swim bladder

A swim bladder, also called an air bladder, is an air-filled sac found in the bodies of modern bony fish. Control of the gas pressure inside the swim bladder enables the fish to remain buoyant in water at any depth. The swim bladder is located above the gut. A connection from the bladder to the gut allows air to enter and leave the bladder. Air may travel to the bladder along capillary vessels. The amount of air in the bladder can be changed so that the fish's specific gravity matches that of the water surrounding it. In lungfish the swim bladder also acts as a rudimentary lung.