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{{Short description|Scientific study of how the component parts of fish function together in the living fish}}
{{use British English|date=August 2021}}
{{use dmy dates|date=May 2017}}
[[File:Tetraodon-hispidus.jpg|thumb|300px|right|When threatened, the toxic [[pufferfish]] fills its extremely elastic stomach with water.<ref>{{cite journal | last1 = Schwab | first1 = Ivan R | year = 2002 | title = More than just cool shades | url= | journal = British Journal of Ophthalmology | volume = 86 | issue = 10| page = 1075 | doi = 10.1136/bjo.86.10.1075| pmid = 12349839 | pmc = 1771297 }}</ref>]]

'''Fish physiology''' is the scientific study of how the component parts of [[fish]] function together in the living fish.<ref name=Prosser>{{cite book |last1 = Prosser |first1 = C. Ladd |title = Comparative Animal Physiology, Environmental and Metabolic Animal Physiology |edition= 4th |publisher = Wiley-Liss |location = Hoboken, NJ |year = 1991 |isbn = 978-0-471-85767-9 |pages=1–12}}</ref> It can be contrasted with [[fish anatomy]], which is the study of the form or [[Morphology (biology)|morphology]] of fishes. In practice, fish anatomy and physiology complement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, such as might be observed on the dissecting table or under the microscope, and the later dealing with how those components function together in the living fish. For this, at first we need to know about their intestinal morphology.

== Respiration ==

{{multiple image
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 | footer = Gills inside the head of a [[tuna]]. The head is snout-down, with the view looking towards the mouth. On the right are the detached gills.
 | footer_align = center
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 | image1 = Tuna Gills in Situ 01.jpg
 | image2 = Tuna Gills in Situ cut.jpg
}}

[[File:Gills (esox).jpg|thumb|right|Gill arches bearing gills in a [[northern pike|pike]]]]
{{see also|Fish gill|Respiratory system#Fish}}

Most fish exchange gases using [[gill]]s on either side of the [[pharynx]] (throat). Gills are tissues which consist of threadlike structures called [[Protein filament|filament]]s. These filaments have many functions and "are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange.<ref>Hoar WS and Randall DJ (1984) [https://books.google.com/books?id=yINDnV4mWi8C&dq=%22FISH+PHYSIOLOGY+V10A%22&pg=PA263 ''Fish Physiology: Gills: Part A – Anatomy, gas transfer and acid-base regulation''] Academic Press. {{ISBN|978-0-08-058531-4}}.</ref><ref>Hoar WS and Randall DJ (1984) [https://books.google.com/books?id=8fuEB7O3IqIC&dq=%22Fish+physiology%22&pg=PA380 ''Fish Physiology: Gills: Part B – Ion and water transfer''] Academic Press. {{ISBN|978-0-08-058532-1}}.</ref> Each filament contains a [[capillary]] network that provides a large [[surface area]] for exchanging [[oxygen]] and [[carbon dioxide]]. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing [[countercurrent exchange]]. The gills push the oxygen-poor water out through openings in the sides of the pharynx.

Fish from multiple groups can live out of the water for extended time periods. [[Amphibious fish]] such as the [[mudskipper]] can live and move about on land for up to several days, or live in stagnant or otherwise oxygen depleted water. Many such fish can breathe air via a variety of mechanisms. The skin of [[anguillidae|anguillid eel]]s may absorb oxygen directly. The [[buccal cavity]] of the [[electric eel]] may breathe air. Catfish of the families [[Loricariidae]], [[Callichthyidae]], and [[Scoloplacidae]] absorb air through their digestive tracts.<ref name="Armbruster1998">{{cite journal|url=http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf|title=Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes|journal=[[Copeia]]|volume=1998|year=1998|issue=3|pages=663–675 | doi = 10.2307/1447796|access-date=25 June 2009|jstor=1447796|last1=Armbruster|first1=Jonathan W.}}</ref> [[Lungfish]], with the exception of the Australian lungfish, and [[bichir]]s have paired lungs similar to those of [[tetrapod]]s and must surface to gulp fresh air through the mouth and pass spent air out through the gills. [[Gar]] and [[bowfin]] have a vascularized swim bladder that functions in the same way. [[Cypriniformes|Loaches]], [[Erythrinidae|trahiras]], and many [[catfish]] breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin (similar to frogs). A number of fish have evolved so-called '''accessory breathing organs''' that extract oxygen from the air. Labyrinth fish (such as [[gourami]]s and [[betta]]s) have a [[labyrinth (organ)|labyrinth organ]] above the gills that performs this function. A few other fish have structures resembling labyrinth organs in form and function, most notably [[Snakehead (fish)|snakeheads]], [[Luciocephalidae|pikeheads]], and the [[Clariidae]] catfish family.

Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. Fish dependent solely on dissolved oxygen, such as perch and [[cichlid]]s, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of [[aestivation]] (summertime hibernation) until water returns.

Air breathing fish can be divided into ''obligate'' air breathers and ''facultative'' air breathers. Obligate air breathers, such as the [[African lungfish]], are obligated to breathe air periodically or they suffocate. Facultative air breathers, such as the catfish ''[[Hypostomus plecostomus]]'', only breathe air if they need to and can otherwise rely on their gills for oxygen. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators.<ref name="Armbruster1998" />

[[File:Comparison of con- and counter-current flow exchange.jpg|250px|thumb|right| {{center|'''Cocurrent and countercurrent flow exchange systems'''}} Red represents a higher value (e.g. of temperature or the partial pressure of a gas) than blue so the property being transported in the channels flows from red to blue. In fish a countercurrent flow (lower diagram) of blood and water in the gills is used to extract oxygen from the environment.<ref name=campbell3 /><ref name="Hughes1972" /><ref name=storer />]]

All [[Basal (phylogenetics)|basal]] vertebrates breathe with [[gills]]. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the [[esophagus]] to the exterior. Each gill is supported by a cartilaginous or bony [[gill arch]].<ref>{{cite book|last=Scott|first=Thomas|title=Concise encyclopedia biology|year=1996|publisher=Walter de Gruyter|isbn=978-3-11-010661-9|page=[https://archive.org/details/conciseencyclope00scot/page/542 542]|url=https://archive.org/details/conciseencyclope00scot/page/542}}</ref> The gills of [[vertebrate]]s typically develop in the walls of the [[pharynx]], along a series of gill slits opening to the exterior. Most species employ a [[countercurrent exchange]] system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other, which increases the efficiency of oxygen-uptake from the water.<ref name=campbell3>{{cite book|last1=Campbell|first1=Neil A.|title= Biology|url=https://archive.org/details/biolog00camp|url-access=limited|edition= Second|publisher= Benjamin/Cummings Publishing Company, Inc|location= Redwood City, California|date= 1990|pages=[https://archive.org/details/biolog00camp/page/836 836]–838|isbn=0-8053-1800-3}}</ref><ref name="Hughes1972">{{Cite journal| author=Hughes GM| title=Morphometrics of fish gills| journal=Respiration Physiology| volume=14| issue=1–2| year=1972| pages=1–25| doi=10.1016/0034-5687(72)90014-x| pmid=5042155}}</ref><ref name=storer>{{cite book|last1=Storer|first1=Tracy I.|last2=Usinger|first2=R. L.|last3=Stebbins|first3=Robert C.|last4=Nybakken|first4=James W.|title=General Zoology|edition=sixth|publisher=McGraw-Hill|location=New York|date=1997|pages=[https://archive.org/details/generalzoolog00stor/page/668 668]–670|isbn=0-07-061780-5|url=https://archive.org/details/generalzoolog00stor|url-access=registration}}</ref> Fresh oxygenated water taken in through the mouth is uninterruptedly "pumped" through the gills in one direction, while the blood in the lamellae flows in the opposite direction, creating the countercurrent blood and water flow, on which the fish's survival depends.<ref name=storer />

The gills are composed of comb-like filaments, the [[gill lamellae]], which help increase their surface area for oxygen exchange.<ref>{{cite book |title=Manual Of Fish Health |last= Andrews |first= Chris |author2=Adrian Exell |author3=Neville Carrington |year= 2003 |publisher= Firefly Books}}</ref> When a fish breathes, it draws in a mouthful of water at regular intervals. Then it draws the sides of its throat together, forcing the water through the gill openings, so that it passes over the gills to the outside. The [[bony fish]] have three pairs of arches, [[cartilaginous fish]] have five to seven pairs, while the primitive [[jawless fish]] have seven. The vertebrate ancestor no doubt had more arches, as some of their [[Chordata|chordate]] relatives have more than 50 pairs of gills.<ref name=VB />

[[Higher vertebrates]] do not develop gills, the gill arches form during [[Prenatal development|fetal development]], and lay the basis of essential structures such as [[jaw]]s, the [[thyroid gland]], the [[larynx]], the ''columella'' (corresponding to the [[stapes]] in [[mammals]]) and in mammals the [[Ossicles|malleus and incus]].<ref name=VB /> Fish gill slits may be the evolutionary ancestors of the [[tonsil]]s, [[thymus gland]], and [[Eustachian tube]]s, as well as many other structures derived from the embryonic [[branchial pouch]]es.{{Citation needed|date=May 2011}}

Scientists have investigated what part of the body is responsible for maintaining the respiratory rhythm. They found that [[neuron]]s located in the [[brainstem]] of fish are responsible for the genesis of the respiratory rhythm.<ref>{{cite journal | url=https://doi.org/10.1007%2FBF00614523 | doi=10.1007/BF00614523 | title=Respiratory pattern generation in adult lampreys (Lampetra fluviatilis): Interneurons and burst resetting | year=1986 | last1=Russell | first1=David F. | journal=Journal of Comparative Physiology A | volume=158 | issue=1 | pages=91–102 | pmid=3723432 | s2cid=19436421 }}</ref> The position of these neurons is slightly different from the centers of respiratory genesis in mammals but they are located in the same brain compartment, which has caused debates about the [[homology (biology)|homology]] of respiratory centers between aquatic and terrestrial species. In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this involuntary rhythm are still not completely understood (see [[Involuntary control of respiration]]).

