The swim bladders of bony fishes

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Swim bladders are gas-filled organs in bony fishes that help them control their buoyancy and maintain their position in the water column. They work by adjusting the volume of gas inside the bladder, which in turn affects the fish's overall density and ability to float or sink. Swim bladders are not used for respiration, and they did not evolve from internalized fins. Some bony fishes do have oil-filled swim bladders, but most have gas-filled swim bladders instead. Finally, swim bladders are not a barrier to existing in a marine environment - in fact, many bony fishes are perfectly adapted to life in saltwater.

The Swim Bladders of Bony Fishes