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.
The swim bladder is evolutionarily homologous to the lungs, and Darwin himself remarked upon this in The Origin Of Species.
Species
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 fishes 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.
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 and filling the swim bladder. Excess gas can be removed in a similar manner.
In more derived varieties of fish, the physoclisti connection to the gastric duct 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. 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.
Human uses
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In some Asian cultures, the swim bladders of certain large sea fishes are considered a food delicacy. It is usually served braised or in stews. Swim bladders are also used in the food industry as a source of collagen. Swim Bladders can also be made into a strong, water-resistant glue. Swim bladders are also used to make
isinglass for the clarification of
beer.
The gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used in sonar equipment to find fish.
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
Swim bladder syndrome
References
Bibliography
Carl E. Bond, Biology of Fishes, 2nd ed., (Saunders, 1996) pp. 283–290.
Category:Organs
Category:Fish anatomy
Category:Chinese ingredients
