Explanation

Fish have special body parts that help them float in water. The swim bladder is a gas-filled sac inside most fish. It works like a balloon to control how deep the fish swims.

The air bladder is another name for the swim bladder. Both terms describe the same organ that keeps fish from sinking. This organ can fill with gas or release gas to change the fish depth.

Fish do not have air sacs like birds or air in spongy bones. Only the swim bladder helps fish stay at the right depth. Without this organ, fish would need to swim constantly to avoid sinking to the bottom.