Editing Teleost (section)

== Evolution and phylogeny ==

{{main|Evolution of fish}}

=== External relationships ===

The teleosts were first recognised as a distinct group by the German [[ichthyologist]] [[Johannes Peter Müller]] in 1845.<ref name=Greenwood/> The name is from [[Ancient Greek|Greek]] ''teleios'', "complete" + ''osteon'', "bone".<ref>{{cite web |title=Teleost |url=http://www.merriam-webster.com/dictionary/teleost |publisher=[[Merriam-Webster]] |access-date=20 April 2016}}</ref> Müller based this classification on certain soft tissue characteristics, which would prove to be problematic, as it did not take into account the distinguishing features of fossil teleosts. In 1966, Greenwood et al. provided a more solid classification.<ref name=Greenwood>{{cite journal |last=Greenwood |first=P. |author2=Rosen, D. |author3=Weitzman, S. |author4=Myers, G. |title=Phyletic studies of teleostean fishes, with a provisional classification of living forms |journal=Bulletin of the American Museum of Natural History |date=1966 |volume=131 |pages=339–456 |url=https://digitallibrary.amnh.org/bitstreams/f4604e7e-adca-4295-9984-1cfd549abf04/download |hdl=2246/1678}}</ref><ref>{{cite journal |last=Arratia |first=G. |year=1998 |title=Basal teleosts and teleostean phylogeny: response to C. Patterson |journal=[[Copeia]] |volume=1998 |issue=4 |pages=1109–1113 |jstor=1447369 |doi=10.2307/1447369}}</ref> The oldest fossils of teleosteomorphs (the [[stem group]] from which teleosts later evolved) date back to the [[Triassic]] [[period (geology)|period]] (''[[Prohalecites]]'', ''[[Pholidophorus]]'').<ref name="Arratia 2015">{{cite journal |author1=Arratia, G. |name-list-style=amp |year=2015 |title=Complexities of early teleostei and the evolution of particular morphological structures through time. |journal=Copeia |volume=103 |issue=4 |pages=999–1025 |doi=10.1643/CG-14-184 |s2cid=85808890 }}</ref><ref name="Romano et al 2016">{{cite journal |last1=Romano |first1=Carlo |last2=Koot |first2=Martha B. |last3=Kogan |first3=Ilja |last4=Brayard |first4=Arnaud |last5=Minikh |first5=Alla V. |last6=Brinkmann |first6=Winand |last7=Bucher |first7=Hugo |last8=Kriwet |first8=Jürgen |title=Permian-Triassic Osteichthyes (bony fishes): diversity dynamics and body size evolution |journal=Biological Reviews |date=February 2016 |volume=91 |issue=1 |pages=106–147 |doi=10.1111/brv.12161 | pmid=25431138  |s2cid=5332637 |url=https://hal.science/hal-01253154 }}</ref> However, it has been suggested that teleosts probably first evolved already during the [[Paleozoic]] [[era (geology)|era]].<ref name=PNAS /> During the [[Mesozoic]] and [[Cenozoic]] eras they diversified widely, and as a result, 96% of all living fish species are teleosts.<ref name=Berra />

The [[cladogram]] below shows the [[phylogeny|evolutionary relationships]] of the teleosts to other [[extant taxa|extant]] [[clade]]s of bony fish,<ref name=PNAS>{{cite journal |title=Resolution of ray-finned fish phylogeny and timing of diversification |last=Near |first=Thomas J. |journal=[[Proceedings of the National Academy of Sciences of the United States of America|PNAS]] |doi=10.1073/pnas.1206625109 | pmid=22869754 |date=2012 |volume=109 |issue=34 |pages=13698–13703 |display-authors=etal | pmc=3427055|bibcode=2012PNAS..10913698N |doi-access=free }}</ref> and to the four-limbed vertebrates ([[tetrapod]]s) that [[Evolution of fish#post devonian|evolved]] from a related group of bony fish during the [[Devonian]] [[period (geology)|period]].<ref name=TOL>{{cite journal |last=Betancur-R. |first=Ricardo |display-authors=etal |year=2013 |title=The Tree of Life and a New Classification of Bony Fishes |journal=[[PLOS Currents|PLOS Currents: Tree of Life]] |volume=5 |edition=1st |doi=10.1371/currents.tol.53ba26640df0ccaee75bb165c8c26288 | pmid=23653398 | pmc=3644299 |hdl=2027.42/150563 |doi-access=free }}</ref><ref name=laurin&reisz1995>{{cite journal |last1=Laurin |first1=M. |last2=Reisz |first2=R.R. |year=1995 |title=A reevaluation of early amniote phylogeny |journal=[[Zoological Journal of the Linnean Society]] |volume=113 |issue=2 |pages=165–223 |doi=10.1111/j.1096-3642.1995.tb00932.x}}</ref><!--The former "[[Chondrostei]]" is seen to be [[paraphyly|paraphyletic]]. --> Approximate [[divergent evolution|divergence dates]] (in millions of years, [[myr|mya]]) are from Near et al., 2012.<ref name=PNAS/>

