swim bladder

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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.

Swim bladder

A gas-filled sac found in the body cavities of most bony fishes (Osteichthyes). The swim bladder has various functions in different fishes, acting as a float which gives the fish buoyancy, as a lung, as a hearing aid, and as a soundproducing organ. In many fishes it serves two or three of these functions, and in the African and Asiatic knife fishes (Notopteridae) it may serve all four. The swim bladder contains the same gases that make up air, but often in different proportions.

Swim Bladder


an unpaired or paired organ in fishes that develops as a process of the anterior part of the intestine. It may function in hydrostasis, respiration, and sound production; it may also serve as a resonator and transformer of sound waves. In Dipnoi, Polypterus, and Holostei, the swim bladder serves as a supplementary organ of respiration. It opens into the ventral part of the anterior end of the esophagus in dipnoans and Polypterus and into the dorsal part in Holostei. In bony fishes (Osteichthyes) the swim bladder is unpaired and departs from the dorsal side of the intestine. It is filled with gases, whose composition may change and is different from the gaseous composition of atmospheric air, and it regulates the density of the fish’s body during immersion and emersion—a hydrostatic function. In physostomous fishes, the swim bladder is always connected with the intestine by a pneumatic duct, through which gases enter and leave the swim bladder. In adult physoclistous fishes, the pneumatic duct becomes atretic; gases are absorbed or excreted through the red gland—a dense network of capillaries on the interior wall of the swim bladder. The quantity of gas in the swim bladder is regulated by reflexes. With the increase in hydrostatic pressure as the fish passively immerses itself deeper, secretion of gas and compression of the swim bladder occur. A decrease in the pressure as the fish floats upward is accompanied by an intake of gas and dilation of the swim bladder.

In some fishes, the swim bladder is connected with the inner ear by blind processes of the swim bladder or by the Weberian ossicles. In such cases, the swim bladder plays a role in sound perception. The fishes hear sounds up to 13,000 hertz. Fishes that do not have this connection hear sounds no higher than 2,500 hertz. The sound-forming function of the swim bladder is characteristic only of males and depends on the organ’s size, shape, and structure. Many benthic bony fishes do not have a swim bladder.


Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivolnykh, 4th ed. Moscow, 1947.
Protasov, V. R. Bioakustika ryb. Moscow, 1965.
Alexander, R. Biomekhanika. Moscow, 1970. (Translated from English.)


swim bladder

[′swim ‚blad·ər]
(vertebrate zoology)
A gas-filled cavity found in the body cavities of most bony fishes; has various functions in different fishes, acting as a float, a lung, a hearing aid, and a sound-producing organ.
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