School › Coral reefs › Ages 10–11
Coral reefs: partners, builders and bleaching
How do tiny polyps and even tinier algae build the largest living structures on Earth? Learn the three kinds of reef, why warm water causes bleaching, and run a fair test on shells and acids.
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Corals are animals made of polyps, usually only 1–3 mm across. A colony can contain hundreds of thousands of polyps and weigh several tons. Coral reefs are the largest structures made by living things on Earth.[5][1]
Each stony coral polyp builds a skeleton of limestone (calcium carbonate). New polyps grow on the skeletons of older ones, so a reef is built up in thin layers. Massive corals grow only about 0.5–2 cm a year.[6]
Most corals have algae called zooxanthellae living in their tissues. The algae photosynthesise and pass up to 90% of the food they make to the coral; in return the coral gives them shelter, carbon dioxide and water. This partnership is why reef corals need clear, sunny water.[2][1]
Reefs grow mostly in warm water (best at 23–29 °C), in the top 70 m of the sea and between about 30° north and 30° south of the equator. There are three main kinds: fringing reefs grow out from the shore, barrier reefs are separated from the land by a lagoon, and atolls are rings of reef around a lagoon where a volcanic island has sunk.[5]
Reefs cover less than 1% of the ocean but are home to nearly a quarter of all ocean species. They also act as natural breakwaters that protect coasts from waves and storms.[4]
When the water stays too warm, corals push out their algae and turn white: coral bleaching. A bleached coral is not dead, but if the stress goes on it can starve and die. The ocean also absorbs about a quarter of the carbon dioxide people release, which makes sea water more acidic and makes it harder for corals to build skeletons.[3][2][7]
Try it: Which liquid dissolves shells? A fair test
You need
- Four clean jars with lids
- White vinegar
- Water
- Carbonated (fizzy) water
- Salt
- Measuring cup and teaspoon
- 12 clean, similar clam or mussel shells
- pH test strips (optional)
- Labels, camera or drawing paper
Safety
Wear safety glasses when pouring vinegar; rinse splashes from skin or eyes with plenty of water. Do not taste any liquid. Keep the closed jars where they will not be knocked over. Wash hands after handling the shells.
What to do
- Make four liquids, the same amount in each jar: vinegar; half vinegar and half water; fizzy water; salt water (stir in salt until dissolved). Label the jars.
- If you have pH strips, measure the pH of each liquid and write it down.
- Photograph or draw the shells. Put three similar shells in each jar and close the lid.
- Write down what you see straight away (bubbles?).
- Leave the jars for one week without moving them. Then take the shells out, rinse them and compare them with your photos.
- Which liquid changed the shells most? Which liquid is most like sea water?
What happens, and why?
Shells and coral skeletons are made of calcium carbonate. Acids react with it, so shells in vinegar fizz and become thin and soft; the weaker acids change them less. Real sea water is only very slightly acidic, but it is slowly becoming more acidic as it absorbs carbon dioxide, which makes building skeletons harder. To be fair, keep the shells, the amount of liquid and the time the same, and only change the liquid.[7][6]
Play: Good or bad for a reef?
Would this help coral reefs grow or harm them? Sort each one.
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Quiz
Other ages: Ages 6–7 · Ages 15–16