{{clade|style=font-size:90%;line-height:90%;
|label1=[[Euteleostomi]]/ |sublabel1=[[Osteichthyes]]
|1={{clade
  |label1=[[Sarcopterygii]] (lobe-fins)
  |1={{clade
    |label1=[[Actinistia]]
    |1=[[Coelacanth]]s [[File:Coelacanth flipped.png|70 px]] 
    |label2=[[Rhipidistia]]
    |2={{clade
      |label1=[[Dipnoi]]
      |1=[[Dipnoi|Lungfish]] <span style="{{MirrorH}}">[[File:Chinle fish Arganodus cropped cropped.png|70 px]]</span>
      |label2=[[Tetrapoda]]
      |2={{clade
        |label1=[[Amphibia]]
        |1=[[Lissamphibia]] [[File:Salamandra salamandra (white background).jpg|70 px]]
        |label2=[[Amniota]]
        |2={{clade
          |1=[[Mammal]]s [[File:Phylogenetic tree of marsupials derived from retroposon data (Paucituberculata).png|60 px]]
          |2=[[Sauropsida]] ([[reptile]]s, [[bird]]s) [[File:British reptiles, amphibians, and fresh-water fishes (1920) (Lacerta agilis).jpg|70px]]
           }}
         }}
       }}
     }}
  |label2=[[Actinopterygii]] (ray-fins)
  |sublabel2=400 mya
  |2={{clade
    |label1=[[Cladistia]]
    |1=[[Polypteriformes]] ([[bichir]]s, [[reedfish]]es) [[File:Cuvier-105-Polyptère.jpg|80px]]
    |label2=[[Actinopteri]]
    |2={{clade
      |label1=[[Chondrostei]]
      |1=[[Acipenseriformes]] ([[sturgeon]]s, [[paddlefish]]es) [[File:Atlantic sturgeon flipped.jpg|80px]]
      |label2=[[Neopterygii]]
      |sublabel2=360 mya
      |2={{clade
        |label1=[[Holostei]]
        |sublabel1=275 mya
        |1={{clade
          |label1=[[Ginglymodi]]
          |1=[[Lepisosteiformes]] ([[gar]]s) [[File:Alligator gar fish (white background).jpg|80px]]
          |label2=[[Halecomorphi]]
          |2=[[Amiiformes]] ([[bowfin]]) [[File:Amia calva (white background).jpg|70px]]
           }}
        |2='''Teleostei''' [[File:Common carp (white background).jpg|70px]]
        |sublabel2=310 mya 
         }}
       }}
     }}
   }}
}}

=== Internal relationships ===

The phylogeny of the teleosts has been subject to long debate, without consensus on either their [[Phylogenetic tree|phylogeny]] or the timing of the emergence of the major groups before the application of modern [[DNA]]-based cladistic analysis. Near et al. (2012) explored the phylogeny and divergence times of every major lineage, analysing the DNA sequences of 9 unlinked genes in 232 species. They obtained well-resolved phylogenies with strong support for the nodes (so, the pattern of branching shown is likely to be correct). They calibrated (set actual values for) branching times in this tree from 36 reliable measurements of absolute time from the fossil record.<ref name=PNAS/> The teleosts are divided into the major clades shown on the cladogram,<ref>{{cite web |website=[[Deepfin]] |author=Betancur-R |display-authors=etal |title=Phylogenetic Classification of Bony Fishes Version 4 |url=https://sites.google.com/site/guilleorti/classification-v-4 |year=2016 |access-date=30 December 2016 |archive-date=11 July 2017 |archive-url=https://web.archive.org/web/20170711171156/https://sites.google.com/site/guilleorti/classification-v-4 |url-status=dead }}</ref> with dates, following Near et al.<ref name=PNAS/><!--<ref name=UCL>{{cite web |title=Vertebrate Diversity: Actinopterygii - ray-finned fishes |url=http://www.ucl.ac.uk/museums-static/obl4he/vertebratediversity/rayfinned_fishes.html |publisher=[[University College London]]|access-date=31 December 2015}}</ref>--> More recent research divide the teleosts into two major groups: Eloposteoglossocephala (Elopomorpha + Osteoglossomorpha) and Clupeocephala (the rest of the teleosts).<ref>[https://www.fau.edu/newsdesk/articles/teleost-fishes-ancestral-lineage.php Study Resolves 50-Year Dispute of Teleost Fishes Ancestral Lineage]</ref><ref>[https://hal.science/hal-03765882/document Genome structures resolve the early diversification of teleost fishes]</ref>

