swim bladder, large, thin-walled sac in some fishes that may function in several ways, e.g., as a buoyant float, a sound producer and receptor, and a respiratory organ. The swim bladder, or air bladder, is located in the dorsal portion of the body cavity and is filled with gases. When gas is added to the swim bladder, by diffusion through the blood vessels in the bladder walls, the fish becomes less dense overall; when gas is removed the fish becomes more dense. The addition and removal of gases is a mechanism by which the density of the fish can be made equal to that of the surrounding water at a given depth. 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.