Editing Swim bladder (section)

{{Short description|Gas-filled organ that contributes to the ability of a fish to control its buoyancy}}
{{Redirect|Air bladder|the special effects technique|Air bladder effect}}
[[File:Swim bladder.jpg|thumb|333px|{{center|The swim bladder of a [[Scardinius|rudd]]}}]]
[[File:PSM V20 D769 Longitudonal section of a bleak.jpg|thumb|333px|{{center|Internal positioning of the swim bladder of a [[Alburnoides bipunctatus|bleak]]<br />S: anterior, S': posterior portion of the air bladder<br />œ: œsophagus; l: air passage of the air bladder}}]]

The '''swim bladder''', '''gas bladder''', '''fish maw''', or '''air bladder''' is an internal gas-filled [[organ (anatomy)|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 (force)|lift]] via swimming, which expends more [[energy]].<ref name="Orr">{{cite encyclopedia | title =Fish | encyclopedia =Microsoft Encarta Encyclopedia Deluxe 1999  | publisher =Microsoft | year =1999}}</ref> Also, the [[Dorsum (biology)|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 (anatomy)|resonating chamber]] to produce or receive sound.

The swim bladder is [[evolution]]arily [[homology (biology)|homologous]] to the [[lung]]s of [[tetrapod]]s and [[lungfish]], and some [[ray-finned fish]] such as [[bowfin]]s have also evolved similar respiratory functions in their swim bladders. [[Charles Darwin]] remarked upon this in ''[[On the Origin of Species]]'',<ref name=origin>{{cite book|author=Darwin, Charles |year=1859|url=https://books.google.com/books?id=IQtjAAAAMAAJ&pg=PA190 |title=Origin of Species|page=190|edition= reprinted 1872|publisher=D. Appleton}}</ref> 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|bottom dwellers]] like the [[weather fish]] and [[Ophioblennius atlanticus|redlip blenny]],<ref>{{cite journal|last=Nursall|first=J. R.|title=Buoyancy is provided by lipids of larval redlip blennies, ''Ophioblennius atlanticus''|journal=Copeia|year=1989|volume=1989|issue=3|pages=614–621|doi=10.2307/1445488|jstor=1445488}}</ref> have secondarily lost the swim bladder during the embryonic stage. Other fish, like the [[opah]] and the [[pomfret]], use their [[pectoral fin]]s 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|find fish]].

[[Cartilaginous fish]] such as [[shark]]s and [[ray (fish)|ray]]s do not have swim bladders,<ref>{{cite web|url=http://www.ucmp.berkeley.edu/vertebrates/actinopterygii/actinomm.html|title=More on Morphology|website=www.ucmp.berkeley.edu}}</ref> 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 (fish)|dynamic lift]]; others store up [[lipid]]s with [[specific density]] less than that of [[seawater]] to produce a neutral or near-neutral buoyancy, which cannot be readily changed with depth.