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.