View source for Biology:Fish

{{Short description|Gill-bearing non-tetrapod aquatic vertebrates}}
{{for multi|fish as eaten by humans|Fish as food|the superclass containing the vast majority of living fish|Osteichthyes|other uses|Fish (disambiguation)}}
{{pp-move}}
{{Paraphyletic group
| name = Fish
| fossil_range = {{fossilrange|535|0}} [[Earth:Middle Cambrian|Middle Cambrian]] – Recent
| image = Fish_diversity.jpg
| image_caption = Diversity of various fish including [[Biology:Shark|shark]]s, [[Biology:Stingray|stingray]]s, bony fish, jawless fish, and [[Biology:Coelacanth|coelacanth]]s.
| auto = yes
| parent = Vertebrata
| includes = :[[Biology:Agnatha|Jawless fish]]
:{{extinct}}[[Biology:Placodermi|Armoured fish]]
:{{extinct}}[[Biology:Acanthodii|Spiny sharks]]
:Cartilaginous fish
:[[Biology:Osteichthyes|Bony fish]]
::[[Biology:Actinopterygii|Ray-finned fish]]
::[[Biology:Sarcopterygii|Lobe-finned fish]]
| excludes = :Tetrapods
:
}}

A '''fish''' ({{plural form}}: '''fish''' or '''fishes''') is an aquatic, [[Biology:Craniate|craniate]], [[Biology:Gill|gill]]-bearing animal that lacks limbs with digits. Included in this definition are the living [[Biology:Hagfish|hagfish]], [[Biology:Lamprey|lamprey]]s, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of living fish species are ray-finned fish, belonging to the class [[Biology:Actinopterygii|Actinopterygii]], with around 99% of those being [[Biology:Teleost|teleost]]s.

The earliest organisms that can be classified as fish were soft-bodied [[Biology:Chordate|chordate]]s that first appeared during the [[Earth:Cambrian|Cambrian]] period. Although they lacked a [[Biology:Vertebrate|true spine]], they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the [[Earth:Paleozoic|Paleozoic]] era, diversifying into a wide variety of forms. Many fish of the [[Earth:Paleozoic|Paleozoic]] developed [[Biology:Placodermi|external armor]] that protected them from predators. The first fish with [[Medicine:Jaw|jaw]]s appeared in the [[Earth:Silurian|Silurian]] period, after which many (such as [[Biology:Shark|shark]]s) became formidable marine predators rather than just the prey of [[Biology:Arthropod|arthropod]]s.

Most fish are [[Biology:Ectotherm|ectotherm]]ic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and [[Biology:Tuna|tuna]] can hold a higher core temperature.<ref>{{cite journal |last=Goldman |first=K.J. |title=Regulation of body temperature in the white shark, ''Carcharodon carcharias'' |journal=Journal of Comparative Physiology |year=1997 |volume=167 |series=B Biochemical Systemic and Environmental Physiology |issue=6 |pages=423–429 |doi=10.1007/s003600050092 |s2cid=28082417 |url=http://www.mendeley.com/research/temperature-and-activities-of-a-white-shark-carcharodon-carcharias/ |access-date=12 October 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120406114844/http://www.mendeley.com/research/temperature-and-activities-of-a-white-shark-carcharodon-carcharias/ |archive-date=6 April 2012}}</ref><ref>{{cite journal |last=Carey |first=F.G. |author2=Lawson, K.D. |title=Temperature regulation in free-swimming bluefin tuna |journal=Comparative Biochemistry and Physiology A |date=February 1973 |volume=44 |issue=2 |pages=375–392 |doi=10.1016/0300-9629(73)90490-8|pmid=4145757 }}</ref> Fish can acoustically communicate with each other, most often in the context of feeding, aggression or courtship.<ref name="Bioacoustics">{{cite journal|last1=Weinmann|first1=S.R.|last2=Black|first2=A.N.|last3=Richter|first3=M. L.|last4=Itzkowitz|first4=M|last5=Burger|first5=R.M|title=Territorial vocalization in sympatric damselfish: acoustic characteristics and intruder discrimination|journal=Bioacoustics|date=February 2017|volume=27|issue=1|pages=87–102|doi=10.1080/09524622.2017.1286263|s2cid=89625932}}</ref>

Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., [[Biology:Salvelinus|char]] and [[Biology:Gudgeon (fish)|gudgeon]]) to the [[Earth:Abyssal zone|abyssal]] and even [[Earth:Hadal zone|hadal]] depths of the deepest oceans (e.g., [[Biology:Cusk-eels|cusk-eels]] and [[Biology:Snailfish|snailfish]]), although no species has yet been documented in the deepest 25% of the ocean.<ref>{{cite journal| pmc=3970477 | pmid=24591588 | doi=10.1073/pnas.1322003111 | volume=111 | issue=12 | title=Marine fish may be biochemically constrained from inhabiting the deepest ocean depths | year=2014 | journal=Proc Natl Acad Sci USA | pages=4461–4465 | last1 = Yancey | first1 = PH | last2 = Gerringer | first2 = ME | last3 = Drazen | first3 = JC | last4 = Rowden | first4 = AA | last5 = Jamieson | first5 = A| bibcode=2014PNAS..111.4461Y | doi-access=free }}</ref> With 34,300 described species, fish exhibit greater species diversity than any other group of vertebrates.<ref>{{cite web |publisher=[[Biology:FishBase|FishBase]] |url=http://www.fishbase.org/search.php |title=FishBase Search |date=March 2020 |access-date=19 March 2020 |url-status=live |archive-url=https://web.archive.org/web/20200303112540/https://www.fishbase.de/search.php |archive-date=3 March 2020}}</ref>

Fish are an important resource for humans worldwide, especially as food. Commercial and subsistence fishers hunt fish in wild fisheries or farm them in ponds or in cages in the ocean (in aquaculture). They are also caught by recreational fishers, kept as pets, raised by [[Biology:Fishkeeping|fishkeepers]], and exhibited in public aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

[[Biology:Cladistics|Cladistically]], fish and [[Biology:Vertebrate|vertebrates]] are synonymous; [[Biology:Tetrapod|tetrapod]]s ([[Biology:Amphibian|amphibian]]s, [[Biology:Reptile|reptile]]s, [[Biology:Bird|bird]]s and [[Biology:Mammal|mammal]]s) emerged within [[Biology:Sarcopterygii|lobe-finned fishes]], so cladistically they are fish as well. However, traditionally fish ('''pisces''' or '''ichthyes''') are rendered paraphyletic by excluding the tetrapods, and are therefore not considered a formal taxonomic grouping in [[Biology:Systematics|systematic biology]], unless it is used in the cladistic sense, including tetrapods,<ref>{{Cite web|url=http://faculty.weber.edu/choagstrom/Zoology1120CH10.pdf|title=Zoology|access-date=19 October 2018|archive-date=28 June 2021|archive-url=https://web.archive.org/web/20210628052208/https://faculty.weber.edu/choagstrom/Zoology1120CH10.pdf|url-status=live}}</ref><ref>{{Cite journal|last=Greene|first=Harry W.|date=1998-01-01|title=We are primates and we are fish: Teaching monophyletic organismal biology|journal=Integrative Biology |language=en|volume=1|issue=3|pages=108–111|doi=10.1002/(sici)1520-6602(1998)1:3<108::aid-inbi5>3.0.co;2-t|issn=1520-6602}}</ref> although usually "vertebrate" is preferred and used for this purpose (fish plus tetrapods) instead. Furthermore, cetaceans, although mammals, have often been considered fish by various cultures and time periods.

==Etymology==
The word for ''fish'' in English and the other Germanic languages ([[Social:German language|German]] {{lang|de|Fisch}}; [[Social:Gothic language|Gothic]] {{lang|got|fisks}}) is inherited from [[Social:Proto-Germanic language|Proto-Germanic]], and is related to the [[Social:Latin language|Latin]] {{lang|la|piscis}} and Old Irish {{lang|sga|īasc}}, though the exact root is unknown; some authorities reconstruct a [[Social:Proto-Indo-European language|Proto-Indo-European]] root {{lang|ine-x-proto|*peysk-}}, attested only in [[Social:Italic languages|Italic]], Celtic, and Germanic.<ref>{{Cite web|title=DWDS – Digitales Wörterbuch der deutschen Sprache|url=https://www.dwds.de/wb/Fisch|access-date=2023-01-21|website=DWDS|language=de|archive-date=31 July 2020|archive-url=https://web.archive.org/web/20200731172104/https://www.dwds.de/wb/Fisch|url-status=live}}</ref><ref>Winfred Philipp Lehmann, Helen-Jo J. Hewitt, Sigmund Feist, ''A Gothic etymological dictionary'', 1986, ''s.v.'' ''fisks'' p. 118</ref><ref>{{Citation|title=fish, n.1|url=https://www.oed.com/view/Entry/70646|work=OED Online|publisher=Oxford University Press|language=en-GB|access-date=2023-01-21|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010607/https://www.oed.com/view/Entry/70646|url-status=live}}</ref><ref>[[Biography:Carl Darling Buck|Carl Darling Buck]], ''A Dictionary of Selected Synonyms in the Principal Indo-European Languages'', 1949, ''s.v.'', section 3.65, p. 184</ref>

The English word once had a much broader usage than its current biological meaning. Names such as [[Biology:Starfish|starfish]], [[Biology:Jellyfish|jellyfish]], shellfish, crayfish/crawfish and [[Biology:Cuttlefish|cuttlefish]] attest to almost any fully aquatic animal (including whales) once being ''fish''. "Correcting" such names (e.g. to ''sea star'') is an attempt to retroactively apply the current meaning of ''fish'' to words that were coined when it had a different meaning.{{cn|date=January 2023}}

==Evolution==

Early fish from the fossil record are represented by a group of small, jawless, armored fish known as [[Biology:Ostracoderm|ostracoderm]]s. Jawless fish lineages are mostly extinct. An extant clade, the [[Biology:Lamprey|lamprey]]s may approximate ancient pre-jawed fish. The first jaws are found in [[Biology:Placodermi|Placodermi]] fossils. They lacked distinct teeth, having instead the oral surfaces of their jaw plates modified to serve the various purposes of teeth. The diversity of jawed vertebrates may indicate the evolutionary advantage of a [[Biology:Fish jaw|jawed mouth]]. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.

Fish may have evolved from a creature similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have [[Biology:Neoteny|kept the larval form into adulthood]] (as some sea squirts do today).

===Phylogeny===
Fishes are a [[Biology:Paraphyly|paraphyletic]] group: that is, any [[Biology:Clade|clade]] containing all fish also contains the [[Biology:Tetrapod|tetrapod]]s. The latter are not fish, though they include fish-shaped forms, such as [[Biology:Cetacea|Whales and Dolphins]] (see [[Biology:Evolution of cetaceans|evolution of cetaceans]]) or the extinct [[Biology:Ichthyopterygia|ichthyosaurs]], both of which acquired a fish-like body shape due to secondary aquatic adaptation. In a cladistic sense, tetrapods are a subset of [[Biology:Osteichthyes|Osteichthyes]].

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The following [[Biology:Cladogram|cladogram]] shows [[Biology:Clade|clade]]s – some with, some without extant relatives – that are traditionally considered as "fishes" ([[Cyan|cyan]] line) and the tetrapods (four-limbed vertebrates), which are mostly terrestrial. Extinct groups are marked with a dagger (†).

