While many have become extinct , others have not changed much at all. There are three goups of fish. The largest group of fish, bony fish, have skeletons. Cartiliganous fish, such as sharks, have cartilage rather than bones. Jawless fish, the third group, are fish with suction cups instead of jaws.
Fish are cold-blooded animals.
This means that their blood temperature changes with the temperature of the surrounding water. Most fish live close to the shore in water that is less than 600 feet deep. In the ocean, the deeper the water, the dimmer the light. Many fish who live at 2000 feet or deeper glow in the dark. All fish have gills .