As mentioned earlier, living in the dense medium of water presents a few problems for fish, and one of these is buoyancy. Maintaining a certain level in the water column without having to expend a lot of energy is very important to fish. Therefore, most species have a special organ called the swim bladder. This gas filled sac, located in the abdominal cavity of the fish, acts as a life vest, keeping the fish at the correct level in the water column.
There are many types of swim bladders. The Trout has a simple single chambered sac, the Goldfish a two-chambered type, and the Angelfish a three chambered bladder. Different species also use different methods to fill the swim bladder with air. Some have a direct connection between the esophagus and the bladder and simply swallow air to fill it. Others must rely on gas exchange from specialized blood vessels in the circulatory system to fill the swim bladder.
In addition to its role in buoyancy control, the swim bladder helps to mechanically amplify sound for better hearing in certain species of fish.
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