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.

A trout swimmingA trout swimmingClick to play video


Fish move by flexing the muscles along their bodies in a wave-like motion. The fins help to balance the fish while the tail, or caudal fin, can be used for steering. The dorsal fin, on the back side of fish, helps keep the animal upright while swimming. The pectoral fins on each side of a fish's body help the fish to steer. The streamlined shape of most fish, as well as a coating of slimy mucus on their bodies, helps them to move easily through the water.

Some fish have an internal organ called a swim bladder that helps them to stay at a certain depth. The swim bladder is filled with air. It enables a fish to stay buoyant (neither sinking nor rising) without needing to swim.

Young fish developing inside their eggsYoung fish developing inside their eggs


In more than 97% of all fish, their eggs develop outside the mother's body. They are known as oviparous. Most fish lay vast numbers of eggs at once, which they leave to hatch out on their own. Newly-hatched fish are tiny replicas of their parents.

A few kinds of fish, such as certain species of shark, carry their eggs inside their bodies, and give birth to live young. They are known as viviparous.

A close up of a shark's skinA close up of a shark's skin

Cartilaginous fish

Cartilaginous fish include sharks, rays and chimaeras. They are found only in the seas and oceans. Cartilaginous fish have skeletons made of soft cartilage, sometimes known as gristle. Instead of flat, overlapping scales, they have tiny, pointed, tooth-like scales. Their gills can be opened and closed, but unlike those of bony fish, they do not usually have protective flaps covering them. Cartilaginous fish must keep moving all the time, to stay afloat in the water.

Bony fish

Sailfish, a bony fishSailfish, a bony fishBony fish are a much larger group than the cartilaginous fish. They have skeletons made of bone. Their bodies are usually covered with overlapping scales. Bony fish also have gas-filled swim bladders inside their bodies that allow them to stay afloat even when they are not moving.

Some bony fish live in fresh water (rivers and lakes), while others live in the seas and oceans. Some feed on plant material, but others are carnivorous. Many ocean-living fish are found close to the surface, where there is warmth and light. Fewer fish live in the deeper, darker waters.Internal organs of a fishInternal organs of a fish



Most bony fish have good eyesight and can see in colour. Their eyes are on the sides of their heads, giving them a wide field of vision to spot both predators and prey. 

Flatfish are a group of bottom-living fish, including flounder and sole. They have both eyes on one side of their flat bodies, so that they can spot predators or prey above them. They are camouflaged to blend in with the sandy sea bed.


Fish such as salmon, which are preyed upon by many other animals, swim in large groups for protection—in the same way that animals such as antelope herd together on land. Shoaling means that fish have a better chance of survival if they are attacked by predators, and a better chance of finding a mate.Shoaling fishShoaling fish

 Chris Jarvis

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

Around one quarter of all fish species spend their whole life shoaling.

Some fishes' bodies contain a special substance to stop them freezing in cold waters. Arctic and Antarctic species have a natural anti-freeze protein in their blood.

Fish were the first vertebrates (backboned animals) to evolve, hundreds of millions of years ago. The oldest species alive today is possibly the coelacanth, fossils of which can be found dating back 360 million years.

Many types of marine animals with "fish" in their name are not fish at all. Examples include: jellyfish, shellfish, cuttlefish, starfish and crayfish.

The ray-finned fish, fish whose fins are supported by spines ("rays"), comprise about 99% of all species of fish. They range from tiny Paedocypris, at 8 mm (0.3 in) the smallest fish—and smallest vertebrate of all—to the massive ocean sunfish and the 11-metre (36-foot) oarfish.

There are more species of fish (over 32,000) than any other vertebrate group.

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