{{clade| style=font-size:85%;line-height:85%;
|grouplabel1={{clade labels  |label1="'''Fishes'''" |top1=50%}}
|label1=Vertebrata/
|1={{clade
   |1={{clade
      |label1=[[Biology:Agnatha|Agnatha]]/
      |sublabel1=Cyclostomes
      |1={{clade
         |1=[[Biology:Hyperoartia|Hyperoartia]] (lampreys)[[File:Nejonöga, Iduns kokbok.jpg|90 px]] |barbegin1=cyan
         |2=Myxini ([[Biology:Hagfish|hagfish]]) [[File:Cuvier-120-Myxine116.jpg|90px]] |bar2=cyan
         }}
      }}
   |2=†Euconodonta<span style="{{MirrorH}}">[[File:ConodontZICA.png|90px]]</span> |bar2=cyan
   |3={{clade
      |1=†[[Biology:Pteraspidomorphi|Pteraspidomorphi]] [[File:Astraspis desiderata.png|50 px]] |bar1=cyan
      |2=†[[Biology:Thelodonti|Thelodonti]] [[File:Sphenonectris turnernae white background.jpg|50 px]] |bar2=cyan
      |3={{clade
         |1=†[[Biology:Anaspida|Anaspida]]<span style="{{MirrorH}}">[[File:Lasanius NT small.jpg|70 px]]</span> |bar1=cyan
         |2={{clade
            |1=†[[Biology:Galeaspida|Galeaspida]] [[File:Galeaspids NT.jpg|70px]] |bar1=cyan
            |2={{clade
               |1=†[[Biology:Pituriaspida|Pituriaspida]]<span style="{{MirrorH}}">[[File:Pituriaspida.gif|70px]]</span> |bar1=cyan
               |2=†[[Biology:Osteostraci|Osteostraci]] [[File:Cephalaspis Lyellii.jpg|70px]] |bar2=cyan
               |label3=[[Biology:Gnathostomata|Gnathostomata]] |sublabel3=(jawed vertebrates)
               |3={{clade
                  |1="†[[Biology:Placodermi|Placodermi]]" (armoured fishes, paraphyletic)<ref name="Giles et al 2015">{{closed access}} {{cite journal |last1=Giles |first1=Sam |last2=Friedman |first2=Matt |last3=Brazeau |first3=Martin D. |title=Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome |journal=Nature |volume=520 |issue=7545 |pages=82–85 |date=2015-01-12 |issn=1476-4687  |doi=10.1038/nature14065 |pmid=25581798 |pmc=5536226 |bibcode=2015Natur.520...82G }}</ref>[[File:Dunkleosteus intermedius.jpg|70 px]]  |bar1=cyan
                  |2={{clade
                     |1="†[[Biology:Acanthodii|Acanthodii]]" ("spiny sharks", paraphyletic or polyphyletic)<ref name=davis>{{cite journal|last1=Davis|first1=S|last2=Finarelli|first2=J|last3=Coates|first3=M|title=Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes|journal=Nature|date=2012|pages=247–250|doi=10.1038/nature11080|volume=486|issue=7402|pmid=22699617|bibcode=2012Natur.486..247D|s2cid=4304310}}</ref> <span style="{{MirrorH}}">[[File:Diplacanthus reconstructed.jpg|40px]]</span>  |bar1=cyan
                       |2={{clade
                        |label1=Chondrichthyes |sublabel1=(cartilaginous fishes)
                          |1={{clade
                          |1="†[[Biology:Acanthodii|Acanthodii]]" ("spiny sharks", paraphyletic or polyphyletic)<span style="{{MirrorH}}">[[File:BrochoadmonesDB15.jpg|70px]]</span>  |bar1=cyan
                        |2={{clade
                         |1=[[Biology:Holocephali|Holocephali]] (ratfish)[[File:Chimaera monstrosa1.jpg|60 px]] |bar1=cyan
                         |2=[[Biology:Euselachii|Euselachii]] (sharks, rays)<span style="{{MirrorH}}">[[File:White shark (Duane Raver).png|75 px]] [[File:Mobula mobular.jpg|75px]]</span> |bar2=cyan
                       }}
                      }}
                     |label2=[[Biology:Euteleostomi|Euteleostomi]]/ |sublabel2=[[Biology:Osteichthyes|Osteichthyes]]
                     |2={{clade
                         |1="†[[Biology:Acanthodii|Acanthodii]]" ("spiny sharks", paraphyletic or polyphyletic)[[File:Acanthodes BW.jpg|90px]] |bar1=cyan
                         |2={{clade
                           |1={{clade
                           |label1=[[Biology:Actinopterygii|Actinopterygii]] |sublabel1=(ray-finned fishes)
                           |1={{clade
                              |1=[[Biology:Cladistia|Cladistia]] (bichirs, [[Biology:Reedfish|reedfish]]) [[File:Cuvier-105-Polyptère.jpg|80px]] |bar1=cyan
                              |2={{clade
                                |1=[[Biology:Chondrostei|Chondrostei]] (sturgeons, [[Biology:Paddlefish|paddlefish]])[[File:Atlantic sturgeon flipped.jpg|90px]] |bar1=cyan
                                |2=[[Biology:Neopterygii|Neopterygii]] (includes Teleostei, 96% of living fish [[Biology:Species|species]])[[File:Common carp (white background).jpg|70px]] |bar2=cyan
                                 }}
                               }}
                          |label2=[[Biology:Sarcopterygii|Sarcopterygii]] |sublabel2=(lobe-finned fish)
                          |2={{clade
                              |1=†[[Biology:Onychodontiformes|Onychodontiformes]] [[File:OnychodusDB15 flipped.jpg|70 px]] |bar1=cyan
                              |2=[[Biology:Actinistia|Actinistia]] ([[Biology:Latimeria|coelacanths]]) [[File:Coelacanth flipped.png|70 px]] |bar2=cyan
                              |label3=[[Biology:Rhipidistia|Rhipidistia]]
                              |3={{clade
                                 |1={{clade
                                   |1=†[[Biology:Porolepiformes|Porolepiformes]] [[File:Reconstruction of Porolepis sp flipped.jpg|70 px]] |bar1=cyan
                                   |2=Dipnoi (lungfishes) <span style="{{MirrorH}}">[[File:Chinle fish Arganodus cropped cropped.png|70 px]]</span>  |bar2=cyan
                                   }}
                                 |label2=[[Biology:Tetrapodomorpha|Tetrapodomorpha]]/ |sublabel2=(Choanata)
                                 |2={{clade
                                     |1={{clade
                                       |1=†Rhizodontimorpha [[File:Gooloogongia loomesi reconstruction.jpg|70 px]] |bar1=cyan
                                       |2={{clade
                                        |1=†[[Biology:Tristichopteridae|Tristichopteridae]] [[File:Eusthenodon DB15 flipped.jpg|70 px]] |bar1=cyan
                                        |2={{clade
                                          |1=†''[[Biology:Tiktaalik|Tiktaalik]]''[[File:Tiktaalik BW white background.jpg|100 px]] |barend1=cyan
                                          |label2=Tetrapoda |sublabel2=four-limbed vertebrates
                                          |2={{clade
                                            |1=†''[[Biology:Ichthyostega|Ichthyostega]]''[[File:Ichthyostega BW (flipped).jpg|80 px]]
                                            |2=crown-group tetrapods [[File:Salamandra salamandra (white background).jpg|80 px]] 80px 35px [[File:Ruskea rotta.png|50px]]
                                           }}
                                         }}
                                       }}
                                     }}
                                  }}
                                }}
                             }}
                          }}
                        }}
                     }}
                   }}
               }}
            }}
         }}
      }}
    }}
  }}
}}
|sublabel1=Craniata}}

===Taxonomy===
[[File:Leedsichthys problematicus.jpg|thumb|''[[Biology:Leedsichthys|Leedsichthys]],'' of the subclass Actinopterygii, is the largest known fish, with estimates in 2005 putting its maximum size at {{convert|16|m}}.]]
Fishes are a paraphyletic group and for this reason, groups such as the class ''Pisces'' seen in older reference works are no longer used in formal classifications. Traditional classification divides fish into three extant [[Biology:Class|classes]], and with extinct forms sometimes classified within the tree, sometimes as their own classes:<ref name="Romer, A 1977">Romer, A.S. & T.S. Parsons. 1977. ''The Vertebrate Body.'' 5th ed. Saunders, Philadelphia. (6th ed. 1985)</ref><ref>Benton, M.J. (1998) The quality of the fossil record of vertebrates. pp. 269–303, in Donovan, S.K. and Paul, C.R.C. (eds), The adequacy of the fossil record, Fig. 2. Wiley, New York, 312 pp.</ref>
* Class [[Biology:Agnatha|Agnatha]] (jawless fish)
** Subclass [[Biology:Cyclostomata|Cyclostomata]] ([[Biology:Hagfish|hagfish]] and [[Biology:Lamprey|lamprey]]s)
** Subclass Ostracodermi (armoured jawless fish) †
* Class Chondrichthyes (cartilaginous fish)
** Subclass [[Biology:Elasmobranchii|Elasmobranchii]] ([[Biology:Shark|shark]]s and [[Biology:Batoidea|rays]])
** Subclass [[Biology:Holocephali|Holocephali]] ([[Biology:Chimaera|chimaera]]s and extinct relatives)
* Class [[Biology:Placodermi|Placodermi]] (armoured fish) †
* Class [[Biology:Acanthodii|Acanthodii]] ("spiny sharks", sometimes classified under Actinopterygii) †
* Superclass [[Biology:Osteichthyes|Osteichthyes]] (bony fish)
** Class [[Biology:Actinopterygii|Actinopterygii]] (ray finned fishes)
** Clade [[Biology:Sarcopterygii|Sarcopterygii]] (lobe finned fishes, ancestors of tetrapods)

The above scheme is the one most commonly encountered in non-specialist and general works. Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnatha are ancestral to Placodermi, who again have given rise to Osteichthyes, as well as to Acanthodii, the ancestors of Chondrichthyes. With the arrival of [[Biology:Phylogenetic nomenclature|phylogenetic nomenclature]], the fishes has been split up into a more detailed scheme, with the following major groups:
* Class Myxini ([[Biology:Hagfish|hagfish]])
* Class [[Biology:Pteraspidomorphi|Pteraspidomorphi]] † (early jawless fish)
* Class [[Biology:Thelodonti|Thelodonti]] †
* Class [[Biology:Anaspida|Anaspida]] †
* Class [[Biology:Hyperoartia|Petromyzontida or Hyperoartia]]
** Petromyzontidae ([[Biology:Lamprey|lamprey]]s)
* Class [[Biology:Conodont|Conodont]]a (conodonts) †
* Class [[Biology:Cephalaspidomorphi|Cephalaspidomorphi]] † (early jawless fish)
** (unranked) [[Biology:Galeaspida|Galeaspida]] †
** (unranked) [[Biology:Pituriaspida|Pituriaspida]] †
** (unranked) [[Biology:Osteostraci|Osteostraci]] †
* Infraphylum [[Biology:Gnathostomata|Gnathostomata]] (jawed vertebrates)
** Class [[Biology:Placodermi|Placodermi]] † (armoured fish)
** Class Chondrichthyes (cartilaginous fish)
** Class [[Biology:Acanthodii|Acanthodii]] † (spiny sharks)
** Superclass [[Biology:Osteichthyes|Osteichthyes]] (bony fish)
*** Class [[Biology:Actinopterygii|Actinopterygii]] (ray-finned fish)
**** Subclass [[Biology:Chondrostei|Chondrostei]]
***** Order [[Biology:Acipenseriformes|Acipenseriformes]] ([[Biology:Sturgeon|sturgeon]]s and [[Biology:Paddlefish|paddlefish]]es)
***** Order [[Biology:Polypteriformes|Polypteriformes]] ([[Biology:Reedfish|reedfish]]es and [[Biology:Bichir|bichir]]s).
**** Subclass [[Biology:Neopterygii|Neopterygii]]
***** Infraclass [[Biology:Holostei|Holostei]] ([[Biology:Gar|gar]]s and [[Biology:Bowfin|bowfin]]s)
***** Infraclass Teleostei (many orders of common fish)
*** Class [[Biology:Sarcopterygii|Sarcopterygii]] (lobe-finned fish)
**** Subclass [[Biology:Actinistia|Actinistia]] ([[Biology:Coelacanth|coelacanth]]s)
**** Subclass Dipnoi ([[Biology:Lungfish|lungfish]], sister group to the tetrapods)

† – indicates extinct taxon<br />Some palaeontologists contend that because Conodonta are [[Biology:Chordate|chordate]]s, they are primitive fish. For a fuller treatment of this taxonomy, see the [[Biology:Vertebrate|vertebrate]] article.

The position of [[Biology:Hagfish|hagfish]] in the phylum Chordata is not settled. Phylogenetic research in 1998 and 1999 supported the idea that the hagfish and the lampreys form a natural group, the [[Biology:Cyclostomata|Cyclostomata]], that is a sister group of the Gnathostomata.<ref>Shigehiro Kuraku, Daisuke Hoshiyama, Kazutaka Katoh, Hiroshi Suga, Takashi Miyata (1999) ''Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA–Coded Genes'' J Mol Evol (1999) 49:729–735</ref><ref>J. Mallatt, J. Sullivan (1998) ''28S and 18S rDNA sequences support the monophyly of lampreys and hagfishes'' Molecular Biology and Evolution V 15, Issue 12, pp. 1706–1718</ref>

The various fish groups account for more than half of vertebrate species. As of 2006,{{sfn|Nelson|2006|pp=4–5}} there are almost 28,000 known extant species, of which almost 27,000 are bony fish, with 970 sharks, rays, and chimeras and about 108 hagfish and lampreys. A third of these species fall within the nine largest families; from largest to smallest, these families are [[Biology:Cyprinidae|Cyprinidae]], [[Biology:Gobiidae|Gobiidae]], [[Biology:Cichlidae|Cichlidae]], [[Biology:Characidae|Characidae]], [[Biology:Loricariidae|Loricariidae]], Balitoridae, [[Biology:Serranidae|Serranidae]], [[Biology:Labridae|Labridae]], and [[Biology:Scorpaenidae|Scorpaenidae]]. About 64 families are monotypic, containing only one species. The final total of extant species may grow to exceed 32,500.{{sfn|Nelson|2006|p=3}} Each year, new [[Biology:Species|species]] are discovered and [[Taxonomy|scientifically described]]. As of 2016,<ref name="auto">{{cite book |last=Nelson |first=Joseph, S. |title=Fishes of the World |year=2016 |publisher=John Wiley & Sons, Inc |isbn=978-1-118-34233-6 }}</ref> there are over 32,000 documented species of bony fish and over 1,100 species of cartilaginous fish. Species are lost through [[Biology:Extinction|extinction]] (see biodiversity crisis). Recent examples are the [[Biology:Psephurus|Chinese paddlefish]] or the [[Biology:Smooth handfish|smooth handfish]].

===Diversity===
{{Main|Biology:Diversity of fish}}

<gallery mode="packed" title="Examples" of="" the="" major="" classes="" fish="">
Pacific_hagfish_Myxine.jpg|Agnatha<br> ([[Biology:Pacific hagfish|Pacific hagfish]])
Hornhai (Heterodontus francisci).JPG|Chondrichthyes <br>([[Biology:Horn shark|Horn shark]])
Salmo trutta.jpg|Actinopterygii<br> ([[Biology:Brown trout|Brown trout]])
Latimeria chalumnae01.jpg|Sarcopterygii <br>([[Biology:Coelacanth|Coelacanth]])
</gallery>

[[File:leafydragon.jpg|thumb|A relative of the seahorses, the [[Biology:Leafy seadragon|leafy seadragon]]'s appendages allow it to camouflage (in the form of crypsis) with the surrounding seaweed.|alt=Photo of fish with many narrow, straight appendages. Some are end in points, and others are longer, ending in two or three approximately flat, triangular flaps, each with a dark spot.]]
[[File:Synchiropus splendidus 2 Luc Viatour cropped.png|thumb| The psychedelic mandarin dragonet is one of only two fish species where the blue colouring has been shown to be due to blue pigment containing chromatophores in the skin.<ref>{{cite journal |author1=Goda, M. |author2=R. Fujii |year=2009 |title=Blue Chromatophores in Two Species of Callionymid Fish |journal=Zoological Science |volume=12 |issue=6 |pages=811–813 |doi=10.2108/zsj.12.811|s2cid=86385679 |doi-access=free }}</ref>]]

The term "fish" most precisely describes any non-[[Biology:Tetrapod|tetrapod]] [[Biology:Craniate|craniate]] (i.e. an animal with a skull and in most cases a backbone) that has [[Biology:Gill|gill]]s throughout life and whose limbs, if any, are in the shape of fins.{{sfn|Nelson|2006|p=2}} Unlike groupings such as birds or [[Biology:Mammal|mammal]]s, fish are not a single [[Biology:Clade|clade]] but a paraphyletic collection of [[Biology:Taxon|taxa]], including [[Biology:Hagfish|hagfish]]es, [[Biology:Lamprey|lamprey]]s, sharks and rays, [[Biology:Actinopterygii|ray-finned fish]], [[Biology:Coelacanth|coelacanth]]s, and [[Biology:Lungfish|lungfish]].{{sfn|Helfman|Collette|Facey|1997|p=3}}<ref>Tree of life web project – [http://tolweb.org/Chordata/2499 Chordates] {{webarchive|url=https://web.archive.org/web/20070224172716/http://tolweb.org/Chordata/2499 |date=24 February 2007 }}.</ref> Indeed, lungfish and coelacanths are closer relatives of [[Biology:Tetrapod|tetrapod]]s (such as mammals, birds, [[Biology:Lissamphibia|amphibians]], etc.) than of other fish such as ray-finned fish or sharks, so the last common ancestor of all fish is also an ancestor to tetrapods. As paraphyletic groups are no longer recognised in modern [[Biology:Systematics|systematic biology]], the use of the term "fish" as a biological group must be avoided.

Many types of aquatic animals commonly referred to as "fish" are not fish in the sense given above; examples include shellfish, [[Biology:Cuttlefish|cuttlefish]], [[Biology:Starfish|starfish]], crayfish and [[Biology:Jellyfish|jellyfish]]. In earlier times, even biologists did not make a distinction&nbsp;– sixteenth century natural historians classified also [[Biology:Pinniped|seals]], whales, [[Biology:Amphibian|amphibian]]s, [[Biology:Crocodile|crocodile]]s, even [[Biology:Hippopotamus|hippopotamus]]es, as well as a host of aquatic invertebrates, as fish.<ref name="integrated">{{cite book | author=Cleveland P. Hickman Jr. |author2=Larry S. Roberts |author3=Allan L. Larson |title=Integrated Principles of Zoology |publisher=McGraw-Hill Publishing Co |year=2001 |isbn=978-0-07-290961-6}}</ref> However, according to the definition above, all mammals, including cetaceans like whales and dolphins, are not fish. In some contexts, especially in aquaculture, the true fish are referred to as '''finfish''' (or '''fin fish''') to distinguish them from these other animals.

