How do we know?
↻ One of the pages with a moving picture
Satellites watched the sea get too warm for coral.
- A coral is made of hundreds to thousands of tiny animals called polyps. Most are no thicker than a nickel.
- Each soft polyp builds a hard skeleton of limestone. Over time, these pile up into a reef.
- Tiny plant-like algae live inside the polyps. They use sunlight to make food and share it with the coral.
- Polyps catch food with little tentacles. Unlike plants, they can’t make their own food. So coral is an animal!
Satellites measure how warm the sea surface is. In 2005, years of satellite data showed Caribbean water was extra warm, and many corals bleached: they turned white.
Most polyps are no thicker than a nickel, but together they build giant reefs. 2,500 separate reefs make up Australia’s Great Barrier Reef, along with more than 900 islands.
Not all corals need sunlight. Over half of all coral species live in deep, cold, dark water.
Wow number
2,500 separate reefs make up Australia’s Great Barrier Reef, along with more than 900 islands!
Myth or fact?
Coral is a kind of plant. Myth. Corals are animals with plant-like algae inside.
Over half of all coral species live in deep, cold, dark water. Fact. They don’t need sunlight.
Think about it
Why do you think most reef corals grow in shallow, sunny water?
Where in the world?
Great Barrier Reef, Australia which stretches about 2,000 km (1,240 miles) along the coast of Queensland. The Belize Barrier Reef in Central America is the largest barrier reef in the Northern Hemisphere — 18.2861° S, 147.7° E.
Where these facts come from
Every number on page 48, and who measured it
This page explains why corals are animals, and tells us about polyps, algae and tentacles.
Are corals animals or plants? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/coral.html
We checked it on 31 October 2026
This page tells us how big the Great Barrier Reef is.
Great Barrier Reef — UNESCO World Heritage, the United Nations' list of the world's most special places
whc.unesco.org/en/list/154
We checked it on 31 October 2026
This page tells us about the Belize Barrier Reef.
Belize Barrier Reef Reserve System — UNESCO World Heritage, the United Nations' list of the world's most special places
whc.unesco.org/en/list/764
We checked it on 31 October 2026
A second source that agrees: the Belize reef is the largest barrier reef in the northern half of the world.
Belize Barrier Reef Reserve System — IUCN, a worldwide nature-protection group that checks on World Heritage sites
worldheritageoutlook.iucn.org/explore-sites/belize-barrier-reef-reserve-system
We checked it on 31 October 2026
A third source for the Belize reef record.
Largest barrier reef system in the northern hemisphere — Guinness World Records, which keeps lists of world records
guinnessworldrecords.com/world-records/largest-barrier-reef-system-in-the-northern-hemisphere
We checked it on 31 October 2026
The Great Barrier Reef's own park managers share its size and how many reefs and islands it has.
Reef facts — The Great Barrier Reef Marine Park Authority, which looks after the reef
www2.gbrmpa.gov.au/learn/reef-facts
We checked it on 31 October 2026
This page tells us how satellites tracked the warm water that bleached corals in 2005.
What is coral bleaching? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/coral_bleach.html
We checked it on 31 October 2026
This page tells us that over half of coral species live in deep, dark, cold water.
Do all corals live in warm water? — NOAA Ocean Exploration, U.S. deep-sea explorers
oceanexplorer.noaa.gov/ocean-fact/coral-water
We checked it on 31 October 2026
Has this changed?
Corals are animals, and that is not going to change.
What does change is the reefs themselves: warm water can make corals turn white, as it did in the Caribbean in 2005.
Words from this page
- polyp
- A tiny soft coral animal with tentacles.
- algae
- Tiny plant-like living things. Some live inside coral.
- bleached
- Turned white, because the coral lost its algae in water that was too warm.
Try it!
Be a coral polyp.
1. Hold up one hand with your fingers spread wide. They’re the polyp’s tentacles. 2. Your palm is its mouth, and your arm is its soft body. 3. Close your fingers around a scrap of paper “food”.
What happens: you’ve caught your food! Real polyps use their tentacles to sweep food into their mouths.
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