Another important feature of the respiratory rhythm is that it is modulated to adapt to the oxygen consumption of the body. As observed in mammals, fish "breathe" faster and heavier when they do [[exercise|physical exercise]]. The mechanisms by which these changes occur have been strongly debated over more than 100 years between scientists.<ref>{{cite journal | url=https://journals.physiology.org/doi/full/10.1152/japplphysiol.01528.2005 | doi=10.1152/japplphysiol.01528.2005 | title=Point:Counterpoint: Supraspinal locomotor centers do/Do not contribute significantly to the hyperpnea of dynamic exercise | year=2006 | last1=Waldrop | first1=Tony G. | last2=Iwamoto | first2=Gary A. | journal=Journal of Applied Physiology | volume=100 | issue=3 | pages=1077–1083 }}</ref> The authors can be classified in 2 schools:

#Those who think that the major part of the respiratory changes are pre-programmed in the brain, which would imply that neurons from [[animal locomotion|locomotion]] centers of the brain connect to [[respiratory center]]s in anticipation of movements.
#Those who think that the major part of the respiratory changes result from the detection of muscle contraction, and that respiration is adapted as a consequence of muscular contraction and oxygen consumption. This would imply that the brain possesses some kind of detection mechanisms that would trigger a respiratory response when muscular contraction occurs.

Many now agree that both mechanisms are probably present and complementary, or working alongside a mechanism that can detect changes in oxygen and/or carbon dioxide blood saturation.

=== Bony fish ===
[[File:breathing in fish.jpg|thumb|right|300 px| {{center|'''Respiratory mechanism in bony fish'''}} The fish draws oxygen-rich water in through the mouth (left). It then pumps it over gills so oxygen enters the bloodstream, and allows oxygen-depleted water to exit through the gill slits (right)]]

In [[Osteichthyes|bony fish]], the gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of evolution. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so that bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Valves inside the mouth keep the water from escaping.<ref name=VB/>

The gill arches of bony fish typically have no septum, so that the gills alone project from the arch, supported by individual gill rays. Some species retain gill rakers. Though all but the most primitive bony fish lack a spiracle, the pseudobranch associated with it often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure.<ref name=VB/>

[[File:Gills.jpg|thumb|left| {{center|Most bony fish have five gills}}]]

Marine [[teleost]]s also use gills to excrete [[electrolyte]]s. The gills' large surface area tends to create a problem for fish that seek to regulate the [[osmolarity]] of their internal fluids. Saltwater is less dilute than these internal fluids, so saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of [[seawater]] and excrete the [[salt]]. Freshwater is more dilute than the internal fluids of fish, however, so freshwater fish gain water [[osmosis|osmotically]] through their gills.<ref name=VB/>

In some primitive bony fishes and [[amphibians]], the [[larva]]e bear external gills, branching off from the gill arches.<ref>{{cite journal|journal=The American Naturalist|year=1957|volume=91|issue=860|page=287|publisher=Essex Institute|jstor = 2458911|title=The Origin of the Larva and Metamorphosis in Amphibia | doi = 10.1086/281990|last1=Szarski|first1=Henryk|s2cid=85231736}}</ref> These are reduced in adulthood, their function taken over by the gills proper in fishes and by [[lung]]s in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of [[tetrapod]]s.<ref name=Gaining_ground>Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. ''Indiana University Press'', Bloomington, Indiana. 369 pp</ref>

=== Cartilaginous fish ===
Like other fish, sharks extract [[oxygen]] from seawater as it passes over their [[gill]]s. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a [[Spiracle (vertebrates)|spiracle]] lies just behind the [[eye]], which assists the shark with taking in water during [[Aquatic respiration|respiration]] and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active [[pelagic]] sharks.<ref name="Gilbertson">{{cite book
| last = Gilbertson| first = Lance
|title = Zoology Laboratory Manual
| publisher = McGraw-Hill Companies, Inc.
| year = 1999
| location = New York
|isbn= 978-0-07-237716-3}}</ref> While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are ''obligate ram ventilators'' and would presumably [[asphyxiate]] if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.<ref>{{cite web
| url = http://www.textbookleague.org/73shark.htm
| title = Deep Breathing
| author = William J. Bennetta
| year = 1996
| access-date = 2007-08-28
| archive-url = https://web.archive.org/web/20070814075030/http://www.textbookleague.org/73shark.htm
| archive-date = 14 August 2007
| url-status = dead
}}</ref>

The respiration and [[Circulatory system|circulation]] process begins when deoxygenated [[blood]] travels to the shark's two-chambered [[heart]]. Here the shark pumps blood to its gills via the ventral [[aorta]] [[artery]] where it branches into [[wikt:afferent#Adjective|afferent]] [[Arm|brachial]] arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the [[wikt:efferent#Adjective|efferent]] brachial arteries, which come together to form the [[dorsal aorta]]. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the [[posterior cardinal vein]]s and enters the posterior cardinal [[Sinus (anatomy)|sinuses]]. From there blood enters the heart [[Ventricle (heart)|ventricle]] and the cycle repeats.<ref>{{cite web|url=http://www.seaworld.org/animal-info/info-books/sharks-&-rays/anatomy.htm|title=SHARKS & RAYS, SeaWorld/Busch Gardens ANIMALS, CIRCULATORY SYSTEM|publisher=Busch Entertainment Corporation|access-date=2009-09-03|archive-url=https://web.archive.org/web/20090424033204/http://www.seaworld.org/animal-info/info-books/sharks-%26-rays/anatomy.htm|archive-date=24 April 2009|url-status=dead}}</ref>
----
[[Shark]]s and [[ray (fish)|rays]] typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six or seven pairs. Adjacent slits are separated by a [[cartilage|cartilaginous]] gill arch from which projects a long sheet-like [[septum]], partly supported by a further piece of cartilage called the ''gill ray''. The individual lamellae of the gills lie on either side of the septum. The base of the arch may also support [[gill raker]]s, small projecting elements that help to filter food from the water.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977|title=The Vertebrate Body |publisher=Holt-Saunders International|location= Philadelphia, PA|pages= 316–327|isbn= 978-0-03-910284-5|author-link=Alfred Romer}}</ref>

A smaller opening, the [[Spiracle (vertebrates)|spiracle]], lies in the back of the first [[gill slit]]. This bears a small ''[[pseudobranch]]'' that resembles a gill in structure, but only receives blood already oxygenated by the true gills.<ref name=VB/> The spiracle is thought to be [[Homology (biology)|homologous]] to the ear opening in higher vertebrates.<ref>Laurin M. (1998): The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I-systematics, middle ear evolution, and jaw suspension. ''Annales des Sciences Naturelles, Zoologie, Paris'', 13e Série 19: pp 1-42.</ref>

Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.<ref name=VB/>

[[Chimaera]]s differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by an [[operculum (fish)|operculum]], developed from the septum of the gill arch in front of the first gill.<ref name=VB/>

=== Lampreys and hagfish ===
[[Lamprey]]s and [[hagfish]] do not have gill slits as such. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally. In adult lampreys, a separate respiratory tube develops beneath the pharynx proper, separating food and water from respiration by closing a valve at its anterior end.<ref name=VB/>

== Circulation ==
[[File:Two chamber heart.svg|thumb|Two-chambered heart of a fish]]

The circulatory systems of all [[vertebrate]]s are ''closed'', just as in humans. Still, the systems of [[fish]], [[amphibian]]s, [[reptile]]s, and [[bird]]s show various stages of the [[evolution]] of the [[circulatory system]]. In fish, the system has only one circuit, with the blood being pumped through the capillaries of the [[gill]]s and on to the capillaries of the body tissues. This is known as ''single cycle'' circulation. The heart of fish is therefore only a single pump (consisting of two chambers). Fish have a [[circulatory system|closed-loop circulatory system]]. The [[heart]] pumps the blood in a single loop throughout the body. In most fish, the heart consists of four parts, including two chambers and an entrance and exit.<ref name="Setaro">{{Cite book | last =Setaro | first =John F. | year =1999 | title =Circulatory System | publisher =Microsoft Encarta 99}}</ref> The first part is the [[sinus venosus]], a thin-walled sac that collects blood from the fish's [[vein]]s before allowing it to flow to the second part, the [[Atrium (heart)|atrium]], which is a large muscular chamber. The atrium serves as a one-way antechamber, sends blood to the third part, [[Ventricle (heart)|ventricle]]. The ventricle is another thick-walled, muscular chamber and it pumps the blood, first to the fourth part, [[bulbus arteriosus]], a large tube, and then out of the heart. The bulbus arteriosus connects to the [[aorta]], through which blood flows to the [[gills]] for oxygenation.

