Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step of reef building. Numbers and scales are simplified to show the idea.
Tap, drag and slide. See what happens!
The Great Barrier Reef is 2,600 km long. Tap it to zoom in on who built it.
A structure visible from space, built by millimetre-scale animals. Tap the reef to zoom in.
Tap the reef. Who lives there?
Drop some plankton into the water. Can the tentacles catch it? Try night and day.
Tentacles armed with stinging cells capture drifting zooplankton, mostly at night. Drop some in.
Tap the water to drop food. Watch the arms grab it!
Add polyps one at a time, or fast-forward. Old cups stay, new ones grow on top.
Each polyp buds clones; dead skeletons become the foundation for the next layer. Fast-forward it.
Tap to add more polyps. Watch the reef grow!
Drag the coral down. What happens to the food its algae make as the light fades?
Zooxanthellae need light; move the colony deeper and watch photosynthetic supply fall.
Move the coral down deep. Is there still sunshine for its helpers?
In this model the usual warmest month is 29 °C. Slide the water up past 30 °C, then cool it again. Can you save it in time?
Push the water 1 °C above the usual warmest-month average. How long can the coral last without its algae?
Make the water hot, then cool it down fast. Can the coral get its colour back?
Slide to sink the island. Fringing reef, barrier reef, atoll: watch each one appear.
Slide to subside the island while the reef keeps pace with sea level: fringing → barrier → atoll.
Slide to sink the island. Look: a ring of coral!
A coral reef looks like colourful rocks under the sea. But it was built by tiny animals called polyps. Most are smaller than a grain of rice!
A polyp has a mouth on top and soft arms all around it. The arms can sting, like a jellyfish. At night the arms catch tiny floating food. Each polyp makes a little cup of stone to live in.
A baby coral floats in the sea and lands on a rock. It makes its first stone cup. Then it splits in two, and two becomes lots. When old polyps die, their cups stay, and new ones grow on top. Very slowly, over a very long time, the reef grows.
Tiny helpers called algae live inside the coral. They use sunshine to make food and share it with the coral. They give coral its bright colours too. That's why reefs grow in clear, sunny water that is not too deep.
If the water gets too hot, the coral pushes its helpers out and turns white. It is not dead, but it is very hungry. If the water cools down soon, the helpers can come back.
More sea stories: how fish breathe · why the sea is salty · volcanoes
Coral reefs are built by animals. Each piece of reef-building coral is covered in tiny soft creatures called polyps, most only 1 to 3 millimetres across.8 Every polyp builds a hard cup of limestone (calcium carbonate) around and under itself.7 Polyps split into copies, the copies share one skeleton, and when polyps die, new ones grow on top of their old cups. Layer by layer, over thousands of years, millions of polyps pile up enough limestone to make a reef.78
They have help: microscopic algae living inside the polyps turn sunlight into food for the coral.3 That's why tropical reefs grow in clear, warm, shallow, sunny seas (a few cold-water corals build mounds in the deep, dark sea without algae).
