefs form? Reef-building corals secrete a hard skeleton of calcium carbonate beneath their polyps. As colonies grow on the remains of earlier ones over centuries, these skeletons accumulate into the vast limestone structures we call reefs. ### How long do corals live? Individual coral colonies can live for decades to centuries because they keep building on their own skeletons over time. Whole reefs are far older still, with some structures thousands of years in the making. ## Sources - [IUCN Red List — Anthozoa.](https://www.iucnredlist.org/search?query=Anthozoa&searchType=species) - [Wikipedia — Coral.](https://en.wikipedia.org/wiki/Anthozoa) --- Source: Global Animal Guide (https://globalanimalguide.org/animals/coral/). Licensed CC BY-NC 4.0 — free for non-commercial reuse with attribution.