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 .
These gills absorb oxygen from the water and pass it along to the bloodstream. A fishes scales are called armor because they protect it. 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 they have swim bladders.