# 12 Amazing Facts About Coral (They Are Animals, and Bleaching Is Starvation)

*Tropical and cold oceans worldwide · JUL 25 2026*

> Corals are animals that host photosynthetic algae in their tissue; bleaching happens when heat stress makes the coral expel those algae, losing both its colour and most of its food supply — a bleached coral is starving, not dead.

Animals that farm algae, build reefs supporting a quarter of marine life, and bleach when the partnership breaks down.

## What are the key takeaways from this briefing?
- Corals are animals — colonies of polyps related to jellyfish and anemones.
- They host photosynthetic algae that supply most of their energy.
- Bleaching is the expulsion of those algae under heat stress; the coral is starving, not yet dead.
- Reefs support around a quarter of all marine species on a tiny fraction of the seafloor.
- Local protection from pollution and overfishing measurably improves recovery from bleaching.

## 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](/wildlife-guides/coral/).

### 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](/wildlife-guides/coral/)** for IUCN status, population context and sources — or **[donate to WARN](/donate/)** to help fund the partner teams protecting wildlife on the ground.

## What do people ask about this briefing?
### Is coral an animal or a plant?
An animal. A coral colony is made of many tiny polyps related to jellyfish and sea anemones, which secrete a calcium carbonate skeleton. The confusion arises because corals host photosynthetic algae in their tissue.

### What is coral bleaching?
When heat stress causes a coral to expel the photosynthetic algae living in its tissue. It loses both its colour and its main food supply, so a bleached coral is starving rather than dead, and can recover if conditions improve quickly enough.

### Why does bleached coral turn white?
The colour comes from the algae, not the coral. Coral tissue is largely transparent and the calcium carbonate skeleton beneath is white, so once the algae are expelled the skeleton shows through.

### How much marine life depends on coral reefs?
Around a quarter of all marine species, supported on well under 1% of the ocean floor. Reefs also dissipate wave energy and protect coastlines, a service that is very expensive to replace with engineering.

### Can anything be done locally about coral loss?
Yes, and it matters. The underlying driver is global warming and ocean acidification, but reducing local pollution, overfishing and physical damage measurably improves a reef's ability to survive and recover from bleaching events.

## Which sources does this briefing use?
- [Smithsonian Ocean — Coral Reefs](https://ocean.si.edu/ecosystems/coral-reefs)
- [IUCN Red List](https://www.iucnredlist.org/)

Human-readable page: https://worldanimalrescuenetwork.org/newsroom/coral-12-amazing-facts/