{{clade |style=font-size:90%;line-height:90%
|label1='''Teleostei'''
|sublabel1=310 mya
|1={{clade
|label1=Eloposteoglossocephala
|1={{clade
   |1={{clade
     |label1=[[Osteoglossomorpha]]
      |1={{clade
      |1=[[Hiodontiformes]] ([[mooneye]]s) [[File:Hiodon tergisus NOAA.jpg|60 px]]
      |sublabel2=250 mya 
      |2=[[Osteoglossiformes]] ([[bonytongue]]s, [[Mormyridae|elephantfishes]]) [[File:Osteoglossum bicirrhosum (white background).jpg|70px]]
      }}
    }}
     |label2=[[Elopomorpha]]
   |2={{clade
       |1=[[Elopiformes]] ([[tenpounder]]s, [[tarpon]]s) <span style="{{MirrorH}}">[[File:Tarpon (PSF).png|70 px]]</span>
    |2={{clade
      |1=[[Albuliformes]] ([[Japanese gissu]]s and [[bonefish]]es) [[File:Albula conorhynchus - 1700-1880 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg|70px]]
      |2={{clade
        |1=[[Notacanthiformes]] (deep sea spiny eels) [[File:Notacanthus sexspinis1.jpg|90px]]
        |2=[[Anguilliformes]] (true [[eel]]s) <span style="{{MirrorH}}">[[File:Conger conger Gervais.jpg|70px]]</span>
           }}
         }}
       }}
     }}
    |label2=[[Clupeocephala]]
    |2={{clade
      |label1=[[Otocephala]] |sublabel1=230 mya
      |1={{clade
        |label1=[[Clupeomorpha|Clupei]]
        |1=[[Clupeiformes]] ([[herrings]]) [[File:Blueback herring fish (white background).jpg|70px]]
        |2={{clade
          |label1=Alepocephali
          |1=[[Alepocephaliformes]] ([[slickhead]]s) [[File:Alepocephalus rostratus Gervais.jpg|70px]]
          |label2=[[Ostariophysi]]
          |2={{clade
            |label1=Anotophysa
            |1=[[Gonorynchiformes]] ([[milkfish]])<span style="{{MirrorH}}">[[File:Chanos salmoneus Achilles 166.jpg|70px]]</span>
            |label2=[[Otophysi|Otophysa]]
            |2={{clade
              |1=[[Cypriniformes]] ([[minnow]]s, [[carp]]s, [[loach]]es)[[File:Common carp (white background).jpg|70px]]
              |2={{clade
                |1=[[Characiformes]] ([[tetras]], [[piranha]]s)<span style="{{MirrorH}}">[[File:Cynopotamus argenteus.jpg|70px]]</span>
                |2={{clade
                  |1=[[Gymnotiformes]] (knifefish and [[electric eel]]s) [[File:Johann Natterer - Ituí-cavalo (Apteronotus albifrons).jpg|70px]]
                  |2=[[Siluriformes]] (catfish) [[File:Black bullhead fish (white background).jpg|70px]]
                   }}
                 }}
               }}
             }}
           }}
         }}
      |label2=[[Euteleostei]] |sublabel2=240 mya
      |2={{clade
        |1={{clade
           |label1=Lepidogalaxii
           |1=[[Lepidogalaxiiformes]] (salamanderfish)
           }}
        |2={{clade
          |1={{clade
             |label1=[[Protacanthopterygii]] |sublabel1=225 mya
             |1={{clade
                |1=[[Argentiniformes]] (marine smelts)[[File:Argentina sphyraena.jpg|70 px]]
                |2={{clade
                   |1=[[Galaxiiformes]] ([[whitebait]], mudfishes)<span style="{{MirrorH}}">[[File:Galaxias maculatus.jpg|70px]]</span>
                   |2={{clade
                      |1=[[Esociformes]] ([[Esox|pike]]) <span style="{{MirrorH}}">[[File:Esox lucius1.jpg|70px]]</span>
                      |2=[[Salmoniformes]] ([[salmon]], [[trout]]) [[File:Salmo salar flipped.jpg|70px]]
                      }}
                   }}
                }}
             }}
          |2={{clade
            |label1=[[Stomiati]]
            |1={{clade
              |1=[[Stomiiformes]] (dragonfish) [[File:Sigmops bathyphilus.jpg|70px]]
              |2=[[Osmeriformes]] ([[smelt (fish)|smelt]]) <span style="{{MirrorH}}">[[File:Southern Pacific fishes illustrations by F.E. Clarke 100 1.jpg|70px]]</span>
               }}
            |sublabel2=175 mya
            |label2=[[Neoteleostei]]
              |2={{clade
              |label1=Ateleopodia
               |1=[[Ateleopodiformes]] (jellynoses)<span style="{{MirrorH}}">[[File:Ijimaia plicatellus1.jpg|60px]]</span>
              |label2=Eurypterygia
               |2={{clade
