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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.

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.

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.

Experiment: Which liquid dissolves shells? A fair test

1. 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.
2. If you have pH strips, measure the pH of each liquid and write it down.
3. Photograph or draw the shells. Put three similar shells in each jar and close the lid.
4. Write down what you see straight away (bubbles?).
5. Leave the jars for one week without moving them. Then take the shells out, rinse them and compare them with your photos.
6. Which liquid changed the shells most? Which liquid is most like sea water?

Liquid	pH (if measured)	Shells after 1 week
Vinegar	Prediction
Half vinegar, half water	Prediction
Fizzy water	Prediction
Salt water	Prediction

Questions

1. Explain how corals and their algae help each other.

2. Draw and label a fringing reef, a barrier reef and an atoll.

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3. In your shell test, what did you change, what did you observe and what did you keep the same?

4. Why does a bleached coral look white?

Sources for every fact on this sheet: echoesof.science/school/coral-reefs/sources/

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