In amphibians and most reptiles, a [[double circulatory system]] is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.

== Digestion ==
[[Fish jaw|Jaws]] allow fish to eat a wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the [[esophagus]]. In the stomach, food is further digested and, in many fish, processed in finger-shaped pouches called [[pyloric caeca]], which secrete digestive [[enzymes]] and absorb nutrients. Organs such as the [[liver]] and [[pancreas]] add enzymes and various chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption.

In most vertebrates, digestion is a four-stage process involving the main structures of the [[digestive tract]], starting with ingestion, placing food into the mouth, and concluding with the excretion of undigested material through the anus. From the mouth, the food moves to the stomach, where as [[Bolus (digestion)|bolus]] it is broken down chemically. It then moves to the intestine, where the process of breaking the food down into simple molecules continues and the results are absorbed as nutrients into the circulatory and lymphatic system.

Although the precise shape and size of the stomach varies widely among different vertebrates, the relative positions of the oesophageal and duodenal openings remain relatively constant. As a result, the organ always curves somewhat to the left before curving back to meet the pyloric sphincter. However, [[lamprey]]s, [[hagfish]]es, [[chimaera]]s, [[lungfish]]es, and some [[teleost]] fish have no stomach at all, with the oesophagus opening directly into the intestine. These animals all consume diets that either require little storage of food, or no pre-digestion with gastric juices, or both.<ref name=VB/>

The [[small intestine]] is the part of the digestive tract following the stomach and followed by the [[large intestine]], and is where much of the [[digestion]] and absorption of food takes place. In fish, the divisions of the small intestine are not clear, and the terms ''anterior'' or ''proximal'' intestine may be used instead of [[duodenum]].<ref name=fish_feeding_book>
{{cite book
 | last=Guillaume
 | first=Jean
 |author2=Praxis Publishing |author3=Sadasivam Kaushik |author4=Pierre Bergot |author5=Robert Metailler
 | title=Nutrition and Feeding of Fish and Crustaceans
 | url=https://books.google.com/books?id=As0flTZo_EAC&q=fish+cytology+jejunum+duodenum&pg=PA31
 | page=31
 | year=2001
 | access-date=2009-01-09
 | publisher=Springer
 | isbn=978-1-85233-241-9
}}</ref> The small intestine is found in all [[teleost]]s, although its form and length vary enormously between species. In teleosts, it is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of ''pyloric caeca'', small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the [[rectum]] being marked only by the end of the digestive epithelium.<ref name=VB/>

There is no small intestine as such in non-teleost fish, such as [[shark]]s, [[sturgeon]]s, and [[lungfish]]. Instead, the digestive part of the gut forms a '''spiral intestine''', connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This valve greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.<ref name=VB/> In [[lamprey]]s, the spiral valve is extremely small, possibly because their diet requires little digestion. [[Hagfish]] have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions.<ref name=VB/>

The [[large intestine]] is the last part of the [[digestive system]] normally found in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless [[feces|waste material]] from the body.<ref name='NCILargeIntestineDef'>{{cite web | url = http://www.cancer.gov/dictionary?cdrid=45097 | title = NCI Dictionary of Cancer Terms — large intestine | access-date = 2012-09-16 | author = National Cancer Institute| author-link = National Cancer Institute | date = 2 February 2011 }}</ref> In fish, there is no true large intestine, but simply a short rectum connecting the end of the digestive part of the gut to the cloaca. In [[shark]]s, this includes a ''rectal gland'' that secretes salt to help the animal maintain [[osmosis|osmotic]] balance with the seawater. The gland somewhat resembles a caecum in structure, but is not a homologous structure.<ref name=VB/>

As with many aquatic animals, most fish release their nitrogenous wastes as [[ammonia]]. Some of the wastes [[diffusion|diffuse]] through the gills. Blood wastes are [[filter (chemistry)|filtered]] by the [[kidney]]s.

Saltwater fish tend to lose water because of [[osmosis]]. Their kidneys return water to the body. The reverse happens in [[freshwater fish]]: they tend to gain water osmotically. Their kidneys produce dilute urine for excretion. Some fish have specially adapted kidneys that vary in function, allowing them to move from freshwater to saltwater.

In sharks, digestion can take a long time. The food moves from the mouth to a J-shaped stomach, where it is stored and initial digestion occurs.<ref name="Digestion">{{cite web
|url=http://elasmo-research.org/education/white_shark/digestion.htm
|title=No Guts, No Glory
|last=Martin|first=R. Aidan
|publisher=ReefQuest Centre for Shark Research
|access-date=2009-08-22}}</ref> Unwanted items may never get past the stomach, and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth. One of the biggest differences between the digestive systems of sharks and mammals is that sharks have much shorter intestines. This short length is achieved by the [[spiral valve]] with multiple turns within a single short section instead of a long tube-like intestine. The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the [[cloaca]].<ref name="Digestion"/>

==Endocrine system==

=== Regulation of social behaviour ===
[[Oxytocin]] is a group of [[neuropeptide]]s found in most vertebrates. One form of oxytocin functions as a [[hormone]] which is associated with 
human love. In 2012, researchers injected [[cichlid]]s from the social species ''[[Neolamprologus pulcher]]'', either with this form of isotocin or with a control saline solution. They found isotocin increased "responsiveness to social information", which suggests "it is a key regulator of social behavior that has evolved and endured since ancient times".<ref>{{cite journal | last1 = Reddon | first1 = AR | last2 = O'Connor | first2 = CM | last3 = Marsh-Rollo | first3 = SE | last4 = Balcshine | first4 = S | year = 2012 | title = Effects of isotocin on social responses in a cooperatively breeding fish | journal = Animal Behaviour | volume = 84 | issue = 4| pages = 753–760 | doi = 10.1016/j.anbehav.2012.07.021| s2cid = 13037173 }}</ref><ref>[http://www.science20.com/news_articles/fish_version_oxytocin_drives_their_social_behavior_says_study-95041 Fish Version Of Oxytocin Drives Their Social Behavior] ''Science 2.0'', 10 October 2012.</ref>

===Effects of pollution===
Fish can [[bioaccumulate]] pollutants that are discharged into waterways. [[Estrogenic]] compounds found in pesticides, birth control, plastics, plants, fungi, bacteria, and synthetic drugs leeched into rivers are affecting the [[endocrine system]]s of native species.<ref>{{cite journal|last1=Pinto|first1=Patricia|last2=Estevao|first2=Maria|last3=Power|first3=Deborah|title=Effects of Estrogens and Estrogenic Disrupting Compounds on Fish Mineralized Tissues|journal=Marine Drugs|date=August 2014|volume=12|issue=8|pages=4474–94|doi=10.3390/md12084474|pmid=25196834|pmc=4145326|doi-access=free}}</ref> In Boulder, Colorado, [[white sucker]] fish found downstream of a municipal waste water treatment plant exhibit impaired or abnormal sexual development. The fish have been exposed to higher levels of estrogen, and leading to feminized fish.<ref>{{cite web|last1=Waterman|first1=Jim|title=Research into wastewater treatment effluent impact on Boulder Creek fish sexual development|url=http://bcn.boulder.co.us/basin/topical/haa.html|website=Boulder Area Sustainability Information Network}}</ref> Males display female reproductive organs, and both sexes have reduced fertility, and a higher hatch mortality.<ref>{{cite journal|last1=Arukwe|first1=Augustine|title=Cellular and Molecular Responses to Endocrine-Modulators and The Impact on Fish Reproduction|journal=Marine Pollution Bulletin|date=August 2001|volume=42|issue=8|pages=643–655 |doi=10.1016/S0025-326X(01)00062-5|pmid=11525282}}</ref>

