The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming. The swim bladder is also of use as a stabilizing agent because in the upright position the center of mass is below the center of volume due to the dorsal position of the swim bladder. Another function of the swim bladder is the use as a resonating chamber to produce or receive sound.
Swim bladders are only found in ray-finned fish. In the embryonic stages some species have lost the swim bladder again, mostly bottom dwellers like the weather fish. Other fish like the Opah and the Pomfret use their pectoral fins to swim and balance the weight of the head to keep a horizontal position. The normally bottom dwelling sea robin can use their pectoral fins to produce lift while swimming. The cartilaginous fish (e.g. sharks and rays) and lobe-finned fish do not have swim bladders. They can control their depth only by swimming (using dynamic lift); others store fats or oils for the purpose.
The gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used in sonar equipment to find fish.
Structure and function 
The swim bladder normally consists of two gas-filled sacs located in the dorsal portion of the fish, although in a few primitive species, there is only a single sac. It has flexible walls that contract or expand according to the ambient pressure. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. By adjusting the gas pressure using the gas gland or oval window the fish can obtain neutral buoyancy and ascend and descend to a large range of depths. Due to the dorsal position it gives the fish lateral stability.
In physostomous swim bladders, a connection is retained between the swim bladder and the gut, the pneumatic duct, allowing the fish to fill up the swim bladder by "gulping" air. Excess gas can be removed in a similar manner.
In more derived varieties of fish, the physoclisti, the connection to the digestive tract is lost. In early life stages, fish have to rise to the surface to fill up their swim bladders, however, in later stages the connection disappears and the gas gland has to introduce gas (usually oxygen) to the bladder to increase its volume and thus increase buoyancy. In order to introduce gas into the bladder, the gas gland excretes lactic acid and produces carbon dioxide. The resulting acidity causes the hemoglobin of the blood to lose its oxygen (Root effect) which then diffuses partly into the swim bladder. The blood flowing back to the body first enters a rete mirabile where virtually all the excess carbon dioxide and oxygen produced in the gas gland diffuses back to the arteries supplying the gas gland. Thus a very high gas pressure of oxygen can be obtained, which can even account for the presence of gas in the swim bladders of deep sea fish like the eel, requiring a pressure of hundreds of bars. Elsewhere, at a similar structure known as the oval window, the bladder is in contact with blood and the oxygen can diffuse back. Together with oxygen other gases are salted out in the swim bladder which accounts for the high pressures of other gases as well.
The combination of gases in the bladder varies. In shallow water fish, the ratios closely approximate that of the atmosphere, while deep sea fish tend to have higher percentages of oxygen. For instance, the eel Synaphobranchus has been observed to have 75.1% oxygen, 20.5% nitrogen, 3.1% carbon dioxide, and 0.4% argon in its swim bladder.
Physoclist swim bladders have one important disadvantage: they prohibit fast rising, as the bladder would burst. Physostomes can "burp" out gas, though this complicates the process of re-submergence.
In some fish, mainly freshwater species (e.g. common carp, wels catfish), the swim bladder is connected to the labyrinth of the inner ear by the Weberian apparatus, a bony structure derived from the vertebrae, which provides a precise sense of water pressure (and thus depth), and improves hearing.
Swim bladders are evolutionarily closely related (i.e. homologous) to lungs. It is believed that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and some fish (e.g. lungfish, gar, and bichir) and into the swim bladders of the ray-finned fish. In embryonal development, both lung and swim bladder originate as an outpocketing from the gut; in the case of swim bladders, this connection to the gut continues to exist as the pneumatic duct in the more "primitive" ray-finned fish, and is lost in some of the more derived teleost orders. There are no animals which have both lungs and a swim bladder.
The cartilaginous fish (e.g. sharks and rays) split from the other fishes about 420 million years ago and lack both lungs and swim bladders, suggesting that these structures evolved after that split. Correspondingly, these fish also have a heterocercal fin which provides the necessary lift needed due to the lack of swim bladders. On the other hand, teleost fish with swim bladders have neutral buoyancy and have no need for this lift.
Human uses 
In some Asian cultures, the swim bladders of certain large fishes are considered a food delicacy. In China they are known as fish maw, 花膠/鱼鳔, and are served in soups or stews. Swim bladders are also used in the food industry as a source of collagen. They can be made into a strong, water-resistant glue, or used to make isinglass for the clarification of beer.
Similar structures in other organisms 
Siphonophores have a special swim bladder that allows the jellyfish-like colonies to float along the surface of the water while their tentacles trail below. This organ is unrelated to the one in fish.
See also 
- "Fish". Microsoft Encarta Encyclopedia Deluxe 1999. Microsoft. 1999.
- "The illustration of the swim bladder in fishes is a good one, because it shows us clearly the highly important fact that an organ originally constructed for one purpose, namely, flotation, may be converted into one for a widely different purpose, namely, respiration. The swim bladder has, also, been worked in as an accessory to the auditory organs of certain fishes. All physiologists admit that the swimbladder is homologous, or “ideally similar” in position and structure with the lungs of the higher vertebrate animals: hence there is no reason to doubt that the swim bladder has actually been converted into lungs, or an organ used exclusively for respiration. According to this view it may be inferred that all vertebrate animals with true lungs are descended by ordinary generation from an ancient and unknown prototype, which was furnished with a floating apparatus or swim bladder." Darwin, Origin of Species.
- Pelster B (December 2001). "The generation of hyperbaric oxygen tensions in fish". News Physiol. Sci. 16 (6): 287–91. PMID 11719607.
- Colleen Farmer (1997). "Did lungs and the intracardiac shunt evolve to oxygenate the heart in vertebrates". Paleobiology
- Kardong, KV (1998) Vertebrates: Comparative Anatomy, Function, Evolution2nd edition, illustrated, revised. Published by WCB/McGraw-Hill, p. 12 ISBN 0-697-28654-1
- Teresa M. (2009) A Tradition of Soup: Flavors from China's Pearl River Delta Page 70, North Atlantic Books. ISBN 9781556437656.
- Clark, F. E.; C. E. Lane (1961). "Composition of float gases of Physalia physalis". Fed. Proc. 107: 673–674.
- Carl E. Bond, Biology of Fishes, 2nd ed., (Saunders, 1996) pp. 283–290.