There are billions of fish from over 25,000 species swimming in the ocean. They vary in size, shape, and physical features. Fish ruled before dinosaurs even came and roamed. Some have changed, others have died out, but some have not changed at all.
There are three groups of fish, bony fish, cartilaginous fish, and jawless fish. Bony fish have skeletons, cartilaginous fish have cartilage instead of bones, and jawless fish have suction cups instead of jaws. All fish are cold-blooded. Their body temperature changes with the water temperatures. Most fish live close to the shore in water less than 600 feet deep. In the ocean, light gets dimmer the farther down you go, and many fish that live deeper than 2000 feet glow.
All fish have gills. They absorb oxygen from the water and put it into the blood stream. A fishes scales are called armor because they protect the body. A slimy substance moistens the scales. This protects the fish from infection and helps the fish slip through the water faster. Fish stay afloat because of their swim bladders. By changing the amount of air, a fish never rises or sinks, but balances. Fins move the fish forward, steer the fish, and keeps it balanced. the pectoral and pelvic fins are used for balance, steering, and stopping. The dorsal fin works with the anal fin to keep the fish from rolling over. The tail provides power, pushing the fish forward.
There are many reasons why fish swim in schools. For small fish, swimming in groups helps protect them from predators. The changing shape confuses and scares large predators to pick out a single fish. Most fish swim in schools to track food to eat.