Freshwater habitats in the United States are widely contaminated by the common pesticide [[atrazine]].<ref name=Reeves2015>{{cite journal | last1 = Reeves | first1 = C | year = 2015 | title = Of Frogs & Rhetoric: The Atrazine wars | url = https://digitalcommons.butler.edu/facsch_papers/954| journal = Technical Communication Quarterly | volume = 24 | issue = 4| pages = 328–348 | doi = 10.1080/10572252.2015.1079333 | s2cid = 53626955 }}</ref> There is controversy over the degree to which this pesticide harms the endocrine systems of freshwater fish and [[Atrazine#Amphibians|amphibians]]. Non-industry-funded researchers consistently report harmful effects while industry-funded researchers consistently report no harmful effects.<ref name=Reeves2015 /><ref>{{cite journal |title=The problem of biased data and potential solutions for health and environmental assessments |journal=Human and Ecological Risk Assessment |year=2015 |last1=Suter |first1=Glenn |last2=Cormier |first2=Susan |volume=21 |issue=7 |doi=10.1080/10807039.2014.974499 |pages=1–17|s2cid=84723794 }}</ref><ref>Rohr, J.R. (2018) "Atrazine and Amphibians: A Story of Profits, Controversy, and Animus". In: D. A. DellaSala, and M. I. Goldstein (eds.) ''Encyclopedia of the Anthropocene'', volume 5, pages 141–148. Oxford: Elsevier.</ref>

In the marine ecosystem, [[organochlorine]] contaminants like pesticides, herbicides (DDT), and [[chlordan]] are accumulating within fish tissue and disrupting their endocrine system.<ref>{{cite web |url=https://portals.iucn.org/library/efiles/documents/2005-029.pdf |last1=Fowler|first1=Sarah|last2=Cavanagh|first2=Rachel|title=Sharks, Rays, and Chimaeras: The Status of Chondrichthyan Fishes|publisher=The World Conservation Union |access-date=3 May 2017}}</ref> High frequencies of infertility and high levels of organochlorines have been found in [[bonnethead shark]]s along the Gulf Coast of Florida. These [[Endocrine disruptor|endocrine-disrupting]] compounds are similar in structure to naturally occurring hormones in fish. They can modulate hormonal interactions in fish by:<ref>{{cite journal|last1=Arukwe|first1=Augustine|title=Cellular and Molecular Responses to Endocrine-Modulators and the Impact on Fish Reproduction|journal=Marine Pollution Bulletin|date=2001|volume=42|issue=8|pages=643–655|doi=10.1016/s0025-326x(01)00062-5|pmid=11525282}}</ref>
* binding to cellular receptors, causing unpredictable and abnormal cell activity
* blocking receptor sites, inhibiting activity
* promoting the creation of extra receptor sites, amplifying the effects of the hormone or compound
* interacting with naturally occurring hormones, changing their shape and impact
* affecting hormone synthesis or metabolism, causing an improper balance or quantity of hormones

Two major types of [[osmoregulation]] are osmoconformers and osmoregulators. [[Osmoconformer]]s match their body osmolarity to their environment actively or passively. Most marine invertebrates are osmoconformers, although their ionic composition may be different from that of seawater.

[[Osmoregulator]]s tightly regulate their body [[osmolarity]], which always stays constant, and are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example is freshwater fish. The gills [[active transport|actively uptake]] salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a very [[Tonicity#Hypotonicity|hypotonic]] (dilute) urine to expel all the excess water. A marine [[fish]] has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt. It actively excretes [[salt]] out from the [[gill]]s. Most fish are [[stenohaline]], which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known as [[euryhaline]] species, e.g., [[salmon]]. Salmon has been observed to inhabit two utterly disparate environments &mdash; marine and fresh water &mdash; and it is inherent to adapt to both by bringing in behavioral and physiological modifications.

In contrast to bony fish, with the exception of the [[coelacanth]],<ref>[https://www.jstor.org/pss/35431 Chemistry of the body fluids of the coelacanth, Latimeria chalumnae]</ref> the blood and other tissue of sharks and [[Chondrichthyes]] is generally [[Isotonicity|isotonic]] to their marine environments because of the high concentration of [[urea]] and [[trimethylamine]] N-oxide (TMAO), allowing them to be in [[osmotic]] balance with the seawater. This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to [[Marine (ocean)|marine]] environments. A few exceptions exist, such as the [[bull shark]], which has developed a way to change its [[kidney]] function to excrete large amounts of urea.<ref name="Collins">{{cite book
| last=Compagno|first=Leonard |author2=Dando, Marc |author3=Fowler, Sarah
| title=Sharks of the World
| publisher=Collins Field Guides
| year=2005
| isbn=978-0-00-713610-0
| oclc=183136093}}</ref> When a shark dies, the urea is broken down to ammonia by bacteria, causing the dead body to gradually smell strongly of ammonia.<ref>{{cite web
| url=http://www.fao.org/docrep/009/a0212e/A0212E18.htm
| title=Management techniques for elasmobranch fisheries: 14. Shark Utilization
| author=John A. Musick
| publisher=FAO: Fisheries and Aquaculture Department
| access-date=2008-03-16
| year=2005}}</ref><ref>{{cite web
| url=http://www.ocean.udel.edu/mas/seafood/mako.html
| title=MAKO SHARK Isurus oxyrinchus
| author=Thomas Batten
| publisher=Delaware Sea Grant, University of Delaware
| access-date=2008-03-16
| archive-url=https://web.archive.org/web/20080311091749/http://www.ocean.udel.edu/mas/seafood/mako.html
| archive-date=11 March 2008
| url-status=dead
}}</ref>

Sharks have adopted a different, efficient mechanism to conserve water, i.e., osmoregulation. They retain urea in their blood in relatively higher concentration. Urea is damaging to living tissue so, to cope with this problem, some fish retain ''[[trimethylamine oxide]]''. This provides a better solution to urea's toxicity. Sharks, having slightly higher solute concentration (i.e., above 1000 mOsm which is sea solute concentration), do not drink water like fresh water fish.

==Thermoregulation==
[[Homeothermy]] and [[poikilothermy]] refer to how stable an organism's temperature is. Most [[endothermic]] organisms are homeothermic, like [[mammal]]s. However, animals with facultative [[endothermy]] are often poikilothermic, meaning their temperature can vary considerably. Similarly, most fish are [[ectotherm]]s, as all of their heat comes from the surrounding water. However, most are homeotherms because their temperature is very stable.

Most organisms have a preferred temperature range, however some can be acclimated to temperatures colder or warmer than what they are typically used to. An organism's preferred temperature is typically the temperature at which the organism's physiological processes can act at optimal rates. When fish become acclimated to other temperatures, the efficiency of their physiological processes may decrease but will continue to function. This is called the [[thermal neutral zone]] at which an organism can survive indefinitely.<ref>{{Cite book|title=Environmental Physiology of Animals|last1=Wilmer|first1=Pat|last2=Stone|first2=Graham|last3=Johnston|first3=Ian|publisher=Wiley|year=2009|isbn=978-1-4051-0724-2|pages=[https://archive.org/details/environmentalphy00will/page/198 198–200]|url=https://archive.org/details/environmentalphy00will/page/198}}</ref>

H.M. Vernon has done work on the death temperature and paralysis temperature (temperature of heat rigor) of various animals. He found that species of the same [[Class (biology)|class]] showed very similar temperature values, those from the [[Amphibia]] examined being 38.5&nbsp;°C, fish 39&nbsp;°C, [[Reptilia]] 45&nbsp;°C, and various [[Mollusc]]s 46&nbsp;°C.

To cope with low temperatures, some [[fish]] have developed the ability to remain functional even when the water temperature is below freezing; some use natural [[antifreeze]] or [[antifreeze proteins]] to resist ice crystal formation in their tissues.

Most sharks are "cold-blooded" or, more precisely, [[poikilotherm]]ic, meaning that their internal [[Thermoregulation|body temperature]] matches that of their ambient environment. Members of the family [[Lamnidae]] (such as the [[shortfin mako shark]] and the [[great white shark]]) are [[homeothermy|homeothermic]] and maintain a higher body temperature than the surrounding water. In these sharks, a strip of [[aerobic metabolism|aerobic]] red muscle located near the center of the body generates the heat, which the body retains via a [[countercurrent exchange]] mechanism by a system of [[blood vessel]]s called the [[rete mirabile]] ("miraculous net"). The [[common thresher|common thresher shark]] has a similar mechanism for maintaining an elevated body temperature, which is thought to have evolved independently{{Failed verification|date=November 2010}}.<ref>{{cite web
|url=http://elasmo-research.org/education/topics/p_warm_body_1.htm
|title=Fire in the Belly of the Beast
|last=Martin
|first=R. Aidan|date=April 1992
|publisher=ReefQuest Centre for Shark Research|access-date=2009-08-21}}</ref>