A typical fish is ectothermic, has a streamlined body for rapid swimming, extracts oxygen from water using gills or uses an accessory breathing organ to breathe atmospheric oxygen, has two sets of paired fins, usually one or two (rarely three) dorsal fins, an anal fin, and a tail fin, has jaws, has skin that is usually covered with [[Biology:Scale (zoology)|scales]], and lays eggs.

Each criterion has exceptions. [[Biology:Tuna|Tuna]], [[Biology:Swordfish|swordfish]], and some species of [[Biology:Shark|shark]]s show [[Biology:Warm-blooded#Temperature control in cold-blooded animals|some warm-blooded adaptations]]&nbsp;– they can heat their bodies significantly above ambient water temperature.{{sfn|Helfman|Collette|Facey|1997|p=3}} Streamlining and swimming performance varies from fish such as tuna, salmon, and [[Biology:Carangidae|jacks]] that can cover 10–20 body-lengths per second to species such as [[Biology:Eel|eel]]s and [[Biology:Batoidea|rays]] that swim no more than 0.5 body-lengths per second.{{sfn|Helfman|Collette|Facey|1997|p=103}} Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. [[Biology:Lungfish|Lungfish]] have paired lungs similar to those of tetrapods, [[Biology:Gourami|gourami]]s have a structure called the labyrinth organ that performs a similar function, while many catfish, such as ''[[Biology:Corydoras|Corydoras]]'' extract oxygen via the intestine or stomach.{{sfn|Helfman|Collette|Facey|1997|pp=53–57}} Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as [[Biology:Seahorse|seahorse]]s, pufferfish, [[Biology:Anglerfish|anglerfish]], and gulpers. Similarly, the surface of the skin may be naked (as in [[Biology:Moray eel|moray eel]]s), or covered with scales of a variety of different types usually defined as placoid (typical of sharks and rays), [[Biology:Scale (zoology)#Cosmoid scales|cosmoid]] (fossil lungfish and coelacanths), ganoid (various fossil fish but also living [[Biology:Gar|gar]]s and [[Biology:Bichir|bichir]]s), [[Biology:Fish scale#Leptoid scales|cycloid]], and ctenoid (these last two are found on most bony fish).{{sfn|Helfman|Collette|Facey|1997|pp=33–36}} There are even fish that live mostly on land or lay their eggs on land near water.<ref>{{cite book |author=Martin, K.L.M. |title=Beach-Spawning Fishes: Reproduction in an Endangered Ecosystem |year=2014 |publisher=CRC Press |isbn=978-1-4822-0797-2}}</ref> [[Biology:Mudskipper|Mudskipper]]s feed and interact with one another on mudflats and go underwater to hide in their burrows.<ref>{{FishBase species|genus=Periophthalmus|species=barbarus|year=2006|month=November|id=12803}}</ref> A single undescribed species of ''[[Biology:Phreatobius|Phreatobius]]'' has been called a true "land fish" as this worm-like catfish strictly lives among waterlogged leaf litter.<ref>{{cite web |url=http://www.planetcatfish.com/catelog/species.php?species_id=646 |title=Cat-eLog: Heptapteridae: ''Phreatobius'': ''Phreatobius'' sp. (1) |author=Planet Catfish |publisher=Planet Catfish |access-date=26 November 2006 |url-status=live |archive-url=https://web.archive.org/web/20061023092123/http://www.planetcatfish.com/catelog/species.php?species_id=646 |archive-date=23 October 2006}}</ref><ref>{{cite journal |last1=Henderson|first1=P.A.|first2=I.|last2=Walker |year=1990 |title=Spatial organization and population density of the fish community of the litter banks within a central Amazonian blackwater stream |journal=Journal of Fish Biology |volume=37 |issue=3 |pages=401–411 |doi=10.1111/j.1095-8649.1990.tb05871.x|bibcode=1990JFBio..37..401H }}</ref> Many species live in [[Earth:Underground lake|underground lake]]s, underground rivers or [[Earth:Aquifer|aquifer]]s and are popularly known as [[Biology:Cavefish|cavefish]].<ref>{{cite book |editor=Aldemaro, R. |title=The Biology of Hypogean Fishes |year=2001 |series=Developments in environmental biology of fishes |volume=21 |isbn=978-1-4020-0076-8}}</ref>

Fish range in size from the huge {{convert|16|m|adj=on}} [[Biology:Whale shark|whale shark]] to the tiny {{convert|8|mm|sigfig=1|adj=on}} stout infantfish.

Fish [[Biology:Species|species]] diversity is roughly divided equally between marine (oceanic) and freshwater ecosystems. [[Earth:Coral reef|Coral reef]]s in the [[Earth:Indo-Pacific|Indo-Pacific]] constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of [[Earth:Tropical rainforest|tropical rainforest]]s, especially the Amazon, [[Place:Congo Basin|Congo]], and [[Earth:Mekong|Mekong]] basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that [[Biology:Neotropical fish|Neotropical fish]]es represent about 10% of all [[Biology:Vertebrate|vertebrate]] species on the Earth. Exceptionally rich sites in the Amazon basin, such as Cantão State Park, can contain more freshwater fish species than occur in all of Europe.<ref>{{Cite web|url=https://central3.to.gov.br/arquivo/24/199|access-date=2023-01-21|website=central3.to.gov.br|title=Estudo das Espécies Ícticas do Parque Estadual do Cantão|archive-date=15 August 2022|archive-url=https://web.archive.org/web/20220815220358/https://central3.to.gov.br/arquivo/24/199|url-status=live}}</ref>

The deepest living fish in the ocean so far found is the snailfish (''Pseudoliparis belyaevi'') which was filmed in the Izu-Ogasawara Trench off the coast of Japan at 8,336 meters in August 2022. The fish was filmed by a robotic lander as part of a scientific expedition funded by Victor Vescovo's Caladan Oceanic with the scientific team led by Professor Alan Jamieson of the University of Western Australia.<ref>{{Cite news |date=2023-04-01 |title=Deepest ever fish caught on camera off Japan |language=en-GB |work=BBC News |url=https://www.bbc.com/news/science-environment-65148876 |access-date=2023-04-06}}</ref>

The [[Biology:Biodiversity|diversity]] of living fish (finfish) is unevenly distributed among the various groups, with teleosts making up the bulk of living fishes (96%), and over 50% of all [[Biology:Vertebrate|vertebrate]] [[Biology:Species|species]].<ref name="auto" /> The following [[Biology:Cladogram|cladogram]]<ref name="Friedman & Sallan 2012">{{cite journal |last1=Friedman |first1=Matt |last2=Sallan |first2=Lauren Cole |title=Five hundred million years of extinczion and recovery: A Phanerozoic survey of large-scale diversity patterns in fishes |journal=Palaeontology |date=June 2012 |volume=55 |issue=4 |pages=707–742 |doi=10.1111/j.1475-4983.2012.01165.x|bibcode=2012Palgy..55..707F |s2cid=59423401 |doi-access=free }}</ref> shows the [[Biology:Phylogeny|evolutionary relationships]] of all groups of living fishes (with their respective diversity<ref name="auto"/><ref name="Brownstein">{{Cite journal |last=Brownstein |display-authors=0 |date=27 July 2022 |title=Hidden species diversity in a living fossil vertebrate |url=https://www.biorxiv.org/content/10.1101/2022.07.25.500718v1.full.pdf |journal=Biology Letters |volume=18 |issue=11 |doi=10.1098/rsbl.2022.0395|pmid=36448369 |pmc=9709656 |biorxiv=10.1101/2022.07.25.500718 |s2cid=251162051 |doi-access=free}}</ref>) and the four-limbed vertebrates (tetrapods).
[[File:Fish evolution.png|thumb|[[Biology:Biodiversity|Diversity]] of various groups of fish (and other vertebrates) through time]]
[[File:Queensland Lungfish (Neoceratodus forsteri).jpg|thumb|Lungfish are the closest living relatives of tetrapods (four-limbed vertebrates).]]
[[File:Amia calva 4.jpg|thumb|The bowfin ''Amia calva'' is one of two surviving species of the halecomorph clade.]]

{{clade| style=font-size:100%;line-height:100%;
| label1=Vertebrates
   |1={{clade
    |1=Jawless fish [[File:Petromyzon marinus Linnaeus 1758 Fig 106 (Matschie et al. 1909).svg|100px]] (118 living species: [[Biology:Hagfish|hagfish]], lampreys)
      |label2=[[Biology:Gnathostomata|Jawed vertebrates]]
      |2={{clade
       |1=Cartilaginous fishes [[File:Notorynchus cepedianus (Broadnose sevengill shark).gif|140px]] (>1,100 living species: sharks, rays, chimaeras)
       |label2= [[Biology:Osteichthyes|Bony fishes]]
       |2={{clade
         | 1={{clade
         |label1=[[Biology:Sarcopterygii|Lobe-finned fish]]
           |1={{clade
             |label1=[[Biology:Rhipidistia|Rhipidistia]]
             |1={{clade
                |1=Tetrapoda [[File:Salamender (PSF).png|90px]] (>30,000 living species: amphibians, mammals, reptiles, birds)
                |2=Dipnoi [[File:Lepidosiren paradoxa2.jpg|140px]] (6 living species: [[Biology:Lungfish#Extant lungfish|lungfish]])
                 }}
             |2=[[Biology:Actinistia|Actinistia]] [[File:Coelacanth-bgiu.png|100px]] (2 living species: [[Biology:Coelacanth|coelacanth]]s)
     }}
         |label2=[[Biology:Actinopterygii|Ray-finned fish]]
         |2={{clade
           |1=[[Biology:Cladistia|Cladistia]] [[File:Polypterus bichir.jpg|130px]] (14 living species: bichirs, [[Biology:Reedfish|reedfish]])
           |label2=[[Biology:Actinopteri|Actinopteri]]
           |2={{clade
             |1=[[Biology:Chondrostei|Chondrostei]] [[File:Acipenser gueldenstaedtii.jpg|130px]] (27 living species: sturgeons, [[Biology:Paddlefish|paddlefish]])
             |label2=[[Biology:Neopterygii|Neopterygii]]
             |2={{clade
                |1={{clade
                |label1=[[Biology:Holostei|Holostei]]
                  |1={{clade
                   |1=[[Biology:Ginglymodi|Ginglymodi]] [[File:Lepisosteus platostomus.jpg|130px]] (7 living species: gars, [[Biology:Alligator gar|alligator gar]]s)
                   |2=[[Biology:Halecomorphi|Halecomorphi]] [[File:Amia calva.jpg|130px]] (2 living species: bowfin, [[Biology:Amia ocellicauda|eyetail bowfin]])
                   }}
                |2=Teleostei [[File:Engraulis ringens.jpg|130px]] (>32,000 living species)
   }} }}
     }} }} }} }} }} }} }}

==Anatomy and physiology==
{{Further|Biology:Fish anatomy|Biology:Fish physiology}}
[[File:Torskens indre organer.jpg|thumb|upright=1.15|[[Biology:Organ (anatomy)|Organs]]: 1.&nbsp;Liver, 2.&nbsp;Gas bladder, 3.&nbsp;Roe, 4.&nbsp;Pyloric caeca, 5.&nbsp;Stomach, 6.&nbsp;Intestine]]

===Respiration===

====Gills====
[[File:Tuna Gills in Situ 01.jpg|thumb|Tuna gills inside the head. The fish head is oriented snout-downwards, with the view looking towards the mouth.|alt=Photo of fish head split in half longitudinally with gill filaments crossing from top to bottom]]
Most fish exchange gases using [[Biology:Gill|gill]]s on either side of the [[Biology:Pharynx|pharynx]]. Gills consist of threadlike structures called [[Biology:Protein filament|filaments]]. Each filament contains a [[Biology:Capillary|capillary]] network that provides a large [[Surface area|surface area]] for exchanging [[Chemistry:Oxygen|oxygen]] and [[Chemistry:Carbon dioxide|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 [[Physics:Countercurrent exchange|countercurrent exchange]]. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like [[Biology:Shark|shark]]s and [[Biology:Lamprey|lamprey]]s, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective bony cover called an [[Biology:Operculum (fish)|operculum]].

Juvenile [[Biology:Bichir|bichir]]s have external gills, a very primitive feature that they share with larval [[Biology:Amphibian|amphibian]]s.

====Air breathing====
Fish from multiple groups can live out of the water for extended periods. [[Biology:Amphibious fish|Amphibious fish]] such as the [[Biology:Mudskipper|mudskipper]] can live and move about on land for up to several days,{{doubtful|date=December 2014}} or live in stagnant or otherwise oxygen-depleted water. Many such fish can breathe air via a variety of mechanisms. The skin of [[Biology:Anguillidae|anguillid eels]] may absorb oxygen directly. The buccal cavity of the [[Biology:Electric eel|electric eel]] may breathe air. Catfish of the families [[Biology:Loricariidae|Loricariidae]], [[Biology:Callichthyidae|Callichthyidae]], and [[Biology:Scoloplacidae|Scoloplacidae]] absorb air through their digestive tracts.<ref name="Armbruster1998">{{cite journal|last1=Armbruster|first1=Jonathan W.|year=1998|title=Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes|url=http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf|url-status=dead|journal=Copeia|volume=1998|issue=3|pages=663–675|doi=10.2307/1447796|jstor=1447796|archive-url=https://web.archive.org/web/20090326130527/http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf|archive-date=26 March 2009|access-date=25 June 2009}}</ref> [[Biology:Lungfish|Lungfish]], with the exception of the [[Biology:Australian lungfish|Australian lungfish]], and [[Biology:Bichir|bichir]]s have paired lungs similar to those of [[Biology:Tetrapod|tetrapod]]s and must surface to gulp fresh air through the mouth and pass spent air out through the gills. [[Biology:Gar|Gar]] and [[Biology:Bowfin|bowfin]] have a vascularized [[Biology:Swim bladder|swim bladder]] that functions in the same way. [[Biology:Cypriniformes|Loaches]], [[Biology:Erythrinidae|trahiras]], and many [[Biology:Catfish|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 [[Biology:Gourami|gourami]]s and [[Biology:Betta|betta]]s) have a labyrinth organ above the gills that performs this function. A few other fish have structures resembling labyrinth organs in form and function, most notably [[Biology:Snakehead (fish)|snakeheads]], pikeheads, and the [[Biology:Clariidae|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 [[Biology:Cichlid|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 extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of [[Biology:Aestivation|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, ''must'' breathe air periodically or they suffocate. Facultative air breathers, such as the catfish ''[[Biology:Hypostomus plecostomus|Hypostomus plecostomus]]'', only breathe air if they need to and will 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"/>

===Circulation===
[[File:Didactic model of a fish heart--FMVZ USP-09.jpeg|thumb|upright|Didactic model of a fish heart]]
Fish have a [[Biology:Circulatory system|closed-loop circulatory system]]. The [[Biology:Heart|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 [[Medicine:Vein|vein]]s before allowing it to flow to the second part, the atrium, which is a large muscular chamber. The atrium serves as a one-way antechamber, sends blood to the third part, [[Medicine:Ventricle (heart)|ventricle]]. The ventricle is another thick-walled, muscular chamber and it pumps the blood, first to the fourth part, [[Biology:Bulbus arteriosus|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.

