Unlike mammals, fish are cold-blooded. This means that they do not maintain a constant internal body temperature; instead, their temperature is greatly influenced by their environment. True fish have a backbone and fins. Most also breathe with gills and have scales that cover their bodies. It is currently believed that fish began to evolve about 480 million years ago. There are about 22,000 known species of fish.

A fish's fins are used for balance and to help propel and steer through the water. Most fish have 2 types of fins: single fins that are found along the centerline (top and bottom) of the fish, and paired fins. The caudal fin, or tail fin, is the main fin used to move the fish forward in the water, while the dorsal and anal fins (on the top and bottom, respectively) help the fish balance and keep it from rolling over. The paired fins help with steering and hovering.

Anatomy of a fish

On the outside of the skin, most fish have scales. These overlap in rows and help protect the fish against injuries and infection. In some species (for example, puffer fish) the skin covers the scales—creating a living surface. Their edges are jagged and sharp in some fish, and smooth and rounded in others. Over the scales, fish secrete a mucous covering to further protect against infection. The mucus traps and immobilizes bacteria and viruses, keeping them from entering the fish's body. This covering also helps reduce friction, allowing the fish to move easily through the water.

In order to breathe underwater, fish have developed special organs called gills. The gills, found on the side of the fish just behind the head, contain thousands of capillaries, or tiny blood vessels. Water is constantly pumped over the gills, which filter the oxygen out of the water and directly into the fish's blood. A gill cover, called the operculum, is a flexible bony plate that helps protect the sensitive gills. Gills are also important for excretion of waste products, particularly ammonia, from the fish's bloodstream.

Fish have a unique internal organ known as the swim bladder or air bladder. It is usually found in the abdomen, and it helps fish move up or down in the water. By adjusting the amount of air in the bladder, fish can adjust the depth at which they float without continuously having to swim. In some fish, the swim bladder is also used to produce sounds. Members of the shark or ray family (elasmobranches) do not have a swim bladder.

Many fish have excellent vision and can see colors. They also have nostrils and are able to detect odors in water. Fish may or may not have teeth, depending on the species. Another organ of sense unique to fish is called the lateral line, located along the side of the fish. It contains small sensory hairs that help detect underwater vibrations and determine their source, enabling fish to navigate even in low light or murky water.

Last full review/revision July 2011 by Ruth Francis-Floyd, DVM, MS, DACZM