Fish are vertebrates (animals with backbones). They live in water all the time. Most kinds cannot survive out of water, because they do not have lungs to breathe air. Instead, they take in oxygen from the water using gills in their heads.
Fish 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. Some species have a special substance in their blood to stop them freezing in cold waters.
Fish move by flexing the muscles along their bodies in a wave-like motion. The fins help to balance and steer the fish while the tail can be used for propulsion. The streamlined shape of most fish, as well as a coating of slimy mucus on their bodies, helps them to swim easily through the water.
Most fish lay vast numbers of eggs at once, which they leave to hatch out on their own. Newly-hatched fish are perfect, tiny replicas of their parents. A few kinds of fish, such as sharks, carry their eggs inside their bodies, and give birth to live young.
Fish were the first vertebrates to evolve, millions of years ago. One of the oldest types alive today is the coelacanth, fossils of which date back 90 million years.
There are two main groups of fish, the cartilaginous fish and the bony fish. Cartilaginous fish include sharks, rays and chimaeras. Most are found only in the seas and oceans. Cartilaginous fish have skeletons made of soft cartilage. Instead of flat, overlapping scales, they have tiny, pointed, tooth-like scales. Their gills can be opened and closed, but, unlike the bony fish, usually do not have protective flaps covering them. Cartilaginous fish must keep moving all the time, to stay afloat in the water.
Sharks are mostly predators, although the largest kinds of all, the whale shark and the basking shark, feed only on plankton, which they filter from the water using a part of their gills. Unlike bony fish, most of which have good vision, sharks rely on smell to hunt their prey. Sharks can detect a single drop of blood in a huge volume of water. They also have sensors on their bodies that can pick up tiny electrical signals produced by the movements of their prey.
Sharks have large, razor-sharp teeth, which arc set in rows. If the shark loses a tooth during feeding, a new one comes forward in the mouth to take its place. Most sharks feed on fish or squid, but the larger sharks, such as the great white, also eat turtles and large mammals such as seals. They slam into their prey, tearing out huge chunks of flesh. Despite their fearsome reputation, only the largest of the predatory sharks, such as the tiger shark or the great white shark, are dangerous to humans.
Rays, and their relatives the skates, are cartilaginous fish with flat bodies and often long, narrow tails. Their gills and mouths are on their undersides. They have large, wing-like fins that they flap as they swim, making them look as if they are flying through the water. Rays feed on fish and shellfish near the sea bed. Sometimes they hide in the sand to ambush passing prey.
Like sharks, a few kinds of rays can be dangerous. Torpedo rays can give off a powerful electric shock, while a jab from the venomous spine on a stingray’s tail can kill a human. Unlike sharks, however, rays do not see humans as prey.
Chimaeras are a group of cartilaginous fish with long bodies and tails. They include the rabbit fish, a bottom-dwelling fish with large eyes and a venomous spine on its dorsal fin.
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