[[Tuna]] can maintain the temperature of certain parts of their body above the temperature of ambient seawater. For example, [[bluefin tuna]] maintain a core body temperature of {{Convert|25|-|33|C|F}}, in water as cold as {{Convert|6|C|F}}. However, unlike typical 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 |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 |url=http://216.172.180.32/~pier/userdocs/images/files/scientific_publications/Sepulveda%20et%20al.%202008%20.pdf |access-date=2 November 2012 |first4=J. B. |url-status=dead |archive-url=https://web.archive.org/web/20130207000108/http://216.172.180.32/~pier/userdocs/images/files/scientific_publications/Sepulveda%20et%20al.%202008%20.pdf |archive-date=7 February 2013}}</ref> Tuna achieve [[endothermy]] by conserving the heat generated through normal [[metabolism]]. The [[rete mirabile]] ("wonderful net"), the intertwining of veins and arteries in the body's periphery, transfers heat from [[venous blood]] to [[arterial blood]] via a [[counter-current exchange]] system, thus mitigating the effects of surface cooling. 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 |first=A.M. |last2=Morrissette |first2=J.M. |last3=Blank |first3=J.M. |last4=Block |first4=B.A. |title=Temperature dependence of the Ca2+-ATPase (SERCA2) in the ventricles of tuna and mackerel|journal=AJP: Regulatory, Integrative and Comparative Physiology|date=16 October 2003|volume=286|issue=2|pages=398R–404|doi=10.1152/ajpregu.00392.2003|pmid=14604842 |citeseerx=10.1.1.384.817 }}</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.{{cn|date=August 2022}} In all tunas, however, the heart operates at [[ambient temperature]], as it receives cooled blood, and coronary circulation is directly from the [[gills]].<ref name=SERCA2/>

* [[Homeothermy]]: Although most fish are exclusively [[ectothermic]], there are exceptions. Certain species of fish maintain elevated body temperatures. [[Warm-blooded|Endothermic]] [[teleost]]s (bony fish) are all in the suborder [[Scombroidei]] and include the [[billfish]]es, tunas, including a "primitive" [[mackerel]] species, ''[[Gasterochisma melampus]]''. All sharks in the family [[Lamnidae]] – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are endothermic, and evidence suggests the trait exists in family [[Alopiidae]] ([[thresher sharks]]). The degree of endothermy varies from the [[billfish]], which warm only their eyes and brain, to [[bluefin tuna]] and [[porbeagle shark]]s who maintain body temperatures elevated in excess of {{cvt|20|C-change}} above ambient water temperatures.<ref>{{cite journal | last1 = Block | first1 = BA | last2 = Finnerty | first2 = JR | year = 1993 | title = Endothermy in fishes: a phylogenetic analysis of constraints, predispositions, and selection pressures | url = http://www.science.siu.edu/zoology/sears/BlockFinnerty1994.pdf | journal = Environmental Biology of Fishes | volume = 40 | issue = 3 | pages = 283–302 | doi = 10.1007/BF00002518 | s2cid = 28644501 }}{{dead link|date=October 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> ''See also [[gigantothermy]]''. Endothermy, though metabolically costly, is thought to provide advantages such as increased muscle strength, higher rates of central [[nervous system]] processing, and higher rates of [[digestion]].

In some fish, a [[rete mirabile]] allows for an increase in muscle temperature in regions where this network of vein and arteries is found. The fish is able to [[thermoregulate]] certain areas of their body. Additionally, this increase in temperature leads to an increase in basal metabolic temperature. The fish is now able to split [[Adenosine triphosphate|ATP]] at a higher rate and ultimately can swim faster.

The eye of a [[swordfish]] can generate [[heat]] to better cope with detecting their [[prey]] at depths of {{convert|2000|ft|m|-2|order=flip|abbr=off}}.<ref>David Fleshler(10-15-2012) ''[https://archive.today/20130203120659/http://www.sun-sentinel.com/news/local/breakingnews/fl-giant-eyeball-mystery-20121015,0,2019024.story South Florida Sun-Sentinel]'',
*[https://www.newscientist.com/article/dn6861-swordfish-heat-their-eyes-for-the-hunt.html Swordfish heat their eyes]</ref>

== Muscular system ==
[[File:Cross section of standard fish.svg|thumb|right|Cross section of a teleost]]
[[File:Pangasius meat.jpg|thumb|right|{{center|[[Iridescent shark]] filets showing myomere structure}}]]
{{Main|Fish locomotion}}
{{see also|Undulatory locomotion}}

[[Undulatory locomotion#Muscle architecture|Fish swim]] by contracting longitudinal red muscle and obliquely oriented white muscles. The red muscle is aerobic and needs oxygen which is supplied by [[myoglobin]]. The white muscle is anaerobic and it does not need oxygen. Red muscles are used for sustained activity such as cruising at slow speeds on ocean migrations. White muscles are used for bursts of activity, such as jumping or sudden bursts of speed for catching prey.<ref>Kapoor 2004</ref>

Mostly fish have white muscles, but the muscles of some fishes, such as [[scombridae|scombroids]] and [[salmonid]]s, range from pink to dark red. The red [[myotome|myotomal]] muscles derive their colour 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/>

Most fish move by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the [[Caudal fin|back fin]], backward force is applied to the water, and in conjunction with the fins, moves the fish forward. The [[Fish fin|fish's fins]] function like an airplane's flaps. Fins also increase the tail's surface area, increasing speed. The streamlined body of the fish decreases the amount of friction from the water.

A typical characteristic of many animals that utilize undulatory locomotion is that they have segmented muscles, or blocks of [[myomere]]s, running from their head to tails which are separated by connective tissue called myosepta. In addition, some segmented muscle groups, such the lateral hypaxial musculature in the salamander are oriented at an angle to the longitudinal direction. For these obliquely oriented fibers the strain in the longitudinal direction is greater than the strain in the muscle fiber direction leading to an architectural gear ratio greater than 1. A higher initial angle of orientation and more dorsoventral bulging produces a faster muscle contraction but results in a lower amount of force production.<ref name="Azizi1">{{cite journal |last1=Brainerd |first1=E. L. |last2=Azizi |first2=E. |year=2005 |title=Muscle Fiber Angle, Segment Bulging and Architectural Gear Ratio in Segmented Musculature |journal=Journal of Experimental Biology |volume=208 |issue=17 |pages=3249–3261 |doi=10.1242/jeb.01770 |pmid=16109887|doi-access=free }}</ref> It is hypothesized that animals employ a variable gearing mechanism that allows self-regulation of force and velocity to meet the mechanical demands of the contraction.<ref name="gearing">{{cite journal |last1=Azizi |first1=E. |last2=Brainerd |first2=E. L. |last3=Roberts |first3=T. J. |year=2008 |title=Variable Gearing in Pennate Muscles |journal=PNAS |volume=105 |issue=5 |pages=1745–1750 |doi=10.1073/pnas.0709212105 |pmid=18230734 |pmc=2234215|bibcode=2008PNAS..105.1745A |doi-access=free }}</ref> When a [[pennate muscle]] is subjected to a low force, resistance to width changes in the muscle cause it to rotate which consequently produce a higher architectural gear ratio (AGR) (high velocity).<ref name="gearing"/> However, when subject to a high force, the perpendicular fiber force component overcomes the resistance to width changes and the muscle compresses producing a lower AGR (capable of maintaining a higher force output).<ref name="gearing"/>

Most fishes bend as a simple, homogenous beam during swimming via contractions of longitudinal red muscle fibers and obliquely oriented white muscle fibers within the segmented axial musculature. The fiber [[Deformation (mechanics)#Engineering strain|strain]] (εf) experienced by the longitudinal red muscle fibers is equivalent to the longitudinal strain (εx). The deeper white muscle fibers fishes show diversity in arrangement. These fibers are organized into cone-shaped structures and attach to connective tissue sheets known as myosepta; each fiber shows a characteristic dorsoventral (α) and mediolateral (φ) trajectory. The segmented architecture theory predicts that, εx > εf. This phenomenon results in an architectural gear ratio, determined as longitudinal strain divided by fiber strain (εx / εf), greater than one and longitudinal velocity amplification; furthermore, this emergent velocity amplification may be augmented by variable architectural gearing via mesolateral and dorsoventral shape changes, a pattern seen in [[pennate muscle]] contractions. A red-to-white gearing ratio (red εf / white εf) captures the combined effect of the longitudinal red muscle fiber and oblique white muscle fiber strains.<ref name="Azizi1"/><ref name="Azizi2">{{cite journal |last1=Brainerd |first1=E. L. |last2=Azizi |first2=E. |year=2007 |title=Architectural Gear Ratio and Muscle Fiber Strain Homogeneity in Segmented Musculature |journal=Journal of Experimental Zoology |volume=307 |issue=A |pages=145–155 |doi=10.1002/jez.a.358|pmid=17397068 }}</ref>

==Buoyancy==
[[File:Swim bladder.jpg|thumb|right|{{center|Swim bladder of a [[common rudd]]}}|alt=Photo of white bladder that consists of a rectangular section and a banana-shaped section connectd by a much thinner element]]
[[File:White shark.jpg|thumb|right|Sharks, like this three tonne [[great white shark]], don't have swim bladders. Most sharks need to keep swimming to avoid sinking.|alt=Photo of shark surrounded by school of other fish]]
{{see also|Swim bladder}}

