- Gas bladder
, and thus to stay at the current water depth, ascend, or descend without having to waste energy in swimming. cite encyclopedia | title =Fish | encyclopedia =Microsoft Encarta Encyclopedia Deluxe 1999 | publisher =Microsoft | date =1999]
The gas bladder is a
gas -filledsac located in the dorsal portion of the fish. It has flexible walls that contract or expand according to the ambientpressure . The walls of the bladder contain very fewblood vessels and are lined withguanine crystals, which make them impermeable to gases. In physostomous gas bladders, a connection is retained between the gas bladder and thegut , allowing the fish to fill up the gas bladder by "gulping" air and filling the gas bladder through thepneumatic duct . In more derived varieties of fish, the bladder has a gas gland that can introduce gases (usually oxygen) to the bladder to increase itsvolume and thus increasebuoyancy . To reduce buoyancy, gases are released from the bladder into the blood stream and then expelled into the water via the gills. In order to introduce gas into the bladder, the gas gland excreteslactic acid ; the resulting acidity causes thehemoglobin of the blood to lose its oxygen, which then diffuses into the bladder while flowing through a complex structure known as the "rete mirabile ". Elsewhere, at a similar structure known as the "oval window", the bladder is in contact with blood and the oxygen can diffuse back.Physoclist gas 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.Gas bladders are only found in
ray-finned fish , but a few of these fish that do not need to change water depth have lost them. Many cartilaginous fish, includingshark s, can control their depth only by swimming (using dynamic lift); others store fats or oils for the purpose.In some fish, mainly freshwater species, the gas bladder is connected to the labyrinth of the inner
ear by theWeberian apparatus , which provides a precise sense ofwater pressure (and thus depth), and may also improve hearing.Fact|date=September 2008The 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 gas bladder.Gas bladders are evolutionarily closely related (i.e. homologous) to
lung s. It is believed that the first lungs, simple sacs that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrialvertebrate s and into the gas bladders of today's fish. In embryonal development, both lung and gas bladder originate as an outpocketing from thegut ; in the case of gas bladders, this connection to the gut continues to exist as thepneumatic duct in more "primitive"teleost s, and is lost in the more derived orders. There are no animals which have both lungs and a gas bladder.Human uses
In some Asian cultures, fish maw of certain large sea fishes is considered a food delicacy (Chinese name: 花膠). It is usually served braised or in stews. Fish maws are also used in the food industry as a source of collagen. Fish maw can also be made into a strong, water-resistant glue.
Gas bladders in other animals
The
Portuguese Man o' War has a special gas bladder that allows its top to float along the surface while its tentacles trail below the water. This organ is unrelated to the one in fish.Footnotes
References
*
Carl E. Bond , "Biology of Fishes", 2nd ed., (Saunders, 1996) pp. 283-290.
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