Twelve coral facts, fact-checked
Coral reefs occupy well under 1% of the ocean floor and support around a quarter of all marine species. Full profile in our coral guide.
1. Coral is an animal
Colonies of tiny polyps, related to jellyfish and sea anemones, that secrete a calcium carbonate skeleton.
2. They farm algae inside their tissue
Zooxanthellae photosynthesise and supply the coral with most of its energy, in exchange for shelter and nutrients.
3. Bleaching is expulsion, not death
Heat stress causes the coral to expel its algae, losing colour and its main food source. A bleached coral is starving and can recover if conditions improve quickly enough.
4. The colour comes from the algae
Coral tissue is largely transparent; the skeleton beneath is white, which is why bleached coral looks bone-coloured.
5. Reefs support around a quarter of marine species
On a tiny fraction of the seafloor.
6. Some spawn once a year, all at once
Mass synchronised spawning triggered by moon phase, day length and temperature releases eggs and sperm across a whole reef in one night.
7. They sting
Polyps carry the same nematocyst stinging cells as jellyfish, used to catch plankton at night and to fight neighbouring colonies for space.
8. Growth is slow
Many reef-building corals grow around 1–2 cm a year, so a large structure represents centuries.
9. Cold-water reefs exist too
Deep, dark, cold reefs occur worldwide without any algal partner, and are damaged principally by bottom trawling.
10. Reefs are natural sea defences
They dissipate a large share of wave energy, protecting coastlines — a service that is expensive to replace with engineering.
11. Acidification compounds heat
Absorbed carbon dioxide lowers ocean pH, making it harder for corals to build skeletons at all.
12. Local protection buys time
Reducing pollution, overfishing and physical damage measurably improves a reef's ability to recover from bleaching, even though the underlying driver is global.
Read the full coral wildlife guide for IUCN status, population context and sources — or donate to WARN to help fund the partner teams protecting wildlife on the ground.