The body of a fish is denser than water, so fish must compensate for the difference or they will sink. Many [[bony fish]]es have an internal organ called a [[swim bladder]], or gas bladder, that adjusts their buoyancy through manipulation of gases. In this way, fish can stay at the current water depth, or ascend or descend without having to waste energy in swimming. The bladder is only found in bony fishes. In the more primitive groups like some [[Leuciscinae|minnows]], [[bichir]]s and [[lungfish]], the bladder is open to the [[esophagus]] and double as a [[lung]]. It is often absent in fast swimming fishes such as the tuna and mackerel families. The condition of a bladder open to the esophagus is called [[physostome]], the closed condition [[Physoclisti|physoclist]]. In the latter, the gas content of the bladder is controlled through the [[rete mirabilis]], a network of blood vessels effecting gas exchange between the bladder and the blood.<ref>{{cite book |last=Kardong |first=K. |year=2008 |title=Vertebrates: Comparative anatomy, function, evolution |edition= 5th |location=Boston |publisher=McGraw-Hill |isbn=978-0-07-304058-5}}</ref>

In some fish, a rete mirabile fills the swim bladder with oxygen. A [[countercurrent exchange system]] is utilized between the venous and arterial capillaries. By lowering the [[pH]] levels in the venous capillaries, oxygen unbinds from blood [[hemoglobin]]. This causes an increase in venous blood oxygen concentration, allowing the oxygen to diffuse through the capillary membrane and into the arterial capillaries, where oxygen is still sequestered to hemoglobin. The cycle of diffusion continues until the concentration of oxygen in the arterial capillaries is [[supersaturation|supersaturated]] (larger than the concentration of oxygen in the swim bladder). At this point, the free oxygen in the arterial capillaries diffuses into the swim bladder via the gas gland.<ref>Kardong, K. (2008). ''Vertebrates: Comparative anatomy, function, evolution, (5th ed.).'' Boston: McGraw-Hill.</ref>

Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains [[squalene]], and their cartilage, which is about half the normal density of bone.<ref name=Cartilagious>{{cite web|url=http://www.elasmo-research.org/education/topics/p_cartilage.htm|title=The Importance of Being Cartilaginous|last=Martin|first=R. Aidan|publisher=ReefQuest Centre for Shark Research|access-date=2009-08-29}}</ref> Their liver constitutes up to 30% of their total body mass.<ref name="Collins" /> The liver's effectiveness is limited, so sharks employ [[dynamic lift (fish)|dynamic lift]] to maintain depth when not swimming. [[Sand tiger shark]]s store air in their stomachs, using it as a form of swim bladder. Most sharks need to constantly swim in order to breathe and cannot sleep very long without sinking (if at all). However, certain species, like the [[nurse shark]], are capable of pumping water across their gills, allowing them to rest on the ocean bottom.<ref>{{cite web|url=http://www.flmnh.ufl.edu/fish/education/questions/basics.html#sleep |title=Do sharks sleep |publisher=Flmnh.ufl.edu |access-date=2010-09-23}}</ref>

==Sensory systems==
{{See also|Sensory systems in fish}}

Most fish possess highly developed sense organs. Nearly all daylight fish have color vision that is at least as good as a human's (see [[vision in fishes]]). Many fish also have chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears, many fish may not hear very well. Most fish have sensitive receptors that form the [[lateral line system]], which detects gentle currents and vibrations, and senses the motion of nearby fish and prey.<ref name="Encarta 99">{{Cite book | last =Orr | first =James | year =1999 | title =Fish | publisher =Microsoft Encarta 99 | isbn =978-0-8114-2346-5 | url =https://archive.org/details/fearsomefishcree00stev }}</ref> Sharks can sense frequencies in the range of 25 to 50&nbsp;[[Hertz|Hz]] through their lateral line.<ref>{{cite journal
| last = Popper
| first = A.N.
|author2=C. Platt
| title = Inner ear and lateral line
| journal = The Physiology of Fishes
| issue = 1st ed
| publisher = CRC Press
| year = 1993}}</ref>

Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish behavior in mazes reveals that they possess spatial memory and visual discrimination.<ref>{{cite web | url= http://juls.sa.utoronto.ca/Issues/JULS-Vol2Iss1/JULS-Vol2Iss1-Review3.pdf | title= Appropriate maze methodology to study learning in fish | author= Journal of Undergraduate Life Sciences | access-date= 28 May 2009 | url-status= dead | archive-url= https://web.archive.org/web/20110706211428/http://juls.sa.utoronto.ca/Issues/JULS-Vol2Iss1/JULS-Vol2Iss1-Review3.pdf | archive-date= 6 July 2011 | df= dmy-all }}</ref>

===Vision===
{{main|Vision in fish}}
[[Visual system|Vision]] is an important [[sensory system]] for most species of fish. Fish eyes are similar to those of [[terrestrial animal|terrestrial]] [[vertebrate]]s like [[bird vision|birds]] and mammals, but have a more [[spherical]] [[lens (anatomy)|lens]]. Their [[retina]]s generally have both [[rod cell]]s and [[cone cell]]s (for [[scotopic vision|scotopic]] and [[photopic vision]]), and most species have [[colour vision]]. Some fish can see [[ultraviolet]] and some can see [[polarized light]]. Amongst [[jawless fish]], the [[lamprey]] has well-developed eyes, while the [[hagfish]] has only primitive [[eyespot (mimicry)|eyespot]]s.<ref>[[Neil A. Campbell|N. A. Campbell]] and [[Jane Reece|J. B. Reece]] (2005). ''Biology'', Seventh Edition. Benjamin Cummings, San Francisco, California.</ref> Fish vision shows [[adaptation]] to their visual environment, for example [[deep sea fish]]es have eyes suited to the dark environment.

===Hearing===
{{see also|Hearing in fish}}

[[Hearing (sense)|Hearing]] is an important sensory system for most species of fish. Hearing threshold and the ability to localize sound sources are reduced underwater, in which the speed of sound is faster than in air. Underwater hearing is by [[bone conduction]], and localization of sound appears to depend on differences in amplitude detected by bone conduction.<ref>{{cite journal | author = Shupak A. Sharoni Z. Yanir Y. Keynan Y. Alfie Y. Halpern P. | title = Underwater Hearing and Sound Localization with and without an Air Interface | url = http://otology-neurotology.com/pt/re/otoneuroto/abstract.00129492-200501000-00023.htm;jsessionid=Hn3GlTRJcB530CTrCxLlgrJLhv6WyCvpgcBmC0FLJCLWgY5yckpm!1138671057!181195629!8091!-1?index=1&database=ppvovft&results=1&count=10&searchid=1&nav=search | journal = Otology & Neurotology | volume = 26 | issue = 1 | pages = 127–130 |date=January 2005 | doi = 10.1097/00129492-200501000-00023 | pmid=15699733| s2cid = 26944504 }}</ref> Aquatic animals such as fish, however, have a more specialized hearing apparatus that is effective underwater.<ref>{{cite journal|last=Graham|first=Michael|title=Sense of Hearing in Fishes|journal=Nature|year=1941|volume=147|pages=779|doi=10.1038/147779b0|issue=3738|bibcode=1941Natur.147..779G|s2cid=4132336|doi-access=free}}</ref>

Fish can sense sound through their [[lateral line]]s and their [[otolith]]s (ears). Some fishes, such as some species of [[carp]] and [[herring]], hear through their swim bladders, which function rather like a hearing aid.<ref>{{cite journal | last1 = Williams | first1 = C | year = 1941| title = Sense of Hearing in Fishes | journal = Nature | volume = 147 | issue = 3731| page = 543 | doi=10.1038/147543b0| bibcode = 1941Natur.147..543W | s2cid = 4095706 | doi-access = free }}</ref>

Hearing is well-developed in [[carp]], which have the [[Weberian organ]], three specialized vertebral processes that transfer vibrations in the swim bladder to the inner ear.

Although it is hard to test sharks' hearing, they may have a sharp [[Hearing (sense)|sense of hearing]] and can possibly hear prey many miles away.<ref>{{cite web
| url = http://www.elasmo-research.org/education/white_shark/hearing.htm
| title = Hearing and Vibration Detection
| first= R. Aidan |last=Martin
| access-date = 2008-06-01}}</ref> A small opening on each side of their heads (not the [[Spiracle (vertebrates)|spiracle]]) leads directly into the [[inner ear]] through a thin channel. The [[lateral line]] shows a similar arrangement, and is open to the environment via a series of openings called lateral line [[Canal pore|pores]]. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fish and [[tetrapod]]s the external opening into the inner ear has been lost.