               |label1=Aulopa
                 |1=[[Aulopiformes]] (lizardfish) [[File:Aulopus filamentosus.jpg|70px]]
               |label2=Ctenosquamata
                 |2={{clade
                   |label1=Scopelomorpha
                   |1=[[Myctophiformes]] ([[lanternfish]])<span style="{{MirrorH}}">[[File:Myctophum punctatum1.jpg|80px]]</span>
                   |label2=[[Acanthomorpha]]
                     |2={{clade
                       |1={{clade
                       |label1=[[Lampripterygii]]
                       |1=[[Lampriformes]] ([[oarfish]], [[opah]], [[ribbonfish]]) [[File:Moonfish 600.jpg|60px]]
                       |label2=[[Paracanthopterygii]]
                         |2={{clade
                           |1=[[Percopsiformes]] (troutperches)<span style="{{MirrorH}}">[[File:Percopsis omiscomaycus.jpg|70px]]</span>
                           |2={{clade
                             |1=[[Zeiformes]] (dories) [[File:Zeus faber.jpg|70px]]
                             |2={{clade
                               |1=[[Stylephoriformes]] (tube-eyes/thread-fins) [[File:Stylephorus chordatus1.jpg|80px]]
                               |2=[[Gadiformes]] ([[cod]]s) <span style="{{MirrorH}}">[[File:Atlantic cod.jpg|70px]]</span>
                               }}
                           }}
                        }}
                      }}
        |2={{clade
          |label1=Polymixiipterygii
          |1=[[Polymixiiformes]] (beardfish) [[File:Polymixia nobilis1.jpg|70px]] 
          |label2=[[Acanthopterygii]]
          |2={{clade
            |1={{clade
            |label1=Berycimorphaceae
            |1={{clade
              |1=[[Beryciformes]] ([[alfonsino]]s, [[whalefish]]es) [[File:Beryx decadactylus (white background).jpg|60px]]
              |2=[[Trachichthyiformes]] ([[pinecone fish]]es, [[slimehead]]s)<span style="{{MirrorH}}">[[File:Anoplogaster cornuta Brauer.jpg|70px]]</span>
               }}
               }}
            |2={{clade
              |label1=Holocentrimorphaceae 
              |1=[[Holocentriformes]] ([[squirrelfish]], [[Myripristinae|soldier fishes]])<span style="{{MirrorH}}">[[File:Plectrypops retrospinis - pone.0010676.g037.png|60px]]</span>
              |2=[[Percomorpha]] <span style="{{MirrorH}}">[[File:Abborre,_Iduns_kokbok.jpg|70px]]</span>
               }}
             }}
           }}
         }}
       }}
     }}
   }}
 }}
           }}
         }}
       }}
     }}
   }}The most [[biodiversity|diverse]] group of teleost fish today are the Percomorpha, which include, among others, the [[Scombroidei|tuna]], [[Syngnathiformes|seahorses]], [[gobies]], [[Cichlidae|cichlids]], [[flatfish]], [[Labridae|wrasse]], [[Perciformes|perches]], [[Lophiiformes|anglerfish]], and [[Tetraodontiformes|pufferfish]].<ref name=deepfin4>{{cite journal |last1=Betancur-R |first1=Ricardo |last2=Wiley |first2=Edward O. |last3=Arratia |first3=Gloria |last4=Acero |first4=Arturo |last5=Bailly |first5=Nicolas |last6=Miya |first6=Masaki |last7=Lecointre |first7=Guillaume |last8=Ortí |first8=Guillermo |title=Phylogenetic classification of bony fishes |journal=BMC Evolutionary Biology |date=6 July 2017 |volume=17 |issue=1 |pages=162 |doi=10.1186/s12862-017-0958-3 | pmid=28683774 | pmc=5501477 |issn=1471-2148 |doi-access=free |bibcode=2017BMCEE..17..162B }}</ref> Teleosts, and percomorphs in particular, thrived during the [[Cenozoic]] [[Era (geology)|era]]. Fossil evidence shows that there was a major increase in size and abundance of teleosts immediately after the [[mass extinction event]] at the [[Cretaceous–Paleogene extinction event|Cretaceous-Paleogene boundary]] ca. 66 [[myr|mya]].<ref name="Sibert2015">{{cite journal |last1=Sibert |first1=E. C. |last2=Norris |first2=R. D. |title=New Age of Fishes initiated by the Cretaceous−Paleogene mass extinction |journal=[[PNAS]] |date=2015-06-29 |pages=8537–8542 |doi=10.1073/pnas.1504985112| pmid=26124114|volume=112|issue=28| pmc=4507219|bibcode=2015PNAS..112.8537S|doi-access=free}}</ref>

