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air bladder

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Definition: Air bladder, also known as the swim bladder, is a large, thin-walled sac that may function as a buoyant float, a sound producer and receptor, and a respiratory organ. Located in the dorsal portion of the body cavity, the swim bladder allows the fish to move up and down through the water. When gas is added by diffusion through the blood vessels in the bladder walls, the fish becomes less dense overall, allowing it to rise. When gas is removed the fish becomes more dense, thus allowing the fish to descend in the water.

The swim bladder produces sound by vibrating; these sounds are probably used in courtship. The organ also amplifies water-borne sounds and thus is an aid to hearing. In most fish the swim bladder has no connection to the digestive tract, but in some, such as the lungfish, there is a connecting tube leading to the pharynx, indicating that the organ may aid in respiration.
Also Known As: Swim bladder
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