Fish, a water-dwelling animal that breathes through gills and has a backbone, fins, and a two-chambered heart. Certain other animals that live in water—such as frogs in the tadpole stage—also have gills and backbones, but fish are the only backboned animals with two-chambered hearts.

Jawless fish were the first of all living vertebrates. There are more species of jawed fish—between 15,000 and 17,000—than of any other group of vertebrates.

Exceptions can be found to almost everything that can be said about fish in general. Some fish have no bones, some lack eyes, others have no jaws, and some have no scales. Some fish are brilliantly colored, some have luminous spots, and some are drab. Of those fish that have lungs and can breathe air, some drown when kept under water. Some species can live a few hours out of water. Although the overwhelming majority of fish are cold-blooded (their body temperature is the same as the temperature of the surrounding water), a few are warmblooded (have a body temperature higher than the temperature of their surroundings).

The saddleback butterflyfishThe saddleback butterflyfish flaunts brilliant colors and patterns.

Many fish are edible and are an important food for humans. The nutritive value of fish is equal to that of meats. Fish is an excellent source of protein. Its fat content ranges from .1 per cent to 26 per cent. Fish is a good source of minerals, especially iodine and phosphorus. Fish-liver oils provide vitamins A and D, and fish is also a source of thiamine, niacin, riboflavin, omega-3 fatty acids, and three of the eight amino acids essential to human nutrition. The belief that fish is “brain food” is without basis.

Catching fish is a popular sport as well as an important industry. Many people keep tropical fish in home aquariums. Fish of many kinds are exhibited in public aquariums and aquariums maintained by educational institutions.

The branch of science concerned with the study of fish is called ichthyology (from the Greek for “fish” and “discourse”). Scientists who study fish are called ichthyologists.