Fish live under water nearly all the time. They do not breathe air. Instead, they use their gills to take in oxygen from the water around them. Fish have fins and tails, and many are covered with scales. Most fish lay eggs, although some, including sharks, give birth to live young. Some kinds of fish live in fresh water (rivers and lakes) while others live in the saltwater of the oceans. There are two main groups of fishes: cartilaginous fish (sharks and rays) which have soft skeletons made of cartilage, and bony fish—all other fish—which have skeletons made of bone.

Living underwater

Many fish have streamlined bodies with fins, a tail and a coat of overlapping scales. They breathe using gills. They draw water, which contains oxygen, into their mouths. When they pump it out through slits in the sides of their head, their gills take in the oxygen. Most fish lay thousands of eggs in the water, although many of these eggs are eaten by predators before they hatch.

Some fish swim about in large groups, called shoals. This helps to protect them from predators. Others hide amongst rocks and seaweed. Many have no need to hide at all, as they are poisonous. Their bright markings warn predators not to eat them.

Dangerous creatures

Fish can be extremely dangerous creatures. Some are venomous while others have a deadly bite. Moray eels hide in holes between rocks and shoot out to grab their prey with their sharp teeth. Stingrays have long, thin tails that they whip round to drive a venomous, sharp spike into their attackers.