===Digestion===
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 [[Biology:Esophagus|esophagus]]. In the stomach, food is further digested and, in many fish, processed in finger-shaped pouches called pyloric caeca, which secrete digestive [[Biology:Enzyme|enzyme]]s and absorb nutrients. Organs such as the [[Biology:Liver|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.

===Excretion===
As with many aquatic animals, most fish release their nitrogenous wastes as [[Chemistry:Ammonia|ammonia]]. Some of the wastes [[Physics:Diffusion|diffuse]] through the gills. Blood wastes are filtered by the kidneys.

Saltwater fish tend to lose water because of [[Physics:Osmosis|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.

===Scales===
{{Main|Biology:Fish scale}}
The scales of fish originate from the [[Biology:Mesoderm|mesoderm]] (skin); they may be similar in structure to teeth.

===Sensory and nervous system===
[[File:Fish brain.png|thumb|upright|Dorsal view of the brain of the rainbow trout|alt=Anatomical diagram showing the pairs of olfactory, telencephalon, and optic lobes, followed by the cerebellum and the mylencephalon]]

====Central nervous system====
Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} However, some fish have relatively large brains, most notably [[Biology:Mormyridae|mormyrids]] and [[Biology:Shark|shark]]s, which have brains about as massive relative to body weight as birds and [[Biology:Marsupial|marsupial]]s.{{sfn|Helfman|Collette|Facey|1997|p=191}}

Fish brains are divided into several regions. At the front are the [[Biology:Olfactory bulb|olfactory lobes]], a pair of structures that receive and process signals from the [[Medicine:Nostril|nostril]]s via the two [[Biology:Olfactory nerve|olfactory nerve]]s.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} The olfactory lobes are very large in fish that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed telencephalon, the structural equivalent to the [[Biology:Cerebrum|cerebrum]] in [[Biology:Amniote|higher vertebrates]]. In fish the telencephalon is concerned mostly with [[Biology:Olfaction|olfaction]].{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} Together these structures form the forebrain.

Connecting the forebrain to the midbrain is the [[Biology:Diencephalon|diencephalon]] (in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated with [[Biology:Hormone|hormone]]s and [[Homeostasis|homeostasis]].{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} The pineal body lies just above the diencephalon. This structure detects light, maintains circadian rhythms, and controls color changes.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}}

The [[Biology:Midbrain|midbrain]] (or mesencephalon) contains the two [[Biology:Midbrain#Corpora quadrigemina|optic lobes]]. These are very large in species that hunt by sight, such as [[Biology:Rainbow trout|rainbow trout]] and [[Biology:Cichlid|cichlid]]s.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}}

The hindbrain (or [[Biology:Metencephalon|metencephalon]]) is particularly involved in swimming and balance.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} The cerebellum is a single-lobed structure that is typically the biggest part of the brain.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} Hagfish and [[Biology:Lamprey|lamprey]]s have relatively small cerebellae, while the mormyrid cerebellum is massive and apparently involved in their [[Biology:Electric fish|electrical sense]].{{sfn|Helfman|Collette|Facey|1997|pp=48–49}}

The brain stem (or [[Biology:Myelencephalon|myelencephalon]]) is the brain's posterior.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs [[Biology:Respiration (physiology)|respiration]] and [[Biology:Osmoregulation|osmoregulation]].{{sfn|Helfman|Collette|Facey|1997|pp=48–49}}

====Sense organs====
{{main|Biology: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 [[Biology:Vision in fishes|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> The sense information obtained from the lateral line system can be considered both a sense of touch and [[Physics:Hearing|hearing]]. Blind cave fish navigate almost entirely through the sensations from their lateral line system.<ref>{{cite book |last1=Godfrey-Smith |first1=Peter |title=Metazoa |publisher=Farrar, Straus, and Giroux |location=New York|isbn=9780374207946 |chapter=Kingfish|date=10 November 2020 }}</ref>  Some fish, such as catfish and sharks, have the [[Biology:Ampullae of Lorenzini|ampullae of Lorenzini]], electroreceptors 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 ed. D.H. Evans and J.B. Claiborne (eds.). CRC Press.</ref> Other fish, like the South American electric fishes [[Biology:Gymnotiformes|Gymnotiformes]], can produce weak electric currents, which they use in navigation and social communication.

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 |last=Sciences |first=Journal of Undergraduate Life |url=http://juls.sa.utoronto.ca/Issues/JULS-Vol2Iss1/JULS-Vol2Iss1-Review3.pdf |title=Appropriate maze methodology to study learning in fish |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}}</ref>

====Vision====
{{main|Biology:Vision in fishes}}
[[Biology:Visual system|Vision]] is an important sensory system for most species of fish. Fish eyes are similar to those of [[Biology:Terrestrial animal|terrestrial]] [[Biology:Vertebrate|vertebrate]]s like [[Biology:Bird vision|birds]] and mammals, but have a more spherical [[Biology:Lens (anatomy)|lens]]. Their [[Biology:Retina|retina]]s generally have both [[Biology:Rod cell|rods]] and [[Biology:Cone cell|cones]] (for [[Biology:Scotopic vision|scotopic]] and [[Biology:Photopic vision|photopic vision]]), and most species have colour vision. Some fish can see [[Physics:Ultraviolet|ultraviolet]] and some can see [[Physics:Polarized light|polarized light]]. Amongst jawless fish, the [[Biology:Lamprey|lamprey]] has well-developed eyes, while the [[Biology:Hagfish|hagfish]] has only primitive [[Biology:Eyespot (mimicry)|eyespots]].<ref>{{cite book |last1=Campbell |first1=Neil A. |last2=Reece |first2=Jane B. |author-link2=Jane Reece |year=2005 |title=''Biology'' |edition=Seventh |publisher=Benjamin Cummings |location=San Francisco}}</ref> Fish vision shows [[Biology:Adaptation|adaptation]] to their visual environment, for example [[Earth:Deep sea fish|deep sea fish]]es have eyes suited to the dark environment.

====Hearing====

Hearing is an important sensory system for most species of fish. Fish sense sound using their [[Biology:Lateral line|lateral line]]s and their [[Biology:Otolith|ears]].

====Cognition====
{{Further|Biology:Fish intelligence}}

New research has expanded preconceptions about the cognitive capacities of fish. For example, manta rays have exhibited behavior linked to [[Philosophy:Self-awareness|self-awareness]] in [[Philosophy:Mirror test|mirror test]] cases. Placed in front of a mirror, individual rays engaged in contingency testing, that is, repetitive behavior aiming to check whether their reflection's behavior mimics their body movement.<ref>{{Cite journal|last1=Ari|first1=Csilla|last2=D’Agostino|first2=Dominic P.|date=2016-05-01|title=Contingency checking and self-directed behaviors in giant manta rays: Do elasmobranchs have self-awareness?|url=https://doi.org/10.1007/s10164-016-0462-z|journal=Journal of Ethology|language=en|volume=34|issue=2|pages=167–174|doi=10.1007/s10164-016-0462-z|s2cid=254134775|issn=1439-5444|access-date=21 January 2023|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010603/https://link.springer.com/article/10.1007/s10164-016-0462-z|url-status=live}}</ref>

Wrasses have also passed the mirror test in a 2018 scientific study.<ref>{{Cite journal|last1=Kohda|first1=Masanori|last2=Hotta|first2=Takashi|last3=Takeyama|first3=Tomohiro|last4=Awata|first4=Satoshi|last5=Tanaka|first5=Hirokazu|last6=Asai|first6=Jun-ya|last7=Jordan|first7=L. Alex|date=2018-08-21|title=Cleaner wrasse pass the mark test. What are the implications for consciousness and self-awareness testing in animals?|url=https://www.biorxiv.org/content/10.1101/397067v1|language=en|pages=397067|doi=10.1101/397067|s2cid=91375693|access-date=26 March 2019|archive-date=23 January 2020|archive-url=https://web.archive.org/web/20200123035714/https://www.biorxiv.org/content/10.1101/397067v1|url-status=live}}</ref><ref>{{Cite web|date=2019-02-07|title=Scientists find some fish can 'recognise themselves' in mirror|url=http://www.theguardian.com/environment/2019/feb/07/scientists-find-some-fish-can-recognise-themselves-in-mirror|access-date=2023-01-21|website=the Guardian|language=en|archive-date=21 January 2023|archive-url=https://web.archive.org/web/20230121015929/https://www.theguardian.com/environment/2019/feb/07/scientists-find-some-fish-can-recognise-themselves-in-mirror|url-status=live}}</ref>

Cases of tool use have also been noticed, notably in the [[Biology:Choerodon|Choerodon]] family, in [[Biology:Archerfish|archerfish]] and [[Biology:Atlantic cod|Atlantic cod]].<ref>[https://www.scientificamerican.com/article/fishes-use-problem-solving-and-invent-tools1/ Fishes Use Problem-Solving and Invent Tools] {{Webarchive|url=https://web.archive.org/web/20230317010617/https://www.scientificamerican.com/article/fishes-use-problem-solving-and-invent-tools1/ |date=17 March 2023 }}- article at [https://www.scientificamerican.com/ Scientific American] {{Webarchive|url=https://web.archive.org/web/20200518110442/https://www.scientificamerican.com/ |date=18 May 2020 }}</ref>

====Capacity for pain====

Experiments done by William Tavolga provide evidence that fish have [[Medicine:Pain|pain]] and fear responses. For instance, in Tavolga's experiments, [[Biology: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 [[Organization:University of Edinburgh|University of Edinburgh]] and the Roslin Institute concluded that rainbow trout exhibit behaviors often associated with [[Medicine:Pain|pain]] in other animals. [[Biology:Bee|Bee]] [[Biology:Venom|venom]] and [[Chemistry:Acetic acid|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>{{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 |url-status=live |archive-url=https://web.archive.org/web/20090215121355/http://news.bbc.co.uk/1/hi/sci/tech/2983045.stm |archive-date=15 February 2009}}</ref><ref name="grandin183">{{cite book |title=Animals in Translation |last=Grandin |first=Temple |author2=Johnson, Catherine |year=2005 |publisher=Scribner |location=New York City |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 [[Organization:University of Wyoming|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}}</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 [[Biology:Neocortex|neocortex]].<ref>{{cite web |last=Rose |first=James D. |url=http://www.coloradotu.org/do-fish-feel-pain/ |title=Do Fish Feel Pain? |year=2002 |access-date=27 September 2007 |url-status=dead |archive-url=https://web.archive.org/web/20130120104558/http://www.coloradotu.org/do-fish-feel-pain/ |archive-date=20 January 2013}}</ref> However, animal behaviorist [[Biography:Temple Grandin|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 [[Social:Suffering|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>{{cite news |last=Leake |first=Jonathan |date=14 March 2004 |title=Anglers to face RSPCA checks |url=https://www.thetimes.co.uk/article/anglers-to-face-rspca-checks-pg53zmc97px |newspaper=The Sunday Times |access-date=13 December 2020 |archive-date=1 May 2021 |archive-url=https://web.archive.org/web/20210501141736/https://www.thetimes.co.uk/article/anglers-to-face-rspca-checks-pg53zmc97px |url-status=live }}</ref>

====Emotion====
In 2019, scientists have shown that members of the monogamous species ''[[Biology:Amatitlania siquia|Amatitlania siquia]]'' exhibit pessimistic behavior when they are prevented from being with their partner.<ref>{{cite journal|first1=Chloé|last1=Laubu|first2=Philippe|last2=Louâpre|first3=François-Xavier|last3=Dechaume-Moncharmont|date=2019|title=Pair-bonding influences affective state in a monogamous fish species|journal=Proc. R. Soc. B|volume=286|issue=1904|at= 20190760|doi= 10.1098/rspb.2019.0760|doi-access=free|pmid= 31185864|pmc=6571461}}</ref>

===Muscular system===
{{Main|Biology:Fish locomotion}}
[[File:Lampanyctodes hectoris (Hector's lanternfish).svg|thumb|upright=1.5|The anatomy of ''Lampanyctodes hectoris'' (1)&nbsp;operculum (gill cover), (2)&nbsp;lateral line, (3)&nbsp;dorsal fin, (4)&nbsp;fat fin, (5)&nbsp;caudal peduncle, (6)&nbsp;caudal fin, (7)&nbsp;anal fin, (8)&nbsp;photophores, (9)&nbsp;pelvic fins (paired), (10)&nbsp;pectoral fins (paired)]]
[[File:Swim bladder.jpg|thumb|left|Swim bladder of a rudd (''Scardinius erythrophthalmus'')|alt=Photo of white bladder that consists of a rectangular section and a banana-shaped section connected by a much thinner element]]

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 back fin, backward force is applied to the water, and in conjunction with the fins, moves the fish forward. The 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. Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a [[Biology:Swim bladder|swim bladder]] that adjusts their buoyancy through manipulation of gases.

====Endothermy====
Although most fish are exclusively ectothermic, there are exceptions. The only known bony fishes (infraclass Teleostei) that exhibit [[Biology:Warm-blooded|endothermy]] are in the suborder [[Biology:Scombroidei|Scombroidei]] – which includes the [[Biology:Billfish|billfish]]es, tunas, and the [[Biology:Butterfly kingfish|butterfly kingfish]], a [[Biology:Basal (phylogenetics)|basal]] species of mackerel<ref name="Block1993">{{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=https://www.researchgate.net/publication/226138605 |format=PDF |journal=Environmental Biology of Fishes |volume=40 |issue=3 |pages=283–302 |doi=10.1007/BF00002518 |s2cid=28644501 |access-date=1 October 2018 |archive-date=6 November 2020 |archive-url=https://web.archive.org/web/20201106144640/https://www.researchgate.net/publication/226138605 |url-status=live }}</ref> – and also the [[Biology:Lampris guttatus|opah]]. The opah, a lampriform, was demonstrated in 2015 to use "whole-body endothermy", generating heat with its swimming muscles to warm its body while countercurrent exchange (as in respiration) minimizes heat loss.<ref>{{Cite journal |last1=Wegner |first1=Nicholas C. |last2=Snodgrass |first2=Owyn E. |last3=Dewar |first3=Heidi |last4=Hyde |first4=John R. |date=2015-05-15 |title=Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus |journal=Science |volume=348 |issue=6236 |pages=786–789 |doi=10.1126/science.aaa8902 |issn=0036-8075 |pmid=25977549|bibcode=2015Sci...348..786W |s2cid=17412022 }}</ref> It is able to actively hunt prey such as squid and swim for long distances due to the ability to warm its entire body, including its heart,<ref>{{Cite web|date=2015-05-12|title=Warm Blood Makes Opah an Agile Predator|url=https://swfsc.noaa.gov/news.aspx?ParentMenuId=39&id=20466|url-status=dead|archive-url=https://web.archive.org/web/20180120000911/https://swfsc.noaa.gov/news.aspx?ParentMenuId=39&id=20466|archive-date=2018-01-20|access-date=2018-03-07|website=Southwest Fisheries Science Center}}</ref> which is a trait typically found in only mammals and birds (in the form of [[Biology:Homeothermy|homeothermy]]).<!-- Mentioned in Wegner 2015 --> In the cartilaginous fishes (class Chondrichthyes), sharks of the families [[Biology:Lamnidae|Lamnidae]] (porbeagle, mackerel, salmon, and great white sharks) and [[Biology:Alopiidae|Alopiidae]] (thresher sharks) exhibit endothermy. The degree of endothermy varies from the billfishes, which warm only their eyes and brain, to the [[Biology:Bluefin tuna|bluefin tuna]] and the porbeagle shark, which maintain body temperatures in excess of {{convert|20|C}} above ambient water temperatures.<ref name="Block1993" />

Endothermy, though metabolically costly, is thought to provide advantages such as increased muscle strength, higher rates of central [[Biology:Nervous system|nervous system]] processing, and higher rates of [[Biology:Digestion|digestion]].