[[File:Evolution of ray-finned fish.png|500px|thumb|Evolution of ray-finned fishes, [[Actinopterygii]], from the [[Devonian]] to the present as a spindle diagram. The width of the spindles are proportional to the number of families as a rough estimate of diversity. The diagram is based on Benton, M. J. (2005) Vertebrate Palaeontology, Blackwell, 3rd edition, Fig 7.13 on page 185.]]

=== Evolutionary trends ===

[[File:Aspidorhynchus acustirostris.jpg|thumb|upright|''[[Aspidorhynchus acustirostris]]'', an early teleost from the [[Middle Jurassic]]]]

The first fossils assignable to this diverse group appear in the [[Early Triassic]],<ref name="Clarke 2018">{{cite journal |last1=Clarke |first1=John T. |last2=Friedman |first2=Matt |date=August 2018 |title=Body-shape diversity in Triassic–Early Cretaceous neopterygian fishes: sustained holostean disparity and predominantly gradual increases in teleost phenotypic variety |journal=Paleobiology|volume=44|issue=3 |pages=402–433 |doi=10.1017/pab.2018.8 |bibcode=2018Pbio...44..402C |s2cid=90207334 |url=http://osf.io/2ytc5/}}</ref> after which teleosts accumulated novel body shapes predominantly gradually for the first 150 million years of their evolution ([[Early Triassic]] through [[early Cretaceous]]).<ref name="Clarke 2018"/>

The most basal of the living teleosts are the [[Elopomorpha]] (eels and allies) and the [[Osteoglossomorpha]] (elephantfishes and allies). There are 800 species of elopomorphs. They have thin leaf-shaped larvae known as [[leptocephalus|leptocephali]], specialised for a marine environment. Among the elopomorphs, eels have elongated bodies with lost pelvic girdles and ribs and fused elements in the upper jaw. The 200 species of osteoglossomorphs are defined by a bony element in the tongue. This element has a basibranchial behind it, and both structures have large teeth which are paired with the teeth on the parasphenoid in the roof of the mouth. The clade [[Otocephala]] includes the [[Clupeiformes]] (herrings) and [[Ostariophysi]] (carps, catfishes and allies). Clupeiformes consists of 350 living species of herring and herring-like fishes. This group is characterised by an unusual abdominal [[scute]] and a different arrangement of the hypurals. In most species, the swim bladder extends to the braincase and plays a role in hearing. Ostariophysi, which includes most freshwater fishes, includes species that have developed some unique adaptations.<ref name=Benton/> One is the [[Weberian apparatus]], an arrangement of bones (Weberian ossicles) connecting the swim bladder to the inner ear. This enhances their hearing, as sound waves make the bladder vibrate, and the bones transport the vibrations to the inner ear. They also have a [[Schreckstoff|chemical alarm system]]; when a fish is injured, the warning substance gets in the water, alarming nearby fish.<ref name=Helfman>Helfman, Collette, Facey and Bowen pp. 268–274</ref>

