There are more than 32,000 fish species, 4,400 of which can be found in Australia. In fact, there are more species of fish than mammals, birds and reptiles combined. Groups of fish are called schools.
Fish have been on Earth for more than 450 million years, much longer than mammals. They can be divided into three main groups - cartilaginous, bony and lobe-finned.
They have a specialised organ call the lateral line that they use to detect movement and vibration, much like a radar.
Fish breathe through scales that extract oxygen from water. Their scales are often coated in slime which assists with movement through the water.
Depending on the species, fish can eat plankton, insects and other smaller fish.
The largest fish in the world is the whale shark, which can reach up to 12 metres long. They can be found off the west coast of Australia.
Scroll through the videos, photos and articles below to find out more about fish.
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