{{Anchor|Reproductive method}}

===Reproductive system===
{{Further|Biology:Fish reproduction|Biology:Spawn}}
[[File:Fish ovary- FMVZ USP-27 (cropped).jpg|thumb|upright|Ovary of fish (Corumbatá)]]

Fish reproductive organs include testicles and [[Biology:Ovary|ovaries]]. In most species, gonads are paired organs of similar size, which can be partially or totally fused.<ref name="Guimaraes" /> There may also be a range of secondary organs that increase reproductive fitness.

In terms of spermatogonia distribution, the structure of [[Biology:Teleost|teleost]]s testes has two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in atherinomorph fish they are confined to the [[Biology:Anatomical terms of location#Proximal and distal|distal]] portion of these structures. Fish can present cystic or semi-cystic [[Biology:Spermatogenesis|spermatogenesis]] in relation to the release phase of germ cells in cysts to the seminiferous tubules [[Biology:Lumen (anatomy)|lumen]].<ref name="Guimaraes">{{cite journal |author=Guimaraes-Cruz, Rodrigo J. |author2=dos Santos, José E. |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 |doi-access=free }}</ref>

Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the [[Biology:Oocyte|oocyte]]s are released directly into the [[Biology:Coelom|coelom]]ic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct.<ref name="Brito">{{cite journal |title=Reproduction of the surubim catfish (Pisces, Pimelodidae) in the São Francisco River, Pirapora Region, Minas Gerais, Brazil |last=Brito |first=M.F.G. |author2=Bazzoli, N. |journal=Arquivo Brasileiro de Medicina Veterinária e Zootecnia |volume=55 |issue=5 |year=2003 |doi=10.1590/S0102-09352003000500018 |issn=0102-0935 |pages=624–633|doi-access=free }}</ref> Gymnovaries are the primitive condition found in [[Biology:Lungfish|lungfish]], [[Biology:Sturgeon|sturgeon]], and [[Biology:Bowfin|bowfin]]. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.<ref name="Guimaraes"/> Secondary gymnovaries are found in salmonids and a few other teleosts.

Oogonia development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilization processes. Changes in the [[Biology:Cell nucleus|nucleus]], ooplasm, and the surrounding layers characterize the oocyte maturation process.<ref name="Guimaraes" />

Postovulatory follicles 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 [[Biology:Apoptosis|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, like the [[Biology:California sheephead|California sheephead]], are [[Biology:Hermaphrodite|hermaphrodite]]s, having both testes and ovaries either at different phases in their life cycle or, as in [[Biology: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, [[Biology:Goldfish|goldfish]], [[Biology:Cichlid|cichlid]]s, [[Biology:Tuna|tuna]], and [[Biology:Eel|eel]]s. In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their [[Biology:Gamete|gamete]]s into the surrounding water. However, a few oviparous fish practice internal fertilization, with the male using some sort of intromittent organ to deliver sperm into the genital opening of the female, most notably the oviparous sharks, such as the [[Biology:Horn shark|horn shark]], and oviparous rays, such as [[Biology:Skate (fish)|skates]]. In these cases, the male is equipped with a pair of modified [[Biology:Pelvic fin|pelvic fin]]s known as [[Biology:Clasper|clasper]]s.

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|sigfig=1}}.
<gallery widths="90" heights="150">
File:Oeufs002b,57.png|Egg of [[Biology:Lamprey|lamprey]]
File:Oeufs002b,54.png|Egg of [[Biology:Catshark|catshark]] (mermaids' purse)
File:Oeufs002b,55.png|Egg of [[Biology:Bullhead shark|bullhead shark]]
File:Oeufs002b,56.png|Egg of [[Biology:Chimaera|chimaera]]
</gallery>

<!-- Deleted image removed: [[File:Fishegg A.Slotwinski TAFI.jpg|thumb|An example of zooplankton|alt=Photo of semi-transparent creature with a darker, yolk-like central structure and other approximately round internal elements]] -->
The newly hatched young of oviparous fish are called [[Biology:Ichthyoplankton|larvae]]. 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 [[Biology:Metamorphosis|metamorphosis]]) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on [[Earth:Zooplankton|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 fertilization 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 [[Biology:Guppy|guppies]], angel sharks, and [[Biology:Coelacanth|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 [[Biology:Placenta|placenta]] seen in mammals connecting the mother's blood supply with that of the embryo. Examples of viviparous fish include the [[Biology:Embiotocidae|surf-perches]], [[Biology:Goodeidae|splitfins]], and [[Biology:Lemon shark|lemon shark]]. Some viviparous fish exhibit [[Biology:Oophagy|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 [[Biology:Porbeagle|porbeagle]], but is known for a few bony fish as well, such as the [[Biology:Halfbeak|halfbeak]] ''Nomorhamphus ebrardtii''.<ref name="Meisner">Meisner, A & Burns, J: Viviparity in the Halfbeak Genera ''Dermogenys'' and ''Nomorhamphus'' (Teleostei: Hemiramphidae)" ''Journal of Morphology'' 234, pp. 295–317, 1997</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" />

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

== Acoustic communication ==

Acoustic communication in fish involves the transmission of acoustic signals from one individual of a species to another. The production of sounds as a means of communication among fish is most often used in the context of feeding, aggression or courtship behaviour.<ref name="Bioacoustics"/> The sounds emitted can vary depending on the species and stimulus involved. Fish can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder.<ref name="Journal">{{cite journal|last1=Bertucci|first1=F.|last2=Ruppé|first2=L.|last3=Wassenbergh|first3=S.V.|last4=Compère|first4=P.|last5=Parmentier|first5=E.|date=29 October 2014|title=New Insights into the Role of the Pharyngeal Jaw Apparatus in the Sound-Producing Mechanism of Haemulon Flavolineatum (Haemulidae).|journal=Journal of Experimental Biology|volume=217|issue=21|pages=3862–3869|doi=10.1242/jeb.109025|pmid=25355850|doi-access=free|hdl=10067/1197840151162165141|hdl-access=free}}</ref>

===Stridulatory===
[[File:French grunts - Haemulon flavolineatum.jpg|thumb|French grunts – ''Haemulon flavolineatum'']]
There are some species of fish that can produce sounds by rubbing or grinding their bones together. These noises produced by bone-on-bone interactions are known as 'stridulatory sounds'.<ref name="Journal" />

An example of this is seen in ''[[Biology:Haemulon flavolineatum|Haemulon flavolineatum]]'', a species commonly referred to as the 'French grunt fish', as it produces a grunting noise by grinding its teeth together.<ref name="Journal" /> This behaviour is most pronounced when the ''H. flavolineatum'' is in distress situations.<ref name="Journal" /> The grunts produced by this species of fishes generate a frequency of approximately 700&nbsp;Hz, and last approximately 47 milliseconds.<ref name="Journal" /> The ''H. flavolineatum'' does not emit sounds with frequencies greater than 1000&nbsp;Hz, and does not detect sounds that have frequencies greater than 1050&nbsp;Hz.<ref name="Journal" />

In a study conducted by Oliveira et al. (2014), the longsnout seahorse, ''Hippocampus reidi'', was recorded producing two different categories of sounds; 'clicks' and 'growls'. The sounds emitted by the ''H. reidi'' are accomplished by rubbing their coronet bone across the grooved section of their neurocranium.<ref>{{cite journal|last1=Colson|first1=D.J.|last2=Patek|first2=S.N.|last3=Brainerd|first3=E.L.|last4=Lewis|first4=S.M.|title=Sound production during feeding in Hippocampus seahorses (Syngnathidae)|journal=Environmental Biology of Fishes|date=February 1998|volume=51|issue=2|pages=221–229|doi=10.1023/A:1007434714122|bibcode=1998EnvBF..51..221C |s2cid=207648816}}</ref> 'Clicking' sounds were found to be primarily produced during courtship and feeding, and the frequencies of clicks were within the range of 50&nbsp;Hz-800&nbsp;Hz.<ref name="Zoology">{{cite journal|last1=Oliveira|first1=T.P.R.|last2=Ladich|first2=F.|last3=Abed-Navandi|first3=D.|last4=Souto|first4=A.S.|last5=Rosa|first5=I.L.|title=Sounds produced by the longsnout seahorse: a study of their structure and functions|journal=Journal of Zoology|date=26 June 2014|volume=294|issue=2|pages=114–121|doi=10.1111/jzo.12160}}</ref> The frequencies were noted to be on the higher end of the range during spawning periods, when the female and male fishes were less than fifteen centimeters apart.<ref name="Zoology" /> Growl sounds were produced when the ''H. reidi'' encountered stressful situations, such as handling by researchers.<ref name="Zoology" /> The 'growl' sounds consist of a series of sound pulses and are emitted simultaneously with body vibrations.<ref name="Zoology" />

===Non-stridulatory===
[[File:C(flipped).jpg|thumb|{{center|Oyster toadfish}}]]
Some fish species create noise by engaging specialized muscles that contract and cause swimbladder vibrations.

[[Biology:Oyster toadfish|Oyster toadfish]] produce loud grunting sounds by contracting muscles located along the sides of their swim bladder, known as sonic muscles<ref name="Experimental">{{cite journal|last1=Fine|first1=L.F.|last2=King|first2=C.B.|last3=Cameron|first3=T.M.|title=Acoustical properties of the swimbladder in the oyster toadfish Opsanus tau|journal=Journal of Experimental Biology|date=16 October 2009|volume=212|issue=21|pages=3542–3552|doi=10.1242/jeb.033423|pmid=19837896|pmc=2762879}}</ref> Female and male toadfishes emit short-duration grunts, often as a fright response.<ref>{{cite journal|last1=Fine|first1=M.L.|last2=Waybright|first2=T.D.|title=Grunt variation in the oyster toadfish Opsanus tau:effect of size and sex|journal=PeerJ|date=15 October 2015|volume=3|issue=1330|pages=e1330|doi=10.7717/peerj.1330|pmid=26623178|pmc=4662586 |doi-access=free }}</ref> In addition to short-duration grunts, male toadfishes produce "boat whistle calls".<ref name="plos" /> These calls are longer in duration, lower in frequency, and are primarily used to attract mates.<ref name="plos" /> The sounds emitted by the ''O. tao'' have frequency range of 140&nbsp;Hz to 260&nbsp;Hz.<ref name="plos">{{cite journal|last1=Ricci|first1=S. W.|last2=Bohnenstiehl|first2=D R.|last3=Eggleston|first3=D.B.|last4=Kellogg|first4=M.L.|last5=Lyon|first5=R.P.|title=Oyster toadfish (Opsanus tau) boatwhistle call detection and patterns within a large-scale oyster restoration site|journal=PLOS ONE|date=8 August 2017|volume=12|issue=8|pages=e0182757|doi=10.1371/journal.pone.0182757|pmid=28792543|pmc=5549733|bibcode=2017PLoSO..1282757R|doi-access=free}}</ref> The frequencies of the calls depend on the rate at which the sonic muscles contract.<ref>{{cite journal|last1=Skoglund|first1=C.R.|title=Functional analysis of swimbladder muscles engaged in sound productivity of the toadfish|journal=Journal of Cell Biology|date=1 August 1961|volume=10|issue=4|pages=187–200|doi=10.1083/jcb.10.4.187|pmid=19866593|pmc=2225107}}</ref><ref name="Experimental" />

The red drum, ''Sciaenops ocellatus'', produces drumming sounds by vibrating its swimbladder.<ref name="Elsevier">{{cite journal|last1=Parmentier|first1=E.|last2=Tock|first2=J.|last3=Falguière|first3=J.C.|last4=Beauchaud|first4=M.|title=Sound production in Sciaenops ocellatus: Preliminary study for the development of acoustic cues in aquaculture|journal=Aquaculture|date=22 May 2014|volume=432|pages=204–211|doi=10.1016/j.aquaculture.2014.05.017|url=https://archimer.ifremer.fr/doc/00190/30127/28589.pdf|access-date=21 January 2019|archive-date=3 June 2021|archive-url=https://web.archive.org/web/20210603172200/https://archimer.ifremer.fr/doc/00190/30127/28589.pdf|url-status=live}}</ref> Vibrations are caused by the rapid contraction of sonic muscles that surround the dorsal aspect of the swimbladder.<ref name="Elsevier" /> These vibrations result in repeated sounds with frequencies that range from 100 to >200&nbsp;Hz.<ref name="Elsevier" /> The ''S. ocellatus'' can produce different calls depending on the stimuli involved.<ref name="Elsevier" /> The sounds created in courtship situations are different from those made during distressing events such as predatorial attacks.<ref name="Elsevier" /> Unlike the males of the ''S. ocellatus'' species, the females of this species do not produce sounds and lack sound-producing (sonic) muscles.<ref name="Elsevier" />

==Diseases==
Like other animals, fish suffer from diseases and parasites. To prevent disease they have a variety of defenses. ''Non-specific'' defenses include the skin and scales, as well as the mucus layer secreted by the epidermis that traps and inhibits the growth of [[Biology:Microorganism|microorganism]]s. If [[Biology:Pathogen|pathogen]]s breach these defenses, fish can develop an [[Medicine:Inflammation|inflammatory response]] that increases blood flow to the infected region and delivers white blood cells that attempt to destroy pathogens. Specific defenses respond to particular pathogens recognised by the fish's body, i.e., an [[Biology:Immune response|immune response]].{{sfn|Helfman|Collette|Facey|1997|pp=95–96}} In recent years, [[Biology:Vaccine|vaccine]]s have become widely used in aquaculture and also with ornamental fish, for example furunculosis vaccines in farmed salmon and koi herpes virus in koi.<ref>{{cite report|author=R.C. Cipriano|date=2001|title=Furunculosis And Other Diseases Caused By ''Aeromonas salmonicida''|series=Fish Disease Leaflet 66|publisher=U.S. Department of the Interior|url= http://www.lsc.usgs.gov/FHB/leaflets/FHB66.pdf|access-date=3 July 2009|url-status=dead|archive-url= https://web.archive.org/web/20090507003140/http://www.lsc.usgs.gov/FHB/leaflets/FHB66.pdf|archive-date=7 May 2009|author2=G.L. Bullock}}</ref><ref>{{cite web|first=K.H.|last=Hartman|year=2004|title=Koi Herpes Virus (KHV) Disease: Fact Sheet VM-149|publisher=University of Florida Institute of Food and Agricultural Sciences|url= http://edis.ifas.ufl.edu/pdffiles/VM/VM11300.pdf|display-authors=0|archive-url= https://web.archive.org/web/20070206131841/http://edis.ifas.ufl.edu/pdffiles/VM/VM11300.pdf|archive-date=6 February 2007|url-status=live}}</ref>