===Chemoreception===
{{see also|Chemoreception in fish}}

[[File:Hammerhead shark.jpg|thumb |240px |The shape of the [[hammerhead shark]]'s head may enhance olfaction by spacing the nostrils further apart.|alt=Eyelevel photo of hammerhead from the front]]
Sharks have keen [[olfactory]] senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one [[parts per million|part per million]] of blood in seawater.<ref>{{cite web
|url=http://elasmo-research.org/education/white_shark/smell.htm
|title=Smell and Taste
|last=Martin
|first=R. Aidan
|publisher=ReefQuest Centre for Shark Research
|access-date=2009-08-21}}</ref>

Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril.<ref>[http://shell.cas.usf.edu/motta/Gardiner%20and%20Atema%202010.pdf The Function of Bilateral Odor Arrival Time Differences in Olfactory Orientation of Sharks] {{webarchive |url=https://web.archive.org/web/20120308032620/http://shell.cas.usf.edu/motta/Gardiner%20and%20Atema%202010.pdf |date=8 March 2012}}, Jayne M. Gardiner, Jelle Atema, Current Biology - 13 July 2010 (Vol. 20, Issue 13, pp. 1187-1191)</ref> This is similar to the method mammals use to determine direction of sound.

They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in [[sewage]] outfalls. Some species, such as [[nurse shark]]s, have external [[barbels]] that greatly increase their ability to sense prey.

===Magnetoception===
{{see also|Magnetoception}}

===Electroreception===
{{See also|Electroreception|Electrocommunication}}
[[File:Electroreceptors in a sharks head.svg|right|300px |thumb |Electromagnetic field receptors (ampullae of Lorenzini) and motion detecting canals in the head of a shark|alt=Drawing of shark head.]]

Some fish, such as catfish and sharks, have organs that detect weak electric currents on the order of millivolt.<ref>Albert, J.S., and W.G.R. Crampton. 2005. Electroreception and electrogenesis. pp. 431–472 in The Physiology of Fishes, 3rd Edition. D.H. Evans and J.B. Claiborne (eds.). CRC Press.</ref> Other fish, like the South American electric fishes [[Gymnotiformes]], can produce weak electric currents, which they use in navigation and social communication. In sharks, the [[ampullae of Lorenzini]] are electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the [[electromagnetic field]]s that all living things produce.<ref>{{cite journal
|doi=10.1126/science.7134985
|author=Kalmijn AJ
|title=Electric and magnetic field detection in elasmobranch fishes
|journal=Science
|volume=218
|issue=4575
|pages=916–8
|year=1982
|pmid=7134985|bibcode=1982Sci...218..916K
}}</ref> This helps sharks (particularly the [[hammerhead shark]]) find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden in sand by detecting the [[electric field]]s they produce. [[Ocean current]]s moving in the [[Earth's magnetic field|magnetic field of the Earth]] also generate electric fields that sharks can use for orientation and possibly navigation.<ref>{{cite journal
|vauthors=Meyer CG, Holland KN, Papastamatiou YP |title=Sharks can detect changes in the geomagnetic field
|journal=Journal of the Royal Society, Interface
|volume=2
|issue=2
|pages=129–30
|year=2005
|pmid=16849172
|pmc=1578252
|doi=10.1098/rsif.2004.0021}}</ref>

* The [[ampullae of Lorenzini]] allow sharks to sense electrical discharges.
* [[Electric fish]] are able to produce electric fields by modified muscles in their body.

===Pain===
{{main|Pain in fish}}

Experiments done by William Tavolga provide evidence that fish have [[pain]] and fear responses. For instance, in Tavolga's experiments, [[Batrachoididae|toadfish]] grunted when electrically shocked and over time they came to grunt at the mere sight of an electrode.<ref>Dunayer, Joan, "Fish: Sensitivity Beyond the Captor's Grasp," The Animals' Agenda, July/August 1991, pp. 12–18</ref>

In 2003, Scottish scientists at the [[University of Edinburgh]] and the Roslin Institute concluded that rainbow trout exhibit behaviors often associated with [[pain]] in other animals. [[Bee]] [[venom]] and [[acetic acid]] injected into the lips resulted in fish rocking their bodies and rubbing their lips along the sides and floors of their tanks, which the researchers concluded were attempts to relieve pain, similar to what mammals would do.<ref>[http://arquivo.pt/wayback/20091014095008/http://www.buzzle.com/editorials/4-30-2003-39769.asp Vantressa Brown, "Fish Feel Pain, British Researchers Say," Agence France-Presse, 1 May 2003]</ref><ref>{{cite news|url=http://news.bbc.co.uk/1/hi/sci/tech/2983045.stm|title=Fish do feel pain, scientists say |work=BBC News | date=30 April 2003 | access-date=4 January 2010 | first=Alex | last=Kirby}}</ref><ref name="grandin183">{{cite book|title= Animals in Translation|last= Grandin|first= Temple|author-link= Temple Grandin|author2= Johnson, Catherine|year= 2005|publisher= Scribner|location= New York, New York|isbn= 978-0-7432-4769-6|pages= [https://archive.org/details/animalsintransla00gran/page/183 183–184]|url= https://archive.org/details/animalsintransla00gran/page/183}}</ref> Neurons fired in a pattern resembling human neuronal patterns.<ref name="grandin183" />

Professor James D. Rose of the [[University of Wyoming]] claimed the study was flawed since it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours".<ref>{{cite web |url=http://www.nal.usda.gov/awic/pubs/Fishwelfare/RoseC.pdf |title=Rose, J.D. 2003. A Critique of the paper: "Do fish have nociceptors: Evidence for the evolution of a vertebrate sensory system" |access-date=21 May 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110608092255/http://www.nal.usda.gov/awic/pubs/Fishwelfare/RoseC.pdf |archive-date=8 June 2011 |df=dmy-all }}</ref> Rose argues that since fish brains are so different from human brains, fish are probably not conscious in the manner humans are, so that reactions similar to human reactions to pain instead have other causes. Rose had published a study a year earlier arguing that fish cannot feel pain because their brains lack a [[neocortex]].<ref>James D. Rose, [http://www.coloradotu.org/do-fish-feel-pain/ Do Fish Feel Pain?] {{Webarchive|url=https://web.archive.org/web/20130120104558/http://www.coloradotu.org/do-fish-feel-pain/ |date=20 January 2013 }}, 2002. Retrieved 27 September 2007.</ref> However, animal behaviorist [[Temple Grandin]] argues that fish could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions."<ref name="grandin183" />

Animal welfare advocates raise concerns about the possible [[suffering]] of fish caused by angling. Some countries, such as Germany have banned specific types of fishing, and the British RSPCA now formally prosecutes individuals who are cruel to fish.<ref>[http://www.timesonline.co.uk/newspaper/0,,176-1037515,00.html Leake, J. "Anglers to Face RSPCA Check," The Sunday Times – Britain, 14 March 2004]</ref>

== Reproductive processes ==

{{see also|Fish reproduction|Spawning|Ichthyoplankton}}

[[Oogonia]] development in teleost fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilisation processes. Changes in the [[Cell nucleus|nucleus]], ooplasm, and the surrounding layers characterize the oocyte maturation process.<ref name="Guimaraes">{{cite journal|last1=Guimaraes-Cruz, Rodrigo J.|author2=Santos, José E. dos|author3=Santos, Gilmar B.|title=Gonadal structure and gametogenesis of ''Loricaria lentiginosa'' Isbrücker (Pisces, Teleostei, Siluriformes)|journal=Rev. Bras. Zool.|date=July–September 2005|volume=22|issue=3|pages=556–564|issn=0101-8175|doi=10.1590/S0101-81752005000300005|first1=Rodrigo J.|doi-access=free}}</ref>

Postovulatory [[Ovarian follicle|follicle]]s are structures formed after oocyte release; they do not have [[endocrine]] function, present a wide irregular lumen, and are rapidly reabsorbed in a process involving the [[apoptosis]] of follicular cells. A degenerative process called [[follicular atresia]] reabsorbs vitellogenic oocytes not spawned. This process can also occur, but less frequently, in oocytes in other development stages.<ref name="Guimaraes" />

Some fish are [[hermaphrodite]]s, having both testes and ovaries either at different phases in their life cycle or, as in [[hamlet (fish)|hamlets]], have them simultaneously.

Over 97% of all known fish are [[oviparous]],<ref name="Scott">Peter Scott: ''Livebearing Fishes'', p. 13. Tetra Press 1997. {{ISBN|1-56465-193-2}}</ref> that is, the eggs develop outside the mother's body. Examples of oviparous fish include [[salmon]], [[goldfish]], [[cichlid]]s, [[tuna]], and [[eel]]s. In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their [[gamete]]s into the surrounding water. However, a few oviparous fish practice internal fertilisation, with the male using some sort of [[Ichthyology terms#I|intromittent organ]] to deliver sperm into the genital opening of the female, most notably the oviparous sharks, such as the [[horn shark]], and oviparous rays, such as [[skate (fish)|skates]]. In these cases, the male is equipped with a pair of modified [[pelvic]] fins known as [[claspers]].

Marine fish can produce high numbers of eggs which are often released into the open water column. The eggs have an average diameter of {{convert|1|mm}}. The eggs are generally surrounded by the extraembryonic membranes but do not develop a shell, hard or soft, around these membranes. Some fish have thick, leathery coats, especially if they must withstand physical force or desiccation. These type of eggs can also be very small and fragile.

