swim bladder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gas-filled structure in many fishes that functions to maintain buoyancy and, in some species, to aid in respiration or to produce sound.

from The Century Dictionary.

• noun Same as swimming-bladder.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy, usually of fish A gas-filled sac within the bodies of most fish that variously acts as a float, lung, sound-producing organ or an aid to hearing.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an air-filled sac near the spinal column in many fishes that helps maintain buoyancy