Some species use [[Earth:Cleaner fish|cleaner fish]] to remove external parasites. The best known of these are the [[Biology:Bluestreak cleaner wrasse|bluestreak cleaner wrasse]]s of the genus ''Labroides'' found on [[Earth:Coral reef|coral reef]]s in the [[Earth:Indian Ocean|Indian]] and [[Earth:Pacific Ocean|Pacific]] oceans. These small fish maintain so-called "cleaning stations" where other fish congregate and perform specific movements to attract the attention of the cleaners.{{sfn|Helfman|Collette|Facey|1997|p=380}} Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, ''[[Biology:Orange chromide|Etroplus maculatus]]'', the cleaner, and the much larger ''Etroplus suratensis''.<ref>{{cite journal |last1=Wyman |first1=Richard L. |last2=Ward |first2=Jack A. |year=1972 |title=A Cleaning Symbiosis between the Cichlid Fishes Etroplus maculatus and Etroplus suratensis. I. Description and Possible Evolution |journal=Copeia |volume=1972 |issue=4 |pages=834–838 |doi=10.2307/1442742|jstor=1442742 }}</ref>

===Immune system===
Immune organs vary by type of fish.<ref>A.G. Zapata, A. Chiba and A. Vara. ''Cells and tissues of the immune system of fish.'' In: The Fish Immune System: Organism, Pathogen and Environment. Fish Immunology Series. (eds. G. Iwama and T.Nakanishi,), New York, Academic Press, 1996, pp. 1–55.</ref> In the jawless fish (lampreys and hagfish), true [[Biology:Lymph|lymph]]oid organs are absent. These fish rely on regions of lymphoid tissue within other organs to produce immune cells. For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature.) They resemble primitive [[Biology:Bone marrow|bone marrow]] in hagfish. Cartilaginous fish (sharks and rays) have a more advanced immune system. They have three specialized organs that are unique to Chondrichthyes; the epigonal organs (lymphoid tissue similar to mammalian bone) that surround the gonads, the [[Biology:Leydig's organ|Leydig's organ]] within the walls of their esophagus, and a [[Biology:Spiral valve|spiral valve]] in their intestine. These organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable [[Biology:Thymus|thymus]] and a well-developed [[Biology:Spleen|spleen]] (their most important immune organ) where various [[Biology:Lymphocyte|lymphocyte]]s, plasma cells and macrophages develop and are stored. Chondrostean fish (sturgeons, paddlefish, and bichirs) possess a major site for the production of granulocytes within a mass that is associated with the [[Medicine:Meninges|meninges]] (membranes surrounding the central nervous system.) Their heart is frequently covered with tissue that contains lymphocytes, [[Biology:Reticular cell|reticular cell]]s and a small number of [[Biology:Macrophage|macrophage]]s. The chondrostean kidney is an important hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop.

Like chondrostean fish, the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney), which houses many different immune cells.<ref>D.P. Anderson. ''Fish Immunology''. (S.F. Snieszko and H.R. Axelrod, eds), Hong Kong: TFH Publications, Inc. Ltd., 1977.</ref> In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads). Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.<ref>{{cite journal |last=Chilmonczyk |first=S. |title=''The thymus in fish: development and possible function in the immune response'' |journal=Annual Review of Fish Diseases |volume=2 |year=1992 |pages=181–200 |doi=10.1016/0959-8030(92)90063-4}}</ref><ref>{{cite journal |last1=Hansen |first1=J.D. |last2=Zapata |first2=A.G. |year=1998 |title=''Lymphocyte development in fish and amphibians'' |journal=Immunological Reviews |volume=166 |pages=199–220 |doi=10.1111/j.1600-065x.1998.tb01264.x|pmid=9914914 |s2cid=7965762 }}</ref> In 2006, a lymphatic system similar to that in mammals was described in one species of teleost fish, the [[Biology:Zebrafish|zebrafish]]. Although not confirmed as yet, this system presumably will be where naive (unstimulated) [[Biology:T cell|T cell]]s accumulate while waiting to encounter an [[Biology:Antigen|antigen]].<ref>{{cite journal|vauthors=Küchler AM, Gjini E, Peterson-Maduro J, Cancilla B, Wolburg H, Schulte-Merker S|title=Development of the Zebrafish Lymphatic System Requires Vegfc Signaling|journal=[[Biology:Current Biology|Current Biology]]|volume=16|issue=12|year=2006|pages=1244–1248|doi=10.1016/j.cub.2006.05.026|pmid=16782017|doi-access=free|url=https://core.ac.uk/download/pdf/82535963.pdf|access-date=5 June 2020|archive-date=6 November 2020|archive-url=https://web.archive.org/web/20201106124655/https://core.ac.uk/download/pdf/82535963.pdf|url-status=live}}</ref>

B and T lymphocytes bearing immunoglobulins and T cell receptors, respectively, are found in all jawed fishes. Indeed, the [[Biology:Adaptive immune system|adaptive immune system]] as a whole [[Biology:Adaptive immune system#Evolution|evolved]] in an ancestor of all jawed vertebrates.<ref>{{cite journal|last1=Flajnik|first1=M. F.|last2=Kasahara|first2=M.|year=2009|title=Origin and evolution of the adaptive immune system: genetic events and selective pressures|journal=Nature Reviews Genetics|volume=11|issue=1|pages=47–59|doi= 10.1038/nrg2703|pmid= 19997068|pmc= 3805090}}</ref>

==Conservation==
The 2006 [[Organization:International Union for Conservation of Nature|IUCN]] [[Organization:IUCN Red List|Red List]] names 1,173 fish species that are threatened with extinction.<ref>{{cite web |url=http://www.iucnredlist.org/info/tables/table1 |title=Table 1: Numbers of threatened species by major groups of organisms (1996–2004) |access-date=18 January 2006 |archive-url=https://web.archive.org/web/20060630054235/http://www.iucnredlist.org/info/tables/table1 |publisher=iucnredlist.org |archive-date=30 June 2006 |url-status=dead}}</ref> Included are species such as Atlantic cod,<ref>{{cite iucn |author=Sobel, J. |date=1996 |title=''Gadus morhua'' |volume=1996 |page=e.T8784A12931575 |doi=10.2305/IUCN.UK.1996.RLTS.T8784A12931575.en |access-date=11 November 2021}}</ref> Devil's Hole pupfish,<ref>{{cite iucn |author=NatureServe |date=2014 |title=''Cyprinodon diabolis'' |volume=2014 |page=e.T6149A15362335 |doi=10.2305/IUCN.UK.2014-3.RLTS.T6149A15362335.en |access-date=11 November 2021}}</ref> [[Biology:Coelacanth|coelacanth]]s,<ref>{{cite iucn |author=Musick, J.A. |date=2000 |title=''Latimeria chalumnae'' |volume=2000 |page=e.T11375A3274618 |doi=10.2305/IUCN.UK.2000.RLTS.T11375A3274618.en |access-date=11 November 2021}}</ref> and [[Biology:Great white shark|great white shark]]s.<ref>{{cite iucn |last1=Rigby |first1=C.L. |last2=Barreto |first2=R. |last3=Carlson |first3=J. |last4=Fernando |first4=D. |last5=Fordham |first5=S. |last6=Francis |first6=M.P. |last7=Herman |first7=K. |last8=Jabado |first8=R.W. |last9=Liu |first9=K.M. |last10=Lowe |first10=C.G. |last11=Marshall |first11=A. |last12=Pacoureau |first12=N. |last13=Romanov |first13=E. |last14=Sherley |first14=R.B. |last15=Winker |first15=H. |year=2019 |title=''Carcharodon carcharias'' |page=e.T3855A2878674 |access-date=19 December 2019}}</ref> Because fish live underwater they are more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small water bodies. For example, the Devil's Hole pupfish occupies only a single {{convert|3|by|6|m|0}} pool.{{sfn|Helfman|Collette|Facey|1997|pp=449–450}}

===Overfishing===
[[File:Whale shark Georgia aquarium.jpg|thumb|[[Biology:Whale shark|Whale shark]]s, the largest species of fish, are classified as endangered.|alt=Photo of shark in profile surrounded by other, much smaller fish in bright sunlight]]

Overfishing is a major threat to edible fish such as cod and [[Biology:Tuna|tuna]].<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/6234881.stm |title=Call to halt cod 'over-fishing' |access-date=18 January 2006 |work=BBC News |date=5 January 2007 |url-status=live |archive-url=https://web.archive.org/web/20070117142906/http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/6234881.stm |archive-date=17 January 2007}}</ref><ref>{{cite news |url=http://news.bbc.co.uk/1/hi/world/asia-pacific/6301187.stm |title=Tuna groups tackle overfishing |access-date=18 January 2006 |work=BBC News |date=26 January 2007 |url-status=live |archive-url=https://web.archive.org/web/20090121111204/http://news.bbc.co.uk/1/hi/world/asia-pacific/6301187.stm |archive-date=21 January 2009}}</ref> Overfishing eventually causes population (known as stock) collapse because the survivors cannot produce enough young to replace those removed. Such '''commercial extinction''' does not mean that the species is extinct, merely that it can no longer sustain a fishery.

One well-studied example of fishery collapse is the Pacific sardine ''Sadinops sagax caerulues'' fishery off the California coast. From a 1937 peak of {{convert|790000|LT}} the catch steadily declined to only {{convert|24000|LT}} in 1968, after which the fishery was no longer economically viable.{{sfn|Helfman|Collette|Facey|1997|p=462}}

The main tension between fisheries science and the fishing industry is that the two groups have different views on the resiliency of fisheries to intensive fishing. In places such as Scotland, Newfoundland, and Alaska the fishing industry is a major employer, so governments are predisposed to support it.<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/uk/6112352.stm |title=UK 'must shield fishing industry' |access-date=18 January 2006 |work=BBC News |date=3 November 2006 |url-status=live |archive-url=https://web.archive.org/web/20061130055349/http://news.bbc.co.uk/1/hi/uk/6112352.stm |archive-date=30 November 2006}}</ref><ref>{{cite news |url=http://news.bbc.co.uk/1/hi/world/europe/6197433.stm |title=EU fish quota deal hammered out |access-date=18 January 2006 |work=BBC News |date=21 December 2006 |url-status=live |archive-url=https://web.archive.org/web/20061226234953/http://news.bbc.co.uk/1/hi/world/europe/6197433.stm |archive-date=26 December 2006}}</ref> On the other hand, scientists and conservationists push for stringent protection, warning that many stocks could be wiped out within fifty years.<ref>{{cite web|title=Ocean study predicts the collapse of all seafood fisheries by 2050|url=http://www.physorg.com/news81778444.html|url-status=live|archive-url=https://web.archive.org/web/20070315014325/http://www.physorg.com/news81778444.html|archive-date=15 March 2007|access-date=13 January 2006|website=phys.org}}</ref><ref>{{cite web |url=http://www.panda.org/about_wwf/where_we_work/europe/what_we_do/mediterranean/about/marine/bluefin_tuna/tuna_at_risk/index.cfm |title=Atlantic bluefin tuna could soon be commercially extinct |access-date=18 January 2006 |archive-url=https://web.archive.org/web/20070430205610/http://www.panda.org/about_wwf/where_we_work/europe/what_we_do/mediterranean/about/marine/bluefin_tuna/tuna_at_risk/index.cfm |archive-date=30 April 2007 |url-status=dead}}</ref>

===Habitat destruction===

A key stress on both freshwater and marine ecosystems is habitat degradation including [[Earth:Water pollution|water pollution]], the building of dams, removal of water for use by humans, and the introduction of [[Earth:Invasive species|exotic]] species.{{sfn|Helfman|Collette|Facey|1997|p=463}} An example of a fish that has become endangered because of habitat change is the [[Biology:Pallid sturgeon|pallid sturgeon]], a North American freshwater fish that lives in rivers damaged by human activity.<ref>{{cite web|title=Threatened and Endangered Species: Pallid Sturgeon ''Scaphirhynchus'' Fact Sheet|url=http://www.mt.nrcs.usda.gov/news/factsheets/pallidsturgeon.html|url-status=dead|archive-url=https://web.archive.org/web/20051126220804/http://www.mt.nrcs.usda.gov/news/factsheets/pallidsturgeon.html|archive-date=26 November 2005|access-date=18 March 2016}}</ref>

===Exotic species===
Introduction of [[Earth:Invasive species|non-native]] species occurs in many habitats. A notable case in point is the [[Place:Mediterranean Sea|Mediterranean Sea]] which has become a major 'hotspot' of exotic invaders since the opening of the Suez Canal in 1869.  Since that time a thousand marine species of all sorts – fishes, seaweeds, invertebrates – originating from the Red Sea and more broadly from the Indo-Pacific have crossed the Canal from south to north to settle in the eastern Mediterranean Basin. Nowadays many of these tropical migrants, also called '''Lessepsian species''', have extended their range towards the west, obviously favoured by the general warming of the Mediterranean. The resulting change in biodiversity is without precedent in human memory and is accelerating: a long-term cross-Basin survey engaged by the Mediterranean Science Commission recently documented that in just twenty years, from 2001 till 2021, no less than 107 alien fish species have reached the Mediterranean from both the tropical Atlantic and the Red Sea, which is more than the total recorded during the whole 130 preceding years.<ref>Atlas of Exotic Fishes in the Mediterranean Sea. 2nd Edition. 2021. (F. Briand Ed.) CIESM Publishers, Paris, Monaco 366 p.[https://ciesm.org/catalog/index.php?article=2021] {{Webarchive|url=https://web.archive.org/web/20221206224806/https://ciesm.org/catalog/index.php?article=2021|date=6 December 2022}}</ref>

Another mode of introduction for marine species is transport across thousands of kms on ship hulls or in ballast waters. Examples abound of marine organisms being transported in [[Earth:Ballast water discharge and the environment|ballast water]], among them the invasive comb jelly ''Mnemiopsis leidyi'', the dangerous bacterium ''Vibrio cholerae'', or the fouling [[Biology:Zebra mussel|zebra mussel]]. The Mediterranean and Black Seas, with their high volume shipping from exotic harbors, are particularly impacted by this problem.<ref>Briand, Frederic and Galil, Bella 2002. Alien marine organisms introduced by ships – An overview https://www.researchgate.net/publication/240305530</ref>

Deliberate introductions of species with market potential are another frequent vector: one of the best studied examples is the introduction of the [[Biology:Nile perch|Nile perch]] into Lake Victoria in the 1960s. Nile perch gradually exterminated the lake's 500 [[Biology:Endemism|endemic]] [[Biology:Cichlid|cichlid]] species. Some of them now survive in captive breeding programmes, but others are probably extinct.<ref>{{cite news |url=https://www.theguardian.com/life/feature/story/0,,1541613,00.html |title=The little fish fight back |access-date=18 January 2006 |work=The Guardian |location=London |first=Laura |last=Spinney |date=4 August 2005 |archive-date=9 November 2020 |archive-url=https://web.archive.org/web/20201109032106/https://www.theguardian.com/science/2005/aug/04/1 |url-status=live }}</ref> Carp, [[Biology:Channidae|snakeheads]],<ref>{{cite news |url=https://www.washingtonpost.com/wp-dyn/articles/A16439-2002Jul2.html |title=Stop That Fish! |access-date=26 August 2007 |newspaper=The Washington Post |date=3 July 2002 |url-status=live |archive-url=https://web.archive.org/web/20121103055232/http://www.washingtonpost.com/wp-dyn/articles/A16439-2002Jul2.html |archive-date=3 November 2012}}</ref> [[Biology:Tilapia|tilapia]], European perch, [[Biology:Brown trout|brown trout]], [[Biology:Rainbow trout|rainbow trout]], and [[Biology:Sea lamprey|sea lamprey]]s are other examples of fish that have caused problems by being introduced into alien environments.

