Editing Tuna (section)

==Biology==
{{See also|Thunnus}}
[[File:Thunnus obesus (Bigeye tuna) diagram cropped.GIF|thumb|[[Bigeye tuna]] ''Thunnus obesus'' showing finlets and keels. Finlets are found between the last dorsal and/or anal fin and the caudal fin. They are rayless and non-retractable.<br/>Drawing by Dr Tony Ayling.]]

=== Description ===
The tuna is a sleek, elongated and streamlined fish, adapted for speed. It has two closely spaced but separated [[dorsal fins]] on its back; The first fin is "depressible"&nbsp;– it can be laid down, flush, in a groove that runs along its back; it is supported by spines.<ref name="FAO">{{cite book |author=<!--Staff writer(s); no by-line.--> |date=n.d. |title=Biological characteristics of tuna |url=https://www.fao.org/figis/pdf/fishery/topic/16082/en?title=FAO%20Fisheries%20%26%20Aquaculture%20-%20Biological%20characteristics%20of%20tuna |publisher=Fisheries and Aquaculture Department, [[Food and Agriculture Organization]] |access-date=17 December 2022 |archive-date=7 June 2023 |archive-url=https://web.archive.org/web/20230607013639/https://www.fao.org/figis/pdf/fishery/topic/16082/en?title=FAO%20Fisheries%20&%20Aquaculture%20-%20Biological%20characteristics%20of%20tuna |url-status=dead }}</ref> Seven to ten yellow [[Pterygiophore|finlet]]s run from the dorsal fins to the tail, which is lunate&nbsp;– curved like a crescent moon&nbsp;– and tapered to pointy tips.<ref name=RISeaGrant>{{cite web|last=Gibbs |first=E. |title=Fact Sheet: Tuna #P1412 |url=http://seagrant.gso.uri.edu/factsheets/tuna.html |publisher=Rhode Island Sea Grant |access-date=20 September 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120712070954/http://seagrant.gso.uri.edu/factsheets/tuna.html |archive-date=12 July 2012}}</ref> A tuna's pelvic fins are located below the base of the pectoral fins. Both dorsal and pelvic fins retract when the fish is swimming fast.<ref name="FAO"/>

The tuna's body is [[countershading|countershaded]] to [[camouflage]] itself in deeper water when seen from above, its dorsal side is generally a metallic dark blue while the ventral or under side is silvery, often with an [[Iridescence | iridescent]] shine.<ref>{{cite web |last1=Argo |first1=Emily |title=Countershading |date=21 April 2017 |url=https://fishionary.fisheries.org/countershading/ |website=Fishionary |publisher=[[American Fisheries Society]] |access-date=17 December 2022}}</ref><ref name=RISeaGrant/> The [[caudal peduncle]], to which the tail is attached, is quite thin, with three stabilizing horizontal [[Homocercal|keel]]s on each side.<ref name=RISeaGrant/>

===Physiology===
''Thunnus'' are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters at [[latitude]]s ranging between about [[45th parallel north|45° north]] and [[45th parallel south|south]] of the equator.<ref name="ISSF"/> All tunas are able to maintain the temperature of certain parts of their body above the temperature of ambient seawater. For example, bluefin can maintain a core body temperature of {{Convert|25|-|33|C|F}}, in water as cold as {{Convert|6|C|F}}. Unlike other endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.<ref name=muscletemp>{{cite journal|last1=Sepulveda |first1=C.A. |last2=Dickson |first2=K.A. |last3=Bernal |first3=D. |last4=Graham |first4=J.B. |title=Elevated red myotomal muscle temperatures in the most basal tuna species, ''Allothunnus fallai'' |journal=Journal of Fish Biology |date=1 July 2008 |volume=73 |issue=1 |pages=241–249 |doi=10.1111/j.1095-8649.2008.01931.x |bibcode=2008JFBio..73..241S }}</ref>

Tunas achieve [[endothermy]] by conserving the heat generated through normal [[metabolism]]. In all tunas, the heart operates at [[ambient temperature]], as it receives cooled blood, and coronary circulation is directly from the [[gills]].<ref name=SERCA2/> The ''[[rete mirabile]]'' ("wonderful net"), the intertwining of veins and arteries in the body's periphery, allows nearly all of the metabolic heat from [[venous blood]]<!-- NOT arterial --> to be "re-claimed" and transferred to the [[arterial blood]]<!-- NOT venous --> via a [[counter-current exchange]] system, thus mitigating the effects of surface cooling.<ref name=Cech1984>{{cite journal |last1=Cech |first1=J.J. |last2=Laurs |first2=R.M. |last3=Graham |first3=J.B. |year=1984 |title=Temperature-induced changes in blood gas equilibria in the albacore, ''Thunnus alalunga'', a warm-bodied tuna |journal=Journal of Experimental Biology |volume=109 |issue=1 |pages=21–34 |doi=10.1242/jeb.109.1.21 |quote=Oxygenated blood that has just reached thermal equilibrium with ambient sea water in the gills enters the rete on the arterial side, while warmed, deoxygenated, and carbon dioxide-laden blood enters on the venous end. In the rete, countercurrent flow and the high surface area contact between the two blood supplies facilitate the transfer of nearly all of the metabolic heat in the venous blood to arterial blood, thus conserving muscle temperature. After exiting the rete, arterial blood continues to the red muscle capillary beds, and cooled venous blood flows to the gills where carbon dioxide is excreted and oxygen is loaded.|doi-access=free |bibcode=1984JExpB.109...21C }}</ref> This allows the tuna to elevate the temperatures of the highly-[[aerobic respiration|aerobic]] tissues of the skeletal muscles, eyes and brain,<ref name=muscletemp/><ref name=SERCA2>{{cite journal|last1=Landeira-Fernandez |first1=A.M. |last2=Morrissette |first2=J.M. |last3=Blank |first3=J.M. |last4=Block |first4=B.A. |title=Temperature dependence of the Ca<sup>2+</sup>-ATPase (SERCA2) in the ventricles of tuna and mackerel|journal=American Journal of Physiology. Regulatory, Integrative and Comparative Physiology|date=16 October 2003|volume=286|issue=2|pages=R398–R404|doi=10.1152/ajpregu.00392.2003|pmid=14604842}}</ref> which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish.{{citation needed|date=August 2022}}

Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red [[Myotome|myotomal]] muscles derive their color from [[myoglobin]], an oxygen-binding molecule, which tuna express in quantities far higher than most other fish. The oxygen-rich blood further enables energy delivery to their muscles.<ref name=muscletemp/>

For powerful swimming animals like [[dolphin]]s and tuna, [[cavitation]] may be detrimental, because it limits their maximum swimming speed.<ref name=speedlimit>{{cite journal|last1=Iosilevskii|first1=G|last2=Weihs|first2=D|title=Speed limits on swimming of fishes and cetaceans|journal=Journal of the Royal Society Interface|date=6 March 2008|volume=5|issue=20|pages=329–338|doi=10.1098/rsif.2007.1073|pmid=17580289|quote=Lacking pain receptors on their caudal fins, scombrids may temporarily cross the cavitation limit, and cavitation-induced damage has been observed (Kishinouye 1923); on the other hand, delphinids probably cannot cross it without pain (Lang 1966)|pmc=2607394}}</ref> Even if they have the power to swim faster, dolphins may have to restrict their speed, because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.<ref name=speedlimit/>