Fish are a group of craniates that live in water.
Fish are usually streamlined in shape, so that they can move through water rapidly.
If a fish has limbs, the limbs are usually in the form of fins.
Many fish have scales.
Most species of fish are oviparous (lay eggs) and practice external fertilization - the male fertilizes the eggs when they are outside the female's body.
There are some species of fish that practice internal fertilization and some species that are viviparous - the young develop inside the body of the female.
Some fish are ovoviviparous - the eggs develop inside the mother's body and the young that are growing inside the egg are nourished by an egg yolk.
Most fish breathe through gills throughout their lives. They use their gills to filter dissolved oxygen from the water.
Some fish, such as lungfish, can breathe oxygen that is in the air.
Fish are usually cold-blooded (ectothermic) but some fish, such as tuna, swordfish and mackerel sharks, are able to regulate the temperature in parts of their bodies.
Classification of Fish
Hagfish are craniates but not vertebrates. They have skulls, like all craniates, but they do not have backbones.
All jawed fish are vertebrates.
Cartilaginous fish include sharks, rays, chimeras and sawfish.
Most fish are ray-finned fish.
Lobe-finned fish have fleshy lobes at the base of their fins. These fleshy lobes have the same structure as the limbs of tetrapods and can move in the same way as the limbs of tetrapods.
Tetrapods and lobe-finned fish share a common ancestor.
Together, lobe-finned fish and tetrapods are known as the clade Sarcopterygii.
A clade is a group of organisms that share a common ancestor.
The jawed fish and the tetrapods make up the clade of jawed vertebrates, which is also known as Gnathostomata.
In folk taxonomy, the word "fish" has often been used to describe any animal that lives in water.
For example, edible mollusks that live in water, such as clams, are sometimes called shellfish.
In the past, people have classified whales as fish, even though they are tetrapods and mammals.