The majority of teleost species belong to the clade [[Euteleostei]], which consists of 17,419 species classified in 2,935 genera and 346 families. Shared traits of the euteleosts include similarities in the embryonic development of the bony or cartilaginous structures located between the head and dorsal fin (supraneural bones), an outgrowth on the stegural bone (a bone located near the neural arches of the tail), and caudal median cartilages located between hypurals of the caudal base. The majority of euteleosts are in the clade [[Neoteleostei]]. A derived trait of neoteleosts is a muscle that controls the pharyngeal jaws, giving them a role in grinding food. Within neoteleosts, members of the [[Acanthopterygii]] have a spiny dorsal fin which is in front of the soft-rayed dorsal fin.<ref>Helfman, Collette, Facey and Bowen pp. 274–276</ref> This fin helps provide thrust in locomotion<ref>{{cite journal |last1=Drucker |first1=E. G. |last2=Lauder |first2=G. V. |year=2001 |title=Locomotor function of the dorsal fin in teleost fishes: experimental analysis of wake forces in sunfish |journal=[[The Journal of Experimental Biology]] |volume=204 |issue=Pt 17 |pages=2943–2958 |doi=10.1242/jeb.204.17.2943 | pmid=11551984 |url=http://jeb.biologists.org/content/204/17/2943|url-access=subscription }}</ref> and may also play a role in defense. Acanthomorphs have developed spiny [[Fish scale#Ctenoid scales|ctenoid scales]] (as opposed to the [[Fish scale#Cycloid scales|cycloid scales]] of other groups), tooth-bearing premaxilla and greater adaptations to high speed swimming.<ref name=Benton/>

The [[adipose fin]], which is present in over 6,000 teleost species, is often thought to have evolved once in the lineage and to have been lost multiple times due to its limited function. A 2014 study challenges this idea and suggests that the adipose fin is an example of [[homoplasy|convergent evolution]]. In [[Characiformes]], the adipose fin develops from an outgrowth after the reduction of the larval fin fold, while in [[Salmoniformes]], the fin appears to be a remnant of the fold.<ref>{{cite journal |author1=Steward, T. A. |author2=Smith, W. L. |author3=Coates, M. I. |year=2014 |title=The origins of adipose fins: an analysis of homoplasy and the serial homology of vertebrate appendages |journal=[[Proceedings of the Royal Society|Proceedings of the Royal Society B]] |volume=281 |issue=1781 |doi=10.1098/rspb.2013.3120 |pmid=24598422 |pmc=3953844 |page=20133120}}</ref>

=== Diversity ===

{{further|Diversity of fish}}<!--mainly but not exclusively teleosts-->

[[File:Piranha jaws.jpg|thumb|left|upright|[[Predator]]y teleost: the flesh-cutting teeth of a piranha ([[Serrasalmidae]])]]

