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swim-bladder (air bladder, gas bladder, hydrostatic organ) In Actinopterygii, a thin-walled, gas-filled sac in the roof of the abdominal cavity, which allows the fish to achieve neutral buoyancy. It may (physostomous) or may not (physoclistous) be connected with the intestinal tract. The sac evolved from an accessory respiratory organ and retains a respiratory function in some fish (e.g. Lepisosteidae). It is also sometimes used for sound production (e.g. Holocentridae), or sound reception (e.g. Gadidae).