How fish live in water
The three groups of fish, how gills take oxygen out of water, how the swim bladder helps a fish float, and how fish sense movement. Then build a diver that rises and sinks like a fish.
Read
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.[1]
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.[4]
Cold water can hold more dissolved oxygen than warm water, so in summer there is often less oxygen for fish.[6]
Many bony fish have a swim bladder, a bag of gas that can fill up or empty. It helps the fish control whether it floats up or sinks down.[1][2]
Along each side, a fish has a lateral line that senses vibrations and movement in the water.[2][1]
Fish are ectotherms (cold-blooded): they cannot control their own body temperature, so it depends on the water around them.[3][8]
Try it: Swim bladder diver
You need
- A clear 1.5 or 2 litre plastic bottle with cap, full of water
- Three plastic pipettes (droppers)
- Three small metal nuts or blobs of modelling clay
- A cup of water
- Towels
Safety
Wipe up spilled water straight away. Do not squeeze the bottle near anyone's face, and keep the cap screwed on.
What to do
- Push a nut onto the end of each pipette so it hangs down. This is your 'fish'.
- 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.
- Put all three divers into the full bottle and screw the cap on tightly.
- Squeeze the bottle slowly. Watch the air bubbles in the divers. Which diver sinks first?
- Let go. What happens?
- 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.
What happens, and why?
Squeezing the bottle squashes the air bubbles in the divers, so each diver takes up less space and pushes aside less water. The upward push of the water (the buoyant force) becomes smaller and the diver sinks; let go, and the bubble grows again. A diver with a smaller bubble sinks with less squeezing. A fish's swim bladder works in a similar way: more gas helps the fish float up, less gas lets it sink.[1][9][10]
Play: Which group of fish?
Is it a jawless fish, a cartilaginous fish or a bony fish?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16