There are over 26,000 species of teleosts, in about 40 [[order (biology)|orders]] and 448 [[family (biology)|families]],<ref>{{cite book |last1=Miller |first1=Stephen |last2=Harley |first2=John P. |title=Zoology|edition=7th|page=297 |publisher=[[McGraw-Hill Education|McGraw-Hill]] |year=2007}}</ref> making up 96% of all [[extant taxon|extant]] species of [[fish]].<ref name=Berra>{{cite book |author=Berra, Tim M. |title=Freshwater Fish Distribution |url=https://books.google.com/books?id=K-1Ygw6XwFQC&pg=PA55 |year=2008 |publisher=[[University of Chicago Press]] |isbn=978-0-226-04443-9|page=55}}</ref> Approximately 12,000 of the total 26,000 species are found in freshwater habitats.<ref name="Lackmann-2019">{{cite journal |last1=Lackmann |first1=Alec R. |last2=Andrews |first2=Allen H. |last3=Butler |first3=Malcolm G. |last4=Bielak-Lackmann |first4=Ewelina S. |last5=Clark |first5=Mark E. |date=2019-05-23 |title=Bigmouth Buffalo Ictiobus cyprinellus sets freshwater teleost record as improved age analysis reveals centenarian longevity |journal=Communications Biology|language=En|volume=2|issue=1|page=197 |doi=10.1038/s42003-019-0452-0|issn=2399-3642| pmid=31149641| pmc=6533251}}</ref> Teleosts are found in almost every aquatic environment and have developed specializations to feed in a variety of ways as carnivores, herbivores, [[filter feeder]]s and [[parasitism|parasites]].<ref name=Dorit>{{cite book |title=Zoology |url=https://archive.org/details/zoology0000dori|url-access=registration |last1=Dorit |first1=R. L. |last2=Walker |first2=W. F. |last3=Barnes |first3=R. D. |year=1991 |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 |pages=[https://archive.org/details/zoology0000dori/page/67 67–69]}}</ref> The longest teleost is the [[giant oarfish]], reported at  {{convert|7.6|m|ft|0|abbr=on}} and more,<ref name=Records>{{cite book |title=Guinness World Records 2015 |url=https://archive.org/details/guinnessworldrec0000unse_f8z3|url-access=registration |year=2014 |publisher=[[Guinness World Records]] |isbn=978-1-908843-70-8 |page=[https://archive.org/details/guinnessworldrec0000unse_f8z3/page/60 60]}}</ref> but this is dwarfed by the extinct ''[[Leedsichthys]]'', one individual of which has been estimated to have a length of {{convert|27.6|m|ft|0|abbr=on}}.<ref>{{cite journal |author=Martill, D.M. |year=1988 |title=''Leedsichthys problematicus'', a giant filter-feeding teleost from the Jurassic of England and France |journal=[[Neues Jahrbuch für Geologie und Paläontologie]] |volume=1988|issue=11 |pages=670–680 |doi=10.1127/njgpm/1988/1988/670 }}</ref> The heaviest teleost is believed to be the [[ocean sunfish]], with a specimen landed in 2003 having an estimated weight of {{convert|2.3|t|abbr=on}},<ref>{{cite news |title=World's Heaviest Bony Fish Discovered? |author=Roach, John |url=http://news.nationalgeographic.com/news/2003/05/0513_030513_sunfish.html |archive-url=https://web.archive.org/web/20030517062722/http://news.nationalgeographic.com/news/2003/05/0513_030513_sunfish.html |url-status=dead |archive-date=17 May 2003 |newspaper=National Geographic News |date=13 May 2003|access-date=9 January 2016}}</ref> while the smallest fully mature adult is the male anglerfish ''[[Photocorynus spiniceps]]'' which can measure just {{convert|6.2|mm|in|2|abbr=on}}, though the female at {{convert|50|mm|in|0|abbr=on}} is much larger.<ref name=Records/> The [[Schindleria brevipinguis|stout infantfish]] is the smallest and lightest adult fish and is in fact the smallest vertebrate in the world; the females measures {{convert|8.4|mm|in|2|abbr=on}} and the male just {{convert|7|mm|in|2|abbr=on}}.<ref>{{cite web |url=https://scripps.ucsd.edu/news/2645 |title=Scientists Describe the World's Smallest, Lightest Fish |date=20 July 2004 |publisher=[[Scripps Institution of Oceanography]]|access-date=9 April 2016|archive-date=5 March 2016|archive-url=https://web.archive.org/web/20160305095456/https://scripps.ucsd.edu/news/2645|url-status=dead}}</ref>

[[File:Giant Oarfish.jpg|thumb|upright=1.7<!--size for long low image-->|A rare [[giant oarfish]] (''Regalecus glesne''), {{convert|23|ft|m|adj=on|sigfig=1|order=flip}} long, captured in 1996]]

Open water fish are usually streamlined like [[torpedo]]es to minimize turbulence as they move through the water. Reef fish live in a complex, relatively confined underwater landscape and for them, manoeuvrability is more important than speed, and many of them have developed bodies which optimize their ability to dart and change direction. Many have laterally compressed bodies (flattened from side to side) allowing them to fit into fissures and swim through narrow gaps; some use their [[Fish fin#AnchPectoral|pectoral fins]] for locomotion and others undulate their dorsal and anal fins.<ref name=Maddock>{{cite book |last1=Maddock |first1=L. |author2=Bone, Q. |author3=Rayner, J.M.V. |title=The Mechanics and Physiology of Animal Swimming |url=https://books.google.com/books?id=orLvpB-EMgEC&pg=PA54 |year=1994 |publisher=[[Cambridge University Press]] |isbn=978-0-521-46078-1 |pages=54–56}}</ref> Some fish have grown dermal (skin) appendages for [[camouflage]]; the [[Chaetodermis penicilligerus|prickly leather-jacket]] is almost invisible among the seaweed it resembles and the [[tasselled scorpionfish]] invisibly lurks on the seabed ready to [[ambush predator|ambush prey]]. Some like the [[foureye butterflyfish]] have eyespots to startle or deceive, while others such as [[Pterois|lionfish]] have [[Aposematism|aposematic coloration]] to warn that they are toxic or have [[venom]]ous spines.<ref name=Ross>{{cite book |author=Ross, David A. |title=The Fisherman's Ocean |url=https://archive.org/details/fishermansocean0000ross |url-access=registration |year=2000 |publisher=[[Stackpole Books]] |isbn=978-0-8117-2771-6 |pages=[https://archive.org/details/fishermansocean0000ross/page/136 136]–138}}</ref>

