Swim bladder
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Swim bladder
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.[1]
The swim bladder of a Rudd
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.
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[2] . 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[3] .
Swim bladder
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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.[3]
Evolution
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.[4] 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.[4] 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.[5]
Human uses
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.
Swim bladder display in a Melaka shopping mall
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.[6]
Swim bladder
See also
• Swim bladder syndrome
References
[1]
[2]
[3]
[4]
"Fish". Microsoft Encarta Encyclopedia Deluxe 1999. Microsoft. 1999.
http:/ / physiologyonline. physiology. org/ cgi/ content/ full/ 16/ 6/ 287
http:/ / www. biolbull. org/ cgi/ content/ abstract/ 161/ 3/ 440
Colleen Farmer (1997), "Did lungs and the intracardiac shunt evolve to oxygenate the heart in vertebrates" (http:/ / www. biology. utah. edu/
farmer/ publications pdf/ 1997 Paleobiology23. pdf), Paleobiology,
[5] Kardong, KV (1998) Vertebrates: Comparative Anatomy, Function, Evolution2nd edition, illustrated, revised. Published by
WCB/McGraw-Hill, p. 12 ISBN 0697286541
[6] Clark, F. E.; C. E. Lane (1961). "Composition of float gases of Physalia physalis". Fed. Proc. 107: 673–674.
Bibliography
• Carl E. Bond, Biology of Fishes, 2nd ed., (Saunders, 1996) pp. 283–290.
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Image:Swim bladder.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Swim_bladder.jpg License: Creative Commons Attribution-Sharealike 2.5 Contributors: User:Uwe Gille
File:Melaka-mall-Fish-maw-kiosk-2267.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Melaka-mall-Fish-maw-kiosk-2267.jpg License: Creative Commons Attribution-Sharealike
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