Fish appeared very early in the history of the Earth. Scientists have found fossilized rock layers over four hundred million years old, from which it has been proved that fish first appeared in the Silurian Period and became numerous in the Age of Fishes, the Devonian Period that followed. Many millions of years passed before land animals appeared.
A true fish is cold-blooded, breathes through gills on each side, and depends almost entirely on water for life. It has a bony skeleton and a long-shaped body, narrowing at the tail. The fins at various parts of its body are used for steering, balancing, and moving it forward. An air bladder, often called swim bladder, helps it maintain balance to rise, descend, and adjust to water pressure. It has a heart which has two principal chambers: the atrium and ventricle. It reproduces by laying eggs.
Many variations, however, occur. Lungfish have limblike fins with air-breathing "lungs;" sharks bring forth their bables alive; the sturgeon has a cartilaginous skeleton. Other examples are the catfish which has no scales, the climbing perch which actually climbs trees, and the flying fish which rises from the water in gliding flight.
Fish have many interesting characteristics. Some mature in a few weeks; others may take up to twenty years. They may range from one-half inch in size to forty feet. Though the maximum age for a fish is generally one year, some live fifty years.
Stinging rays, spines in the fins or on the head, speed of movement, armor coating, hardscales with tiny needlelike spines, and powerful electric shocks are typical of some of the varied protective measures evolution has given fish.
The most common food eaten by fish is plankton; a few eat large plants attached to rocks. Most of them devour worms, crabs, and shrimps, and some will attack oyster beds and shellfish. Many fish consume other smaller fish.
The fish have many enemies. Other fish and animals prey upon them. In fresh water the bladderwort traps and digests baby fish. Insect larvae seek them out as well as squids and jellyfish. Seals, whales, and porpoises eat fish, as do marine birds.
Fish are very useful. They provide an abundant food supply—nearly thirty billion pounds a year valued at seventy-five million dollars. The fishing industry is particuarly important in countries like Norway and Japan and on a smaller scale in the U.S.A. Fish also control the increase of harmful insects. Their oils provide man with vitamins A and D; the swim bladder is used for the production of isinglass, and the skin provides shagreen, a type of leather.
There are two main groups of fish, those with cartilaginous skeletons (sharks and rays) and those with bony skeletons (perch, garpike, seahorse). The fish make up one of the large classes of vertebrates.
