The rainforests of the sea
Coral reefs cover less than 1% of the ocean floor, yet they’re home to about 25% of all marine species. Their incredible biodiversity has earned them the nickname "rainforests of the sea."
What are corals?
Corals look like plants or rocks, but they’re animals! A coral is made of thousands of tiny animals called coral polyps, each only a few millimetres wide. Polyps are related to jellyfish and sea anemones.
Each polyp has:
- a soft, tube-shaped body
- a mouth surrounded by stinging tentacles to catch tiny plankton
- a hard limestone (calcium carbonate) skeleton that it builds underneath itself
When polyps die, their skeletons remain, and new polyps grow on top. Over thousands of years, these skeletons build up into massive reefs.
A partnership with algae
Inside coral tissues live tiny algae called zooxanthellae (zoh-zan-THEL-ee). This is a great example of symbiosis:
| Coral gives algae | Algae give coral |
|---|---|
| A safe home | Up to 90% of the coral’s food, made by photosynthesis |
| Carbon dioxide and nutrients | Oxygen |
| Bright colours |
Because the algae need sunlight, most reef-building corals live in shallow, clear water.
Where coral reefs grow
Reef-building corals need:
- warm water, about 23–29 °C
- clear water so sunlight can reach them
- shallow water, usually less than 50 m deep
- salty water
Most reefs are found in tropical oceans between about 30° N and 30° S.
| Reef | Location | Notes |
|---|---|---|
| Great Barrier Reef | Australia | The largest reef system, about 2300 km long; visible from space |
| Mesoamerican Reef | Mexico, Belize, Guatemala, Honduras | Second-largest barrier reef |
| Coral Triangle | Indonesia, Philippines, Papua New Guinea | Highest coral diversity |
| Red Sea reefs | Egypt, Saudi Arabia | Corals tolerate unusually warm water |
Cold-water corals
Some corals live in cold, dark, deep water without algae. Canada has cold-water coral and sponge reefs, including ancient glass sponge reefs off the BC coast, once thought to have died out with the dinosaurs.
Types of reefs
| Type | Description |
|---|---|
| Fringing reef | Grows right along a coastline |
| Barrier reef | Separated from the shore by a lagoon |
| Atoll | A ring of coral around a lagoon, often where a volcanic island has sunk |
Life on the reef
| Animal | Role |
|---|---|
| Parrotfish | Scrape algae off coral; their droppings make white sand |
| Clownfish | Live safely among stinging sea anemones |
| Sea turtles | Eat sponges and seagrass |
| Sharks | Top predators that keep the ecosystem balanced |
| Cleaner wrasse | Eat parasites off larger fish at "cleaning stations" |
| Giant clams | Can live over 100 years |
| Octopuses | Masters of camouflage |
Why reefs matter
- Protect coastlines from waves and storms
- Provide food for hundreds of millions of people
- Support tourism, diving and fishing jobs
- Medicines: reef organisms are sources of new drugs
Threats
Coral bleaching
When the water gets too warm, corals become stressed and expel their algae. Without the algae, the corals turn white. This is coral bleaching. Bleached corals aren’t dead yet, but if the heat lasts too long, they starve. Mass bleaching events have become more frequent as oceans warm. In 2023–2024, scientists reported a global bleaching event affecting reefs in dozens of countries.
Ocean acidification
The ocean absorbs carbon dioxide from the air, forming carbonic acid. This ocean acidification makes it harder for corals to build their limestone skeletons.
Other threats
| Threat | Effect |
|---|---|
| Pollution and runoff | Fertilizers cause algae to smother coral |
| Overfishing | Removes fish that keep algae in check |
| Destructive fishing | Dynamite and cyanide fishing destroy reefs |
| Coastal development | Sediment clouds the water |
| Careless tourism | Anchors and touching damage coral |
Protecting reefs
- Marine protected areas
- Reducing greenhouse gas emissions
- Reef-safe sunscreen
- Coral restoration: scientists grow corals in nurseries and replant them
- Sustainable seafood choices
Common mistakes
- Thinking corals are plants or rocks.
- Thinking bleached coral is dead. It’s stressed and can recover if conditions improve.
- Thinking coral reefs grow quickly. Many grow only a few millimetres to centimetres a year.
Try it yourself
- Put a piece of chalk (calcium carbonate) in vinegar. Relate what happens to ocean acidification.
- Draw a coral reef food web with at least eight organisms.
- Research a coral restoration project and present how it works.
Quick recap
- Coral reefs are built by tiny animals called coral polyps.
- Corals live in symbiosis with algae that make their food.
- Reefs hold about a quarter of all ocean species.
- Warming water causes coral bleaching; acidification weakens skeletons.
- Protecting reefs requires reducing pollution, overfishing and climate change.