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There are about 31,000 kinds of fish, more than any other group of animals with a backbone. They form three groups: jawless fish such as lampreys and hagfish, cartilaginous fish such as sharks and rays, whose skeleton is made of cartilage, and bony fish, with a skeleton of bone.

Water holds much less oxygen than air, so a fish has to pass a lot of water over its gills. Each gill has many thin filaments with thousands of tiny folds. Oxygen dissolved in the water passes through their thin walls into the blood.

Cold water can hold more dissolved oxygen than warm water, so in summer there is often less oxygen for fish.

Experiment: Swim bladder diver

1. Push a nut onto the end of each pipette so it hangs down. This is your 'fish'.
2. Suck a little water into the first pipette so that it only just floats in the cup. This diver has a big air bubble. Give the second diver a medium bubble and the third a small bubble.
3. Put all three divers into the full bottle and screw the cap on tightly.
4. Squeeze the bottle slowly. Watch the air bubbles in the divers. Which diver sinks first?
5. Let go. What happens?
6. Fair test: keep the bottle, water and nuts the same; change only the size of the bubble. Repeat three times and note the order in which the divers sink.

Diver	Try 1: order sunk	Try 2: order sunk	Try 3: order sunk
Big bubble	Which diver will sink first?
Medium bubble	Which diver will sink first?
Small bubble	Which diver will sink first?

Questions

1. Name the three groups of fish and give one example of each.

2. Explain how a fish gets oxygen from water. Use the words gills, dissolved and blood.

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

4. Why might fish in a pond find it harder to get oxygen on a hot summer day?

Sources for every fact on this sheet: echoesof.science/school/fish/sources/

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