==Importance to humans==
===Economic===
[[File:Community fish-farming ponds in the rural town of Masi Manimba, DRC (7609946524).jpg|alt=A row of square artificial ponds, with trees on either side|thumb|These fish-farming ponds were created as a [[Finance:Cooperative|cooperative]] project in a rural village.]]
[[File:Oulu Market Hall 20170617 04.jpg|thumb|Fish counter display at the Oulu Market Hall in Oulu, Finland.]]
Throughout history, humans have used fish as a food source for dietary protein. Historically and today, most fish harvested for human consumption has come by means of catching wild fish. However, fish farming, which has been practiced since about 3,500 BCE in ancient China,<ref>{{cite magazine |last1=Spalding |first1=Mark |title=Sustainable Ancient Aquaculture |url=http://voices.nationalgeographic.com/2013/07/11/sustainable-ancient-aquaculture/ |magazine=National Geographic |access-date=13 August 2015 |date=11 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150518210243/http://voices.nationalgeographic.com/2013/07/11/sustainable-ancient-aquaculture/ |archive-date=18 May 2015}}</ref> is becoming increasingly important in many nations. Overall, about one-sixth of the world's protein is estimated to be provided by fish.<ref name="Helfman">{{cite book |last1=Helfman |first1=Gene S. |title=Fish Conservation: A Guide to Understanding and Restoring Global Aquatic Biodiversity and Fishery Resources |date=2007 |publisher=Island Press |isbn=978-1-59726-760-1 |pages=11}}</ref> That proportion is considerably elevated in some developing nations and regions heavily dependent on seafood. In a similar manner, fish have been tied to primary industry and associated food, feed, [[Medicine:Pharmaceutical industry|pharmaceutical]] production and service industries.

Catching fish for the purpose of food or sport is known as fishing, while the organized effort by humans to catch fish is called a fishery (which also describes the area where such enterprise operates). Fisheries are a huge global business and provide income for millions of people.<ref name="Helfman" /> The annual yield from all fisheries worldwide is about 154 million tons,<ref>{{cite web |title=World Review of Fisheries and Aquaculture |url=http://www.fao.org/docrep/016/i2727e/i2727e01.pdf |website=fao.org |publisher=Food and Agriculture Organization of the United Nations |access-date=13 August 2015 |url-status=live |archive-url=https://web.archive.org/web/20150828131307/http://www.fao.org/docrep/016/i2727e/i2727e01.pdf |archive-date=28 August 2015}}</ref> with popular species including [[Biology:Herring|herring]], [[Biology:Cod|cod]], [[Biology:Anchovy|anchovy]], [[Biology:Tuna|tuna]], [[Biology:Flounder|flounder]], and salmon. However, the term fishery is broadly applied, and includes more organisms than just fish, such as [[Biology:Mollusca|mollusks]] and [[Biology:Crustacean|crustacean]]s, which are often collectively called "shellfish" when used as food.

The [[Organization:Environmental Defense Fund|Environmental Defense Fund]] (EDF) has a guide<ref name=EDFfish>[https://seafood.edf.org/ EDF Seafood Selector: Fish Choices that are Good for You and the Oceans] on the Environmental Defense Fund (EDF) website.  Last access 1/21/2024.</ref> to which fish are healthiest (or least safe) to eat, given the state of pollution in today's world, and also which fish are obtained in a way that does not lead towards extinction of the fish.

===Recreation===
[[File:Vendor in Fish Market - Sylhet - Bangladesh (12968716714).jpg|thumb|A [[Social:Bengalis|Bengali]] fish vendor from Sylhet]]
==== Fishkeeping ====
{{main|Biology:Fishkeeping}}
Fish have been recognized as a source of beauty for almost as long as used for food, appearing in cave art, being raised as ornamental fish in ponds, and displayed in aquariums in homes, offices, or public settings.

==== Recreational fishing ====
''Recreational fishing'' is fishing primarily for pleasure or competition; it can be contrasted with commercial fishing, which is fishing for profit, or artisanal fishing, which is fishing primarily for food. The most common form of recreational fishing is done with a [[Engineering:Fishing rod|rod]], [[Engineering:Fishing reel|reel]], [[Engineering:Fishing line|line]], [[Engineering:Fish hook|hooks]], and any one of a wide range of [[Engineering:Bait (luring substance)|baits]].  Recreational fishing is particularly popular in North America and Europe and state, provincial, and federal government agencies actively management target fish species.<ref>{{Cite book|title=The Angler in the Environment: Social, Economic, Biological, and Ethical Dimensions|publisher=American Fisheries Society|year=2011|isbn=978-1-934874-24-0|editor-last=Beard|editor-first=T. Douglas|location=Bethesda, MD|pages=365}}</ref><ref>{{Cite book|title=Recreational Fisheries: Social, Economic and Management Aspects|publisher=Wiley-Blackwell|year=1998|isbn=978-0-852-38248-6|editor-last=Hickley|editor-first=Phil|pages=328|editor-last2=Tompkins|editor-first2=Helena}}</ref> Angling is a method of fishing, specifically the practice of catching fish by means of an "angle" (hook). Anglers must select the right hook, cast accurately, and retrieve at the right speed while considering water and weather conditions, species, fish response, time of the day, and other factors.

=== Culture ===
[[File:Matsya painting.jpg|thumb|upright|right|[[Philosophy:Avatar|Avatar]] of Vishnu as a Matsya|alt=Photo of painting showing blue-skinned, 4-armed upper body of man standing in the opened mouth of a fish with bent tail with other, paler men facing him with hands raised together]]
[[File:Goldfish Matisse.jpg|thumb|upright|''Goldfish'' by Henri Matisse, 1912, Pushkin Museum of Fine Arts, Moscow]]
Fish themes have symbolic significance in many religions. In ancient [[Place:Mesopotamia|Mesopotamia]], fish offerings were made to the gods from the very earliest times.<ref name="BlackGreen1992">{{cite book|last1=Black|first1=Jeremy|first2=Anthony|last2=Green|title=Gods, Demons and Symbols of Ancient Mesopotamia: An Illustrated Dictionary|url=https://books.google.com/books?id=05LXAAAAMAAJ&q=Inana|publisher=The British Museum Press|year=1992|isbn=978-0-7141-1705-8|pages=82–83|url-status=live|archive-url=https://web.archive.org/web/20180220073403/https://books.google.com/books?id=05LXAAAAMAAJ&q=Inana|archive-date=20 February 2018}}</ref> Fish were also a major symbol of [[Religion:Enki|Enki]], the god of water.<ref name="BlackGreen1992"/> Fish frequently appear as filling motifs in cylinder seals from the Old Babylonian ({{circa}} 1830 BC – {{circa}} 1531 BC) and [[History:Neo-Assyrian Empire|Neo-Assyrian]] (911–609 BC) periods.<ref name="BlackGreen1992"/> Starting during the Kassite Period ({{circa}} 1600 BC – {{circa}} 1155 BC) and lasting until the early [[Place:Achaemenid Empire|Persian Period]] (550–30 BC), healers and exorcists dressed in ritual garb resembling the bodies of fish.<ref name="BlackGreen1992"/> During the [[Place:Seleucid Empire|Seleucid Period]] (312–63 BC), the legendary Babylonian culture hero Oannes, described by [[Biography:Berossus|Berossus]], was said to have dressed in the skin of a fish.<ref name="BlackGreen1992"/> Fish were sacred to the Syrian goddess Atargatis<ref name="Hyde2008">{{cite book|last=Hyde|first=Walter Woodburn|date=2008|orig-year=1946|title=Paganism to Christianity in the Roman Empire|url=https://books.google.com/books?id=H_VLAwAAQBAJ&q=Atargatis|location=Eugene, Oregon|publisher=Wipf and Stock Publishers|isbn=978-1-60608-349-9|pages=57–58|access-date=12 December 2020|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010624/https://books.google.com/books?id=H_VLAwAAQBAJ&q=Atargatis|url-status=live}}</ref> and, during her festivals, only her priests were permitted to eat them.<ref name="Hyde2008"/>

[[File:Ichthus.svg|thumb|left|The [[Ichthys|ichthus]] is a Christian symbol of a fish signifying that the person who uses it is a Christian.<ref name="Hyde2008"/><ref name="Coffman2008"/>]]

In the Book of Jonah, a work of Jewish literature probably written in the fourth century BC, the central figure, a [[Religion:Prophet|prophet]] named Jonah, is swallowed by a giant fish after being thrown overboard by the crew of the ship he is travelling on.<ref name="Sherwood">{{citation|last=Sherwood|first=Yvonne|date=2000|title=A Biblical Text and Its Afterlives: The Survival of Jonah in Western Culture|url=https://books.google.com/books?id=Kivijc6ZX5QC&q=Jonah|location=Cambridge, England|publisher=Cambridge University Press|isbn=978-0-521-79561-6|pages=1–8|access-date=12 December 2020|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010619/https://books.google.com/books?id=Kivijc6ZX5QC&q=Jonah|url-status=live}}</ref><ref name="Ziolkowski2007">{{cite book|last=Ziolkowski|first=Jan M.|date=2007|title=Fairy Tales from Before Fairy Tales: The Medieval Latin Past of Wonderful Lies|url=https://books.google.com/books?id=mJ12yPfBL4IC&q=Lucian%27s%20True%20Story%20and%20the%20Book%20of%20Jonah&pg=PA80|location=Ann Arbor, Michigan|publisher=University of Michigan Press|isbn=978-0-472-03379-9|page=80|access-date=12 December 2020|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010623/https://books.google.com/books?id=mJ12yPfBL4IC&q=Lucian%27s%20True%20Story%20and%20the%20Book%20of%20Jonah&pg=PA80|url-status=live}}</ref><ref name="Gaines">{{cite book|last=Gaines|first=Janet Howe|date=2003|title=Forgiveness in a Wounded World: Jonah's Dilemma|url=https://books.google.com/books?id=eLpZ83tJVX4C&q=parody%20in%20the%20Book%20of%20Jonah&pg=PA22|location=Atlanta, Georgia|publisher=Society of Biblical Literature|isbn=978-1-58983-077-6|pages=8–9|access-date=12 December 2020|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010612/https://books.google.com/books?id=eLpZ83tJVX4C&q=parody%20in%20the%20Book%20of%20Jonah&pg=PA22|url-status=live}}</ref> The fish later vomits Jonah out on shore after three days.<ref name="Sherwood"/><ref name="Ziolkowski2007"/><ref name="Gaines"/> This book was later included as part of the [[Religion:Hebrew Bible|Hebrew Bible]], or Christian [[Religion:Old Testament|Old Testament]],<ref>{{cite book|last=Band|first=Arnold J.|date=2003|title=Studies in Modern Jewish Literature|series=JPS Scholar of Distinction Series|url=https://books.google.com/books?id=mHo50J7fGe4C&q=The%20Book%20of%20Jonah%20as%20a%20work%20of%20satire&pg=PA107|location=Philadelphia, Pennsylvania|publisher=The Jewish Publication Society|isbn=978-0-8276-0762-0|pages=106–107|access-date=12 December 2020|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010629/https://books.google.com/books?id=mHo50J7fGe4C&q=The%20Book%20of%20Jonah%20as%20a%20work%20of%20satire&pg=PA107|url-status=live}}</ref><ref>{{cite book|last=Person|first=Raymond|date=1996|title=In Conversation with Jonah: Conversation Analysis, Literary Criticism, and the Book of Jonah|location=Sheffield, England|publisher=Sheffield Academic Press|url=https://books.google.com/books?id=rYKtAwAAQBAJ&q=The%20Book%20of%20Jonah%20as%20a%20work%20of%20satire&pg=PA155|isbn=978-1-85075-619-4|page=155|access-date=12 December 2020|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010623/https://books.google.com/books?id=rYKtAwAAQBAJ&q=The%20Book%20of%20Jonah%20as%20a%20work%20of%20satire&pg=PA155|url-status=live}}</ref> and a version of the story it contains is summarized in [[Religion:Surah|Surah]] 37:139-148 of the [[Religion:Quran|Quran]].<ref>{{citation|last=Vicchio|first=Stephen J.|date=2008|title=Biblical Figures in the Islamic Faith|url=https://books.google.com/books?id=GULkDAAAQBAJ&q=Islamic%20views%20on%20Jonah&pg=PA69|location=Eugene, Oregon|publisher=Wipf & Stock|isbn=978-1-55635-304-8|page=67|access-date=12 December 2020|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010613/https://books.google.com/books?id=GULkDAAAQBAJ&q=Islamic%20views%20on%20Jonah&pg=PA69|url-status=live}}</ref> [[Religion:Early Christianity|Early Christians]] used the ''[[Ichthys|ichthys]]'', a symbol of a fish, to represent Jesus,<ref name="Hyde2008"/><ref name="Coffman2008">{{cite magazine |url=http://www.christianitytoday.com/ch/asktheexpert/oct26.html |first=Elesha |last=Coffman |title=What is the origin of the Christian fish symbol? |magazine=Christianity Today |date=8 August 2008 |access-date=13 August 2015 |archive-date=30 January 2016 |archive-url=https://web.archive.org/web/20160130092733/http://www.christianitytoday.com/ch/asktheexpert/oct26.html |url-status=live }}</ref> because the Greek word for fish, ΙΧΘΥΣ Ichthys, could be used as an acronym for "Ίησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ" (Iesous Christos, Theou Huios, Soter), meaning "Jesus Christ, Son of God, Saviour".<ref name="Hyde2008"/><ref name="Coffman2008"/> The [[Religion:Gospel|gospel]]s also refer to "fishers of men"<ref>Matthew 4:19</ref> and feeding the multitude. In the dhamma of [[Religion:Buddhism|Buddhism]], the fish symbolize happiness as they have complete freedom of movement in the water.

