Editing Swim bladder (section)

==Evolution==
[[File:PSM V20 D769 Lepidosiren annectens using the air bladder as a lung.jpg|thumb|left|The [[West African lungfish]] possesses a lung homologous to swim bladders]]

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 |quote = The illustration of the swim bladder in fishes ... shows us clearly the highly important fact that an organ originally constructed for one purpose, namely, flotation, may be converted into one for a widely different purpose, namely, respiration. The swim bladder has, also, been worked in as an accessory to the auditory organs of certain fishes. All physiologists admit that the swimbladder is homologous, or “ideally similar” in position and structure with the lungs of the [[higher vertebrate]] animals: hence there is no reason to doubt that the swim bladder has actually been converted into lungs, or an organ used exclusively for respiration.  According to this view it may be inferred that all vertebrate animals with true lungs are descended by ordinary generation from an ancient and unknown prototype, which was furnished with a floating apparatus or swim bladder.
 |source = [[Charles Darwin]], 1859<ref name=origin />
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Swim bladders are evolutionarily closely related (i.e., [[homology (biology)|homologous]]) to [[lung]]s. The first lungs originated in the last common ancestor of the [[Actinopterygii]] (ray-finned fish) and [[Sarcopterygii]] (lobe-finned fish and the [[tetrapod]]s) as expansions of the upper digestive tract which allowed them to gulp air under oxygen-poor conditions.<ref>{{cite book | url=https://books.google.com/books?id=YRcAVvmE6eMC&dq=Lungs+primitive+condition+bony+fishes+upper+digestive+tract+pharynx&pg=PA162 | title=Encyclopedia of Evolution | isbn=978-1-4381-1005-9 | last1=Rice | first1=Stanley A. | date=2009 | publisher=Infobase }}</ref> In the [[Actinopteri]] (ray-finned fish minus the [[bichir]]s) the lungs evolved into a swim bladder (secondary absent in some lineages), which unlike lungs that bud ventrally, buds dorsally from the anterior foregut.<ref>{{cite journal | url=https://pubmed.ncbi.nlm.nih.gov/33538055/ | pmid=33538055 | doi=10.1002/jmor.21330 | title=Does the bowfin gas bladder represent an intermediate stage during the lung-to-gas bladder evolutionary transition? | date=2021 | last1=Funk | first1=Emily C. | last2=Birol | first2=Eda B. | last3=McCune | first3=Amy R. | journal=Journal of Morphology | volume=282 | issue=4 | pages=600–611 }}</ref><ref>{{Cite journal |last1=Bi |first1=Xupeng |last2=Wang |first2=Kun |last3=Yang |first3=Liandong |last4=Pan |first4=Hailin |last5=Jiang |first5=Haifeng |last6=Wei |first6=Qiwei |last7=Fang |first7=Miaoquan |last8=Yu |first8=Hao |last9=Zhu |first9=Chenglong |last10=Cai |first10=Yiran |last11=He |first11=Yuming |last12=Gan |first12=Xiaoni |last13=Zeng |first13=Honghui |last14=Yu |first14=Daqi |last15=Zhu |first15=Youan |date=4 March 2021 |title=Tracing the genetic footprints of vertebrate landing in non-teleost ray-finned fishes |journal=Cell |language=en |volume=184 |issue=5 |pages=1377–1391.e14 |doi=10.1016/j.cell.2021.01.046|pmid=33545088 |doi-access=free }}</ref> [[Coelacanth]]s have a "fatty organ" that have sometimes been referred to as a swim bladder, but is structurally different and have a separate evolutionary history.<ref>{{cite journal | doi=10.1038/ncomms9222 | title=Allometric growth in the extant coelacanth lung during ontogenetic development | date=2015 | last1=Cupello | first1=Camila | last2=Brito | first2=Paulo M. | last3=Herbin | first3=Marc | last4=Meunier | first4=François J. | last5=Janvier | first5=Philippe | last6=Dutel | first6=Hugo | last7=Clément | first7=Gaël | journal=Nature Communications | volume=6 | page=8222 | pmid=26372119 | pmc=4647851 | bibcode=2015NatCo...6.8222C }}</ref>

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.<ref name="farmer">{{Cite journal|last1=Farmer|first1=Colleen|title=Did lungs and the intracardiac shunt evolve to oxygenate the heart in vertebrates|journal=Paleobiology|volume=23|issue=3|pages=358–372|year=1997|url=http://biologylabs.utah.edu/farmer/manuscripts/1997%20Paleobiology23.pdf|doi=10.1017/S0094837300019734|bibcode=1997Pbio...23..358F |s2cid=87285937 }}</ref> In [[embryo]]nal 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 [[wax ester]]s as a way to cope with [[Boyle's law]].<ref>{{cite book | url=https://books.google.com/books?id=e2N4AgAAQBAJ&dq=swimbladders+low-density+wax+esters+lift&pg=PA109 | title=Biology of Fishes | isbn=978-1-134-18631-0 | last1=Bone | first1=Quentin | last2=Moore | first2=Richard | date=19 March 2008 | publisher=Taylor & Francis }}</ref>

The [[Chondrichthyes|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.<ref name="farmer"/> Correspondingly, these fish also have both [[Protocercal|heterocercal]] and stiff, wing-like [[pectoral fin]]s  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.<ref>Kardong, KV (1998) ''Vertebrates: Comparative Anatomy, Function, Evolution''2nd edition, illustrated, revised. Published by WCB/McGraw-Hill, p. 12 {{ISBN|0-697-28654-1}}</ref>