Fish.
I INTRODUCTION
  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.
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, a tiny goby, measuring only 1 cm (0.4 in) long.
Fishes are generally streamlined with a pointed snout and pointed posterior and a broadpropulsive 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.
Fishes are an important source of protein for millions of people worldwide. Since the early1970s, 70 to 100 million metric tons of fish are caught each year for food (see Fisheries). People consume about 70 percent of fish caught, and nearly 30 percent are used as animal feed that helps produce other forms of protein. Fish protein represents about 25 percent of the total animal protein consumed by the world’s population, second only to beef.
II TYPES OF FISH
Fishes may be divided into two distinct groups, jawless fish and jawed fish. The jawless fish are represented by two families of distantly related eel-like fish, the hagfish and the lampreys. Both fishes have tongues equipped with numerous small teeth and lack paired fins and a bony skeleton. Although these two families include only a handful of living species, the fossil record shows they were once a highly diverse group that also included fish whose head and trunk were covered with a hard bony shell. Hagfish are the vultures of the abyss, feeding on carcasses of dead fish and other animals. Lampreys, in contrast, feed on live fish by attaching their sucking disk to their host and rasping away tissue with their toothed tongue.
The jawed fish may also be separated into two major groups: bony fish, which have skeletons made of rigid bone, and cartilaginous fish, which have skeletons made of elastic cartilage. There are nearly 1000 species of cartilaginous fish, including sharks, rays, and chimaeras, or ratfish. Sharks and rays live in relatively shallow ocean waters and occasionally freshwater, while chimaeras are found only in the ocean, mostly in deep water. Sharks have an age-old reputation for savagery, but only a few of the approximately 370 species deserve this reputation. Most sharks, like the spiny dogfish, are predators of small fish and invertebrates, while the largest, such as megamouths, whale sharks, and basking sharks, feed by filtering tiny invertebrates from the water. The nearly 200 species of rays are essentially sharks flattened like a pancake that have adapted to life on the ocean floor.
The bony fishes encompass by far the largest diversity of fish, with about 24,000 species inhabiting nearly every body of water on the earth. They are divided into two groups—the lobe-finned fish and the ray-finned fish. Lobe-finned fishes include the lungfish, a small group of primitive air-breathing fish, and the coelacanth, the single livin...