Flatfish are [[demersal fish]] (bottom-feeding fish) that show a greater degree of asymmetry than any other vertebrates. The larvae are at first [[Symmetry in biology#Bilateral symmetry|bilaterally symmetrical]] but they undergo [[metamorphosis]] during the course of their development, with one eye migrating to the other side of the head, and they simultaneously start swimming on their side. This has the advantage that, when they lie on the seabed, both eyes are on top, giving them a broad field of view. The upper side is usually [[disruptive coloration|speckled and mottled]] for camouflage, while the underside is pale.<ref>{{cite journal |last=Schreiber |first=Alexander M. |year=2006 |title=Asymmetric craniofacial remodeling and lateralized behavior in larval flatfish |journal=The Journal of Experimental Biology |volume=209 |issue=Pt 4 |pages=610–621 |doi=10.1242/jeb.02056| pmid=16449556 |doi-access=free}}</ref>

Some teleosts are parasites. [[Remora]]s have their front dorsal fins modified into large suckers with which they cling onto a [[host (biology)|host animal]] such as a [[whale]], [[sea turtle]], [[shark]] or [[Batoidea|ray]], but this is probably a [[commensalism|commensal]] rather than parasitic arrangement because both remora and host benefit from the removal of [[Parasitism#Basic concepts|ectoparasites]] and loose flakes of skin.<ref>{{cite news |title=How does the Remora develop its sucker? |last=Jackson |first=John |url=http://www.nhm.ac.uk/natureplus/blogs/science-news/2012/11/30/how-does-the-remora-develop-its-sucker?fromGateway=true |publisher=[[Natural History Museum, London|National History Museum]] |date=30 November 2012 |access-date=2 January 2016}}</ref> More harmful are the [[Vandelliinae|catfish]] that enter the gill chambers of fish and feed on their blood and tissues.<ref name=Combes>{{cite book |last=Combes |first=Claude |title=Parasitism: The Ecology and Evolution of Intimate Interactions |url=https://books.google.com/books?id=LovrfCYloxgC&pg=PA23 |year=2001 |publisher=University of Chicago Press |isbn=978-0-226-11446-0 |page=23}}</ref> The [[snubnosed eel]], though usually a [[scavenger]], sometimes bores into the flesh of a fish, and has been found inside the heart of a [[shortfin mako shark]].<ref>{{cite journal |journal=[[Environmental Biology of Fishes]] |volume=49 |pages=139–144 |year=1997 |title=Pugnose eels, ''Simenchelys parasiticus'' (Synaphobranchidae) from the heart of a shortfin mako, ''Isurus oxyrinchus'' (Lamnidae) |last1=Caira |first1=J.N. |last2=Benz |first2=G.W. |author3=Borucinska, J. |author4=Kohler, N.E. |issue=1 |doi=10.1023/a:1007398609346 |bibcode=1997EnvBF..49..139C |s2cid=37865366 }}</ref>

Some species, such as [[electric eel]]s, can produce powerful electric currents, strong enough to stun prey.<!--should explain the specially adapted muscles forming a stack of electric cells here--> Other fish, such as [[Gymnotiformes|knifefish]], [[Electroreception and electrogenesis|generate and sense weak electric fields]] to detect their prey; they swim with straight backs to avoid distorting their electric fields. These currents are produced by modified muscle or nerve cells.<ref name=Helfman/>

<gallery class=center mode=nolines widths=220 heights=160>
File:Pseudopleuronectes americanus.jpg|The [[winter flounder]] is asymmetrical, with both eyes lying on the same side of the head.
File:Remora Belize Reef.jpg|[[Commensalism|Commensal]] fish: a [[remora]] holds on to its host with a sucker-like organ (detail inset)
File:Gymnarque du Nil.JPG|The knifefish ''[[Gymnarchus niloticus]]'' [[electric fish|generates weak electric fields]] enabling it to [[Electroreception and electrogenesis|detect and locate prey]] in turbid water.
</gallery>