Fish, diverse group of animals that live and breathe in water. All fishes are vertebrates (animals with backbones) with gills for breathing. Most fish have fins for swimming, scales for protection, and a streamlined body for moving easily through the water.

Fishes live in nearly every underwater habitat, from near-freezing Arctic waters to hot desert springs; from mud in dried-up tropical ponds to the deepest ocean abyss. Special antifreeze chemicals in the blood of Antarctic icefish enable them to survive in water below 0° C (32° F). Desert pupfish found in hot springs of western North America live in temperatures higher than 40° C (100° F). Killifish release their eggs, or spawn, as the dry season begins in the tropics of South America and Africa, leaving their eggs to dry in the ground until the rains return six months later. In the deep ocean, where sunlight never reaches, many fishes cooperate with glowing bacteria to create their own light for communication and to attract mates and prey (see Bioluminescence).

With approximately 25,000 recognized species, fishes make up the most diverse vertebrate group, comprising about half of all known vertebrate species. New fishes continue to be discovered and named at the rate of 200 to 300 species per year. With this vast number of different fishes comes a diversity of sizes and shapes, from huge whale sharks that reach 12 m (40 ft) in length to the smallest vertebrate, the stout infantfish (Schindleria brevipinguis), measuring only about 7 mm (0.3 in) long.

Fishes are generally streamlined with a pointed snout and pointed posterior and a broad propulsive tail. Unlike the shape of a human body, a fish’s body shape is ideal for speeding through the water without creating excess resistance. This torpedo-shaped body is typical of the fastest-swimming fishes, the billfish and the tunas. One billfish, the sailfish, can swim in bursts of over 110 km/h (70 mph). Tunas are built for long-distance endurance as well as speed, swimming as fast as 50 km/h (30 mph) and migrating as far as 12,500 km (7700 mi) in only four months. Other fishes come in a wide variety of shapes. The snakelike eels, flat halibuts, and boxy puffers are all slower swimmers that have evolved distinctive bodies best adapted to their specific habitats. Unlike fishes that swim through the open water, these fishes have adapted to life in caves, on the ocean floor, and among coral reefs where speed is less important than camouflage or maneuverability.