Among the [[Religion:Deity|deities]] said to take the form of a fish are Ika-Roa of the Polynesians, [[Unsolved:Dagon#Fish-god tradition|Dagon]] of various ancient Semitic peoples, the shark-gods of Hawai{{okina}}i and [[Unsolved:Matsya|Matsya]] of the Hindus. The [[Unsolved:Astrology|astrological]] symbol [[Unsolved:Pisces (astrology)|Pisces]] is based on a constellation of the [[Astronomy:Pisces (constellation)|same name]], but there is also a second fish constellation in the night sky, [[Astronomy:Piscis Austrinus|Piscis Austrinus]].<ref>{{cite web |title=Piscis Austrinus |url=http://www.allthesky.com/constellations/piscisaustrinus/constell.html |website=allthesky.com |publisher=The Deep Photographic Guide to the Constellations |access-date=1 November 2015 |url-status=live |archive-url=https://web.archive.org/web/20151125044838/http://www.allthesky.com/constellations/piscisaustrinus/constell.html |archive-date=25 November 2015}}</ref>

Fish feature prominently in art and literature, in movies such as ''Finding Nemo'' and books such as ''The Old Man and the Sea''. Large fish, particularly sharks, have frequently been the subject of horror movies and thrillers, most notably the novel ''Jaws'', which spawned a series of films of the same name that in turn inspired similar films or parodies such as ''Shark Tale'' and ''Snakehead Terror''. Piranhas are shown in a similar light to sharks in films such as ''Piranha''; however, contrary to popular belief, the [[Biology:Red-bellied piranha|red-bellied piranha]] is actually a generally timid scavenger species that is unlikely to harm humans.<ref>{{cite web |last1=Zollinger |first1=Sue Anne |title=Piranha – Ferocious Fighter or Scavenging Softie? |url=http://indianapublicmedia.org/amomentofscience/piranhaferocious-fighter-scavenging-softie/ |website=A Moment of Science |publisher=Indiana Public Media |access-date=1 November 2015 |date=3 July 2009 |url-status=live |archive-url=https://web.archive.org/web/20151017010303/http://indianapublicmedia.org/amomentofscience/piranhaferocious-fighter-scavenging-softie/ |archive-date=17 October 2015}}</ref> Legends of half-human, half-fish [[Unsolved:Mermaid|mermaid]]s have featured in folklore, including the stories of Hans Christian Andersen.

{{clear}}

==Terminology==
==="Fish" or "fishes"===
Though often used interchangeably, in biology these words have different meanings. ''Fish'' is used as a singular noun, or as a plural to describe multiple individuals from a single species. ''Fishes'' is used to describe different species or species groups.<ref name="Pauly2004">{{cite encyclopedia|last=Pauly|first=Daniel|title=Fish(es)|encyclopedia=Darwin's Fishes: An Encyclopedia of Ichthyology, Ecology, and Evolution|url= https://books.google.com/books?id=lTDa0fM4vQIC&pg=PA77|date=13 May 2004|publisher=Cambridge University Press|isbn=978-1-139-45181-9|page=77|url-status=live|archive-url= https://web.archive.org/web/20160208060837/https://books.google.com/books?id=lTDa0fM4vQIC&pg=PA77|archive-date=8 February 2016}}</ref><ref>{{cite book|last1=Nelson|first1=Joseph S.|last2=Paetz|first2=Martin Joseph|date=1992|url= http://www.uap.ualberta.ca/images/pdfs/9780888642363.pdf|title=The Fishes of Alberta|url-status=dead|archive-url= https://web.archive.org/web/20140407085249/http://www.uap.ualberta.ca/images/pdfs/9780888642363.pdf|archive-date=7 April 2014|page=400|publisher=University of Alberta|isbn=978-0-88864-236-3}}</ref>{{sfn|Helfman|Collette|Facey|1997|p=5}} Thus a pond would be said to contain 120 fish if all were from a single species or 120 fishes if these included a mix of several species. The distinction is similar to that between people and peoples.

==="True fish" or "finfish"=== <!-- Courtesy note per WP:RSECT: True fish, Fin-fish and Finfish redirect here -->
* In biology, the term ''fish'' is most strictly used to describe any animal with a [[Biology:Vertebral column|backbone]], [[Biology:Gill|gill]]s throughout life, and limbs (if any) in the shape of [[Biology:Fin|fin]]s.{{sfn|Nelson|2006|p=2}} Many types of aquatic animals with common names ending in "fish" are not fish in this [[Biology:Fish#Diversity|sense]]; examples include shellfish, [[Biology:Cuttlefish|cuttlefish]], [[Biology:Starfish|starfish]], crayfish and [[Biology:Jellyfish|jellyfish]]. In earlier times, even biologists did not make a distinction&nbsp;– sixteenth century natural historians classified also [[Biology:Pinniped|seals]], whales, [[Biology:Amphibian|amphibian]]s, [[Biology:Crocodile|crocodile]]s, even [[Biology:Hippopotamus|hippopotamus]]es, as well as a host of aquatic invertebrates, as fish.<ref name="integrated" />
* In fisheries, the term ''fish'' is used as a collective term, and includes mollusks, [[Biology:Crustacean|crustacean]]s and any aquatic animal which is harvested.<ref name="fao glossary">{{cite web|publisher=FAO|url=http://www.fao.org/fi/glossary/default.asp|title=Fisheries|website=FAO TERM PORTAL|access-date=15 January 2014|archive-date=8 July 2007|archive-url=https://web.archive.org/web/20070708081658/http://www.fao.org/fi/glossary/default.asp|url-status=live}}</ref>
* The strict biological definition of a fish, above, is sometimes called a ''true fish''. True fish are also referred to as ''finfish'' or ''fin fish'' to distinguish them from other aquatic life harvested in fisheries or aquaculture.

==="Shoal" or "school"===
{{Main|Biology:Shoaling and schooling}}
[[File:School of Pterocaesio chrysozona in Papua New Guinea 1.jpg|thumb|These [[Biology:Goldband fusilier|goldband fusilier]]s are schooling because their swimming is synchronised.|alt=Photo of thousands of fish separated from each other by distances of {{convert|2|in}} or less]]

An assemblage of fish merely using some localised resource such as food or nesting sites is known simply as an ''aggregation''. When fish come together in an interactive, social grouping, then they may be forming either a ''shoal'' or a ''school'' depending on the degree of organisation. A ''shoal'' is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. ''Schools'' of fish are much more tightly organised, synchronising their swimming so that all fish move at the same speed and in the same direction. Shoaling and schooling behaviour is believed to provide a variety of advantages.{{sfn|Helfman|Collette|Facey|1997|p=375}}

Examples:
* Cichlids congregating at lekking sites form an ''aggregation''.
* Many minnows and characins form ''shoals''.
* Anchovies, herrings and silversides are classic examples of ''schooling'' fish.

The most common collective nouns for a group of fish in general are school and shoal. Both the words have evolved from the same common Dutch root 'schole' meaning a troop or crowd. While the words "school" and "shoal" have different meanings within biology, the distinctions are often ignored by non-specialists who treat the words as synonyms. Thus speakers of British English commonly use "shoal" to describe any grouping of fish, and speakers of American English commonly use "school" just as loosely.<ref>{{cite book|last1=Pitcher|first1=T.J.|last2=Parish|first2=J.K.|date=1993|chapter-url= https://books.google.com/books?id=10lElCT7v5wC&pg=PA363|chapter=Functions of shoaling behaviour in teleosts|editor-last=Pitcher|editor-first=T.J.|title=Behaviour of teleost fishes|publisher=Chapman and Hall|location=New York|isbn=9780412429309|pages=363–440|url-status=live|archive-url= https://web.archive.org/web/20170405001403/https://books.google.com/books?id=10lElCT7v5wC&pg=PA363|archive-date=5 April 2017}}</ref>

==See also==
{{Main|Biology:Outline of fish}}
<!-- Please keep alphabetical -->
{{columns-list|colwidth=22em|
* Angling (sport fishing)
* Aquaculture
* Aquarium
* Catch and release
* [[Earth:Deep sea fish|Deep sea fish]]
* Fish acute toxicity syndrome
* [[Biology:Fish anatomy|Fish anatomy]]
* Fish as food
* [[Biology:Fish development|Fish development]]
* Fishing (fishing for food)
* [[Biology:Fishkeeping|Fishkeeping]]
* [[Biology:Forage fish|Forage fish]]
* [[Biology:Ichthyology|Ichthyology]]
* [[Biology:List of fish common names|List of fish common names]]
* [[Biology:List of fish families|List of fish families]]
* [[Biology:Marine biology|Marine biology]]
* Marine vertebrates
* [[Chemistry:Mercury in fish|Mercury in fish]]
* [[Biology:Otolith|Otolith]] (Bone used for determining the age of a fish)
* Pregnancy (fish)
* Seafood
* [[Biology:Shoaling and schooling|Shoaling and schooling]]
* [[Biology:Walking fish|Walking fish]]
}}

==Notes==
{{Reflist}}

==References==
{{refbegin}}
* {{cite book |last1=Helfman |first1=G. |last2=Collette |first2=B. |last3=Facey |first3=D. |title=The Diversity of Fishes |publisher=Wiley-Blackwell |year=1997 |isbn=978-0-86542-256-8 |edition=1st}}
* {{cite book |last=Nelson |first=Joseph S. |year=2006 |title=Fishes of the World |edition=4th |publisher=John Wiley & Sons |isbn=978-0-471-75644-6 |url= http://www.somas.stonybrook.edu/~conover/Textbooks/Fishes%2520of%2520the%2520World%2C%25204th%2520ed.%2520Joseph%2520S.%2520Nelson.pdf |access-date=30 April 2013 |url-status=dead |archive-url= https://web.archive.org/web/20130305052159/http://www.somas.stonybrook.edu/~conover/Textbooks/Fishes%20of%20the%20World%2C%204th%20ed.%20Joseph%20S.%20Nelson.pdf |archive-date=5 March 2013}}
{{refend}}

==Further reading==
{{refbegin}}
* {{cite web|ref=none|last1=Eschmeyer|first1=William N.|last2=Fong|first2=Jon David|year=2013|url=http://researcharchive.calacademy.org/research/ichthyology/|title=Catalog of Fishes|publisher=California Academy of Sciences|access-date=28 February 2013|archive-date=21 November 2018|archive-url=https://web.archive.org/web/20181121203659/http://researcharchive.calacademy.org/research/ichthyology/|url-status=live}}
* {{cite book|ref=none|last1=Helfman|first1=G.|last2=Collette|first2=B.|last3=Facey|first3=D.|last4=Bowen|first4=B.|year=2009|url=http://www.blackwellpublishing.com/helfman/|title=The Diversity of Fishes: Biology, Evolution, and Ecology|publisher=Wiley-Blackwell|isbn=978-1-4051-2494-2|edition=2nd|access-date=26 January 2010|archive-date=26 August 2021|archive-url=https://web.archive.org/web/20210826212914/http://www.blackwellpublishing.com/helfman/|url-status=live}}
* Moyle, Peter B. (1993) [https://books.google.com/books?id=cu-J9tqy4IQC&q=Fish%3A+An+Enthusiast%27s+Guide ''Fish: An Enthusiast's Guide''] {{Webarchive|url=https://web.archive.org/web/20230317010703/https://books.google.com/books?id=cu-J9tqy4IQC&q=Fish%3A+An+Enthusiast%27s+Guide |date=17 March 2023 }}  University of California Press. {{ISBN|978-0-520-91665-4}} – good lay text.
* {{cite book|ref=none|last1=Moyle|first1=Peter B.|last2=Cech|first2=Joseph J.|year=2003|title=Fishes, An Introduction to Ichthyology|edition=5th|publisher=Benjamin Cummings|isbn=978-0-13-100847-2}}
* {{cite book|ref=none|last=Scales|first=Helen|year=2018|title=Eye of the shoal: A Fishwatcher's Guide to Life, the Ocean and Everything|publisher=Bloomsbury Sigma|isbn=978-1-4729-3684-4}}
* {{cite book|ref=none|last=Shubin|first=Neil|year=2009|url=https://books.google.com/books?id=c008kdNwR1cC|title=Your inner fish: A journey into the 3.5 billion year history of the human body|publisher=Vintage Books|isbn=978-0-307-27745-9|access-date=15 December 2015|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317010712/https://books.google.com/books?id=c008kdNwR1cC|url-status=live}} [http://www.uctv.tv/search-details.aspx?showID=16412 ''UCTV'' interview] {{Webarchive|url=https://web.archive.org/web/20210114070358/https://books.google.com/books?id=c008kdNwR1cC |date=14 January 2021 }}
{{refend}}

==External links==
{{EB1911 poster|Fish}}
* [https://db.angfa.org.au/ ANGFA] – Illustrated database of freshwater fishes of Australia and New Guinea
* {{Webarchive |url=https://web.archive.org/web/20111130234923/http://fischinfos.de/ |date=30 November 2011 |title=Fischinfos.de – Illustrated database of the freshwater fishes of Germany}} {{in lang|de}}
* [https://www.fishbase.se/search.php FishBase online] – Comprehensive database with information on over 29,000 fish species
* {{Webarchive |url=https://archive.today/20121215023321/http://fishdata.siu.edu/ |date=15 December 2012 |title=Fisheries and Illinois Aquaculture Center – Data outlet for fisheries and aquaculture research center in the central US}}
* [https://www.poppe-images.com/ Philippines Fishes] – Database with thousands of Philippine Fishes photographed in natural habitat
* {{Webarchive |url=https://web.archive.org/web/20080312223952/http://www.nativefish.org/ |date=12 March 2008 |title=The Native Fish Conservancy – Conservation and study of North American freshwater fishes}}
* [https://www.fao.org/fishery/en/topic/2888 United Nation] – Fisheries and Aquaculture Department: Fish and seafood utilization
* [https://web.archive.org/web/20121101120959/http://www.tacoma.uw.edu/library/digital-collections University of Washington Libraries Digital Collections] – Digital collection of freshwater and marine fish images
* {{Cite NIE |last=Davenport |first=Charles B. |last2=Ingersoll |first2=Ernest |wstitle=Fish |year=1905 |short=x}}
* {{cite web | title=Fish | website=Better Health Channel | url=http://www.betterhealth.vic.gov.au/node/1990 | access-date=26 Aug 2023}} – Health tips from the State government of Victoria, Australia

{{diversity of fish|state=expanded}}
{{Aquatic ecosystems}}
{{Toxins}}
{{Brood parasite}}
{{Taxonbar|from=Q152}}

[[Category:Aquatic ecology]]
[[Category:Ichthyology]]
[[Category:Paraphyletic groups]]

{{Sourceattribution|Fish}}