Coral is an animal, even though it often looks like a plant or a rock. Corals belong to the same big animal group as jellyfish and sea anemones (the cnidarians).1115 They count as animals because they don't make their own food the way plants do: they catch it with tentacles.7 Like plants, though, they stay fixed in one place for life, attached to the sea floor.7
What we call "a coral" is usually a colony: hundreds to hundreds of thousands of polyps joined together.2
The Smithsonian describes a polyp as being like a tin can open at just one end.14 The open end is the mouth, and it's surrounded by a ring of tentacles. Food goes in through the mouth, and waste comes out through the same opening.2
Most corals feed at night, stretching out their tentacles to catch zooplankton (tiny drifting animals) and sometimes even small fish.2
Reefs grow very slowly. According to NOAA:18
| What's growing | How fast / how long |
|---|---|
| Massive (boulder-shaped) corals | about 0.3 to 2 cm a year |
| Branching corals | up to about 10 cm a year |
| A reef, from a group of larvae | up to 10,000 years |
| A large barrier reef or atoll | 100,000 to 30,000,000 years |
Reefs themselves grow even more slowly than single corals, because dead coral breaks into pieces and gets packed down.14 Corals even lay down yearly growth bands, a bit like tree rings.14
Most reef-building corals have microscopic algae called zooxanthellae (say "zo-uh-zan-thuh-lay") living inside their tissues.37 It's a deal that helps both sides:
This extra food is what powers the coral to build so much limestone. It's also why reef corals behave a bit like plants: they need clear water so sunlight can reach their algae.3 The algae give many corals their colours, too.3
Reef-building corals live mostly in tropical and subtropical seas.4 They need:
The biggest reef system is Australia's Great Barrier Reef. It stretches about 2,600 km (1,600 miles), is made of around 2,500 separate reefs and over 900 islands, and is one of the few living structures visible from space.1416
| Type | What it looks like |
|---|---|
| Fringing reef | The most common type. Grows right along a shoreline.1 |
| Barrier reef | Runs along a coast but farther out, separated from land by a lagoon.1 |
| Atoll | A ring of reef around a central lagoon, with no island in the middle.1 |
How does a ring of reef end up in the middle of the ocean? Charles Darwin worked it out on his voyage on HMS Beagle.18 A fringing reef grows around a volcanic island. Over millions of years the volcano slowly sinks, but the coral keeps growing upward toward the light. A lagoon opens between the reef and the shrinking island (a barrier reef). When the island disappears below the waves, only the ring of reef, an atoll, is left.114 Waves can pile up sand and broken coral on the ring, making small islands.14
When the water gets too warm, corals get stressed and push out their algae. Without the algae the coral loses most of its colour and turns white. That's coral bleaching.9 NOAA's Coral Reef Watch says corals start to get stressed and bleach when the water is about 1 °C warmer than the usual warmest month.12
A bleached coral is not dead, but it has lost its main food supply.9 If the water cools in time, the algae can come back and the coral can recover. If the polyps go too long without their algae, the coral dies.317 In 2005, the U.S. lost half of its coral reefs in the Caribbean in a single year to a huge bleaching event.9 NOAA calls climate change the greatest global threat to coral reefs. Extra carbon dioxide also makes seawater more acidic, which makes it harder for corals to build their skeletons.10
Reefs cover less than 1% of the ocean floor,11 yet scientists estimate that about a quarter of all ocean species live in and around them. That's why reefs are called "the rainforests of the sea".1417 All the nooks and crannies in a reef give fish and other animals places to shelter, feed, breed and raise their young.13 Reefs support about 4,000 species of fish and 800 species of hard coral.6
People need reefs too. Reefs can absorb up to 97% of the energy of waves, protecting coasts from storms.6 Over half a billion people depend on reefs for food, income and protection.13
A baby reef-building coral (a larva) settles on hard rock in warm, shallow, clear water and turns into a polyp. The polyp builds a little cup of limestone around itself, then splits into copies of itself. Thousands of polyps share one skeleton (a colony). When polyps die their stony cups stay, and new polyps grow on top. Over thousands of years, those layers of limestone build a reef.
Coral is an animal. Most corals are colonies of tiny animals called polyps, relatives of jellyfish and sea anemones. Reef-building corals make a rock-like skeleton of limestone (calcium carbonate), and many have microscopic algae living inside them, which is why they can look like plants or rocks.
Most reef-building corals get most of their food from the algae (zooxanthellae) living in their tissues, which make sugar from sunlight and pass up to 90% of it to the coral. Corals also catch tiny drifting animals (zooplankton) with stinging tentacles, mostly at night.
When the water gets too warm, stressed corals push out the algae that live in their tissues. Without the algae, the coral loses most of its colour and its main food source, and turns white. A bleached coral is not dead, but if the heat lasts too long, it can starve and die. If the water cools in time, the algae can return.
A long time. Most corals grow less than 2.5 cm (an inch) a year; branching corals can grow up to about 10 cm a year. NOAA says it can take up to 10,000 years for a reef to form from a group of larvae, and 100,000 to 30 million years for a large barrier reef or atoll to fully form.
Shallow coral reefs cover less than 1% of the ocean floor (about 284,300 square kilometres), yet scientists estimate that about a quarter of all ocean species live in and around them. That's why reefs are called the rainforests of the sea.
The hard skeleton is covered by a thin layer of living polyps. Touching, standing on or kicking coral can damage that living tissue. Reef-building corals grow very slowly, so damage can take many years to heal.