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Yellow perchYellow perchFish are vertebrates (animals with backbones) that live in water all of the time. They have hairless, streamlined bodies with fins and a tail. Many have a protective layer of over­lapping scales. Fish are cold-blooded animals, so their body temperature depends on their surroundings. There are two main groups of fish: the cartilaginous fish, including sharks and their relatives, and the bony fish.

Parts of a fishParts of a fish
How a fish breathesHow a fish breathes


Most kinds of fish cannot survive out of water, because they do not have lungs to breathe air. Instead, they are able to filter oxygen from the water around them using slits called gills on their heads. To breathe, fish open their mouths, and take in water that contains oxygen. When they pump it out through slits in the side of their head, it passes over the gills and tiny blood vessels extract the oxygen. Bony fish usually have a hard protective flap over their gills.

At top speed, a bluefin tuna could outsprint a greyhound, zebra or hyena.

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