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.