<gallery>
File:Oeufs002b,57.png|Egg of [[lamprey]]
File:Oeufs002b,54.png|Egg of [[catshark]] ([[mermaids' purse]])
File:Oeufs002b,55.png|Egg of [[bullhead shark]]
File:Oeufs002b,56.png|Egg of [[chimaera]]
</gallery>

The newly hatched young of oviparous fish are called [[larva]]e. They are usually poorly formed, carry a large [[yolk sac]] (for nourishment) and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed [[metamorphosis]]) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on [[zooplankton]] prey, a process which depends on typically inadequate zooplankton density, starving many larvae.

In [[ovoviviparous]] fish the eggs develop inside the mother's body after internal fertilisation but receive little or no nourishment directly from the mother, depending instead on the [[yolk]]. Each embryo develops in its own egg. Familiar examples of ovoviviparous fish include [[Guppy|guppies]], [[angel shark]]s, and [[coelacanth]]s.

Some species of fish are [[viviparous]]. In such species the mother retains the eggs and nourishes the embryos. Typically, viviparous fish have a structure analogous to the [[placenta]] seen in [[placental mammals|mammals]] connecting the mother's blood supply with that of the embryo. Examples of viviparous fish include the [[Embiotocidae|surf-perches]], [[Goodeidae|splitfins]], and [[lemon shark]]. Some viviparous fish exhibit [[oophagy]], in which the developing embryos eat other eggs produced by the mother. This has been observed primarily among sharks, such as the [[shortfin mako]] and [[porbeagle]], but is known for a few bony fish as well, such as the [[halfbeak]] ''Nomorhamphus ebrardtii''.<ref name="Meisner">{{cite journal |last1=Meisner |first1=A. |last2=Burns |first2=J. |date=December 1997 |title=Viviparity in the Halfbeak Genera ''Dermogenys'' and ''Nomorhamphus'' (Teleostei: Hemiramphidae) |journal=Journal of Morphology |volume=234 |issue=3 |pages=295–317 |doi=10.1002/(SICI)1097-4687(199712)234:3<295::AID-JMOR7>3.0.CO;2-8|pmid=29852651 |s2cid=46922423 }}</ref> [[Intrauterine cannibalism]] is an even more unusual mode of vivipary, in which the largest embryos eat weaker and smaller siblings. This behavior is also most commonly found among sharks, such as the [[grey nurse shark]], but has also been reported for ''Nomorhamphus ebrardtii''.<ref name="Meisner" />

In many species of fish, fins have been modified to allow [[Fish fin#Internal fertilization|Internal fertilisation]].

[[Fishkeeping|Aquarists]] commonly refer to ovoviviparous and viviparous fish as [[livebearers]].

* Many fish species are [[hermaphrodite]]s. ''Synchronous hermaphrodites'' possess both [[ovary|ovaries]] and [[testicle|testes]] at the same time. ''Sequential hermaphrodites'' have both types of tissue in their [[gonad]]s, with one type being predominant while the fish belongs to the corresponding gender.

==Social behaviour==
{{see also|Shoaling and schooling}}Fish social behaviour called ‘shoaling’ involves a group of fish swimming together. This behaviour is a defence mechanism in the sense that there is safety in large numbers, where chances of being eaten by predators are reduced. Shoaling also increases mating and foraging success. Schooling on the other hand, is a behaviour within the shoal where fish can be seen performing various manoeuvres in a synchronised manner.<ref name=":0">{{Cite book|title=Shoaling and Schooling in Fishes|last=Pitcher|first=T.J.|publisher=Greenberg, G. and Hararway. M.M.|year=1998|location=Garland, New York, USA|pages=748–760}}</ref> The parallel swimming is a form of ‘social copying’ where fish in the school replicate the direction and velocity of its neighbouring fishes.<ref>{{Cite journal|last=Pitcher|first=T.J.|date=1983|title=Heuristic definitions of fish shoaling behaviour|journal=Animal Behaviour|volume=31|issue=2|pages=611–613|doi=10.1016/S0003-3472(83)80087-6|s2cid=53195091}}</ref>

Experiments done by D.M. Steven, on the shoaling behaviour of fish concluded that during the day, fish had a higher tendency to stay together as a result of a balance between single fish leaving and finding their own direction and the mutual attraction between fishes of the same species. It was found that at night the fish swam noticeably faster however, often singly and in no co-ordination. Groups of two or three could be seen frequently formed although were dispersed after a couple of seconds.<ref>{{Cite journal|last=Steven|first=D.M.|date=1959|title=Studies on the shoaling behaviour of fish: I. Responses of two species to changes of illumination and to olfactory stimuli.|journal=Journal of Experimental Biology|volume=36|issue=2|pages=261–280|doi=10.1242/jeb.36.2.261}}</ref>

Theoretically, the amount of time that a fish stays together in a shoal should represent their cost of staying instead of leaving.<ref>{{Cite journal|last=Partridge|first=B.L.|date=1982|title=Rigid definitions of schooling behaviour are inadequate.|journal=Animal Behaviour|volume=30|pages=298–299|doi=10.1016/S0003-3472(82)80270-4|s2cid=53170935}}</ref> A past laboratory experiment done on cyprinids has established that the time budget for social behaviour within a shoal varies proportionally to the quantity of fishes present. This originates from the cost/benefit ratio which changes accordingly with group size, measured by the risk of predation versus food intake.<ref>{{Cite journal|last=Pitcher|first=T.J|date=1982|title=Evidence for position preferences in schooling mackerel.|journal=Animal Behaviour|volume=30|issue=3|pages=932–934|doi=10.1016/S0003-3472(82)80170-X|s2cid=53186411}}</ref> When the cost/benefit ratio is favourable to shoaling behaviour then decisions to stay with a group or join one is favourable. Depending on this ratio, fish will correspondingly decide to leave or stay. Thus, shoaling behaviour is considered to be driven by an individual fish's constant stream of decisions.<ref name=":0" />

==See also==
{{Portal|Fish}}
{{div col|colwidth=30em}}
* [[Anatomical terms of location]]
* [[Digital Fish Library]]
* [[Evolution of fish]]
* [[Fish anatomy]]
* [[Fish development]]
* [[Fish measurement]]
* [[Ichthyology terms]]
* [[Digit (anatomy)#Evolution|Panderichthys digits]]
{{div col end}}

==References==
{{reflist|33em}}

==Further reading==
* Bernier NJ, Van Der Kraak G, Farrell AP and Brauner CJ (2009) [https://books.google.com/books?id=vIOm8QXb7JEC&q=%22Fish+physiology%22 ''Fish Physiology: Fish Neuroendocrinology''] Academic Press. {{ISBN|978-0-08-087798-3}}.
* Eddy FB and Handy RD (2012) [https://books.google.com/books?id=VpWllLJ8INYC&q=%22Ecological+and+Environmental+Physiology+of+Fishes%22 '' Ecological and Environmental Physiology of Fishes''] Oxford University Press. {{ISBN|978-0-19-954095-2}}.
* Evans DH, JB Claiborne and S Currie (Eds) (2013) [https://books.google.com/books?id=KHtcAgAAQBAJ&q=%22The+Physiology+of+Fishes%22 ''The Physiology of Fishes''] 4th edition, CRC Press. {{ISBN|978-1-4398-8030-2}}.
* Grosell M, Farrell AP and Brauner CJ (2010) [https://books.google.com/books?id=eddWw_nyRlYC&q=%22Fish+physiology%22 ''Fish Physiology: The Multifunctional Gut of Fish''] Academic Press. {{ISBN|978-0-08-096136-1}}.
* Hara TJ and Zielinski B (2006) [https://books.google.com/books?id=KpXBu4y4XNIC&q=%22Fish+physiology%22 ''Fish Physiology: Sensory Systems Neuroscience''] Academic Press. {{ISBN|978-0-08-046961-4}}.
* Kapoor BG and Khanna B (2004) [https://books.google.com/books?id=I7WhoPBdAooC&q=%22Ichthyology+handbook%22 "Ichthyology handbook"] Pages 137–140, Springer. {{ISBN|978-3-540-42854-1}}.
* McKenzie DJ, Farrell AP and Brauner CJ (2007) [https://books.google.com/books?id=gfBc_omOIeAC&q=%22Fish+physiology%22 ''Fish Physiology: Primitive Fishes''] Academic Press. {{ISBN|978-0-08-054952-1}}.
* Sloman KA, Wilson RW and Balshine S (2006) [https://books.google.com/books?id=CBE0-T2ADFoC&q=%22Fish+physiology%22 ''Behaviour And Physiology of Fish''] Gulf Professional Publishing. {{ISBN|978-0-12-350448-7}}.
* Wood CM, Farrell AP and Brauner CJ (2011) [https://books.google.com/books?id=Nnh1WpHZbysC&q=%22Fish+physiology%22 ''Fish Physiology: Homeostasis and Toxicology of Non-Essential Metals''] Academic Press. {{ISBN|978-0-12-378634-0}}.

==External links==
{{Commons category|Fish physiology}}

[[Category:Fish physiology| ]]

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