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The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift via swimming, which expends more energy. Also, the dorsal position of the swim bladder means that the expansion of the bladder moves the center of mass downwards, allowing it to act as a stabilizing apparatus. Additionally, the swim bladder functions as a resonating chamber to produce or receive sound.
The swim bladder is homologous to the of and lungfish, and some ray-finned fish such as have also evolved similar respiratory functions in their swim bladders. Charles Darwin remarked upon this in On the Origin of Species, and reasoned that the lung in air-breathing vertebrates had derived from a more primitive swim bladder as a specialized form of enteral respiration.
Some species, such as mostly benthos like the weather fish and redlip blenny, have secondarily lost the swim bladder during the embryonic stage. Other fish, like the opah and the pomfret, use their 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 like cartilaginous fish do.
The gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used by sonar equipment to fishfinder.
Cartilaginous fish such as and rays do not have swim bladders, as they belong to a completely different evolutionary clade. Without swim bladders to modular buoyancy, most cartilaginous fish can only control depth by actively swimming, which produce dynamic lift; others store up with specific density less than that of seawater to produce a neutral or near-neutral buoyancy, which cannot be readily changed with depth.
In physostome 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, these fish must rise to the surface to fill up their swim bladders; in later stages, the pneumatic duct disappears, and the gas gland has to introduce gas (usually oxygen) to the bladder to increase its volume and thus increase buoyancy. This process begins with the acidification of the blood in the rete mirabile when the gas gland excretes lactic acid and produces carbon dioxide, the latter of which acidifies the blood via the bicarbonate buffer system. The resulting acidity causes the hemoglobin of the blood to lose its oxygen (Root effect) which then diffusion partly into the swim bladder. Before returning to the body, the blood re-enters the rete mirabile, and as a result, virtually all the excess carbon dioxide and oxygen produced in the gas gland diffuses back to the arteries supplying the gas gland via a countercurrent multiplication loop. 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 out again. 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. can "burp" out gas, though this complicates the process of re-submergence.
The swim bladder in some species, mainly fresh water fishes (common carp, catfish, bowfin) is interconnected with the inner ear of the fish. They are connected by four bones called the Weberian ossicles from the Weberian apparatus. These bones can carry the vibrations to the saccule and the lagena. They are suited for detecting sound and vibrations due to its low density in comparison to the density of the fish's body tissues. This increases the ability of sound detection. The swim bladder can radiate the pressure of sound which help increase its sensitivity and expand its hearing. In some deep sea fishes like the Antimora, the swim bladder maybe also connected to the macula of saccule in order for the inner ear to receive a sensation from the sound pressure. In red-bellied piranha, the swim bladder may play an important role in sound production as a resonator. The sounds created by piranhas are generated through rapid contractions of the sonic muscles and is associated with the swim bladder.
are thought to lack a sense of absolute hydrostatic pressure, which could be used to determine absolute depth. (2025). 9780415375627, Taylor & Francis. ISBN 9780415375627 However, it has been suggested that teleosts may be able to determine their depth by sensing the rate of change of swim-bladder volume.
In 1997, Farmer proposed that lungs evolved to supply the heart with oxygen. In fish, blood circulates from the gills to the skeletal muscle, and only then to the heart. During intense exercise, the oxygen in the blood gets used by the skeletal muscle before the blood reaches the heart. Primitive lungs gave an advantage by supplying the heart with oxygenated blood via the cardiac shunt. This theory is robustly supported by the fossil record, the ecology of extant air-breathing fishes, and the physiology of extant fishes. In 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.
As an adaptation to migrations between the surface and deeper waters, some fish have evolved a swim bladder where the gas is replaced with low-density as a way to cope with Boyle's law.
The Chondrichthyes (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 both Protocercal and stiff, wing-like which provide the necessary lift needed due to the lack of swim bladders. Teleost fish with swim bladders have neutral buoyancy, and have no need for this lift.Kardong, KV (1998) Vertebrates: Comparative Anatomy, Function, Evolution2nd edition, illustrated, revised. Published by WCB/McGraw-Hill, p. 12
Most mesopelagic fish make daily vertical migrations, moving at night into the epipelagic zone, often following similar migrations of zooplankton, and returning to the depths for safety during the day. (2025). 9780131008472, Pearson/Prentice Hall. ISBN 9780131008472 (2025). 9780203885222, Taylor & Francis. ISBN 9780203885222 These vertical migrations often occur over large vertical distances, and are undertaken with the assistance of a swim bladder. The swim bladder is inflated when the fish wants to move up, and, given the high pressures in the mesopelagic zone, this requires significant energy. As the fish ascends, the pressure in the swimbladder must adjust to prevent it from bursting. When the fish wants to return to the depths, the swimbladder is deflated. Some mesopelagic fishes make daily migrations through the thermocline, where the temperature changes between , thus displaying considerable tolerance for temperature change.
Sampling via deep trawling indicates that lanternfish account for as much as 65% of all deep sea fish biomass. Indeed, lanternfish are among the most widely distributed, populous, and diverse of all , playing an important ecology role as prey for larger organisms. The estimated global biomass of lanternfish is 550–660 million , several times the annual world fisheries catch. Lanternfish also account for much of the biomass responsible for the deep scattering layer of the world's oceans. Sonar reflects off the millions of lanternfish swim bladders, giving the appearance of a false bottom.
The vanity price of a vanishing kind of maw is behind the imminent extinction of the vaquita, the world's smallest porpoise species. Found only in Mexico's Gulf of California, the once numerous vaquita are now critically endangered. Vaquita die in gillnets set to catch totoaba (the world's largest drum fish). Totoaba are being hunted to extinction for its maw, which can sell for as much $10,000 per kilogram.
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.Bridge, T. W. (1905) [2] "The Natural History of Isinglass" In earlier times, they were used to make .
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