View source for Fish

[[File:Blue shark.jpg|thumb|230px|[[Blue shark]]]]
[[File:Pterois volitans Manado-e edit.jpg|thumb|230px|Head-on view of a red lion-fish]]
[[File:Grass carp fexx.jpg|thumb|right|230px|[[Carp]]]]
[[File:leafydragon.jpg|thumb|230px|Fish come in many shapes and sizes. This is a sea dragon (kira), a close relative of the [[seahorse]]. They are [[camouflage]]d as floating seaweed.<ref>[http://animals.nationalgeographic.com/animals/fish/sea-dragon.html Leafy and Weedy Sea Dragon] National Geographic Profile. Retrieved 20 July 2009.</ref><ref>{{IUCN2006|assessors=Connolly R|year=2006|id=17096|title=Phycodurus eques|downloaded=20 July 2009}}</ref>]]
[[File:Dunkleosteus skull QM email.jpg|thumb|right|230px|''[[Dunkleosteus]]'', a giant armoured arthrodire of the later [[Devonian]]. Up to 6 metres long, it lived in the [[Inland sea]]s of [[North America]].<ref name="Long">Long J.A. 1995. ''The rise of fishes: 500 million years of evolution''. Johns Hopkins, Baltimore.</ref>]]
[[File:Vogelherd Fisch 2008 Eiszeit.jpg|thumb|This is one of the first works of art: a sculpture of a fish; age:about 40.000 years.]]

'''Fish''' (plural: fish or fishes) are an aquatic group of [[vertebrate]]s which live in water and [[respiration|respire]] (get [[oxygen]]) with [[gills]]. They do not have limbs, like arms or legs, and they do have digits (fingers & toes). This is a [[definition]] which does not quite work: some [[amphibia]] also live in water and have external gills, but they are not fish.

Fish used to be a [[class (biology)|class]] of [[vertebrates]].  Now the term covers five classes of aquatic vertebrates:
:[[Agnatha|Jawless fish]]
:[[Placodermi|Armoured fish]]
:[[Chondrichthyes|Cartilaginous fish]]
:[[Actinopterygii|Ray-finned fish]]
:[[Sarcopterygii|Lobe-finned fish]]

There are more fish than [[tetrapods]] (land vertebrates): there are over 33,000 described species of fish.<ref>[http://www.fishbase.org/search.php FishBase]</ref> Fish are usually covered with scales. They have two sets of paired [[fins]] and several unpaired fins. Most fish are cold-blooded ([[poikilotherm]]). A fish takes in the oxygen from the water using gills. There are many different kinds of fish. They live in [[fresh water]] in [[lake]]s and [[river]]s, and in [[salt water]] in the [[ocean]]. Some fish are less than one [[centimeter]] long. The largest fish is the [[whale shark]], which can be almost 15 [[metre|meters]] long and weigh 15 [[ton]]s. Most fish live in the water. A group of fish called the [[lungfish]] have developed [[lung]]s because they live in rivers and pools which dry up in certain parts of the year. They burrow into mud and [[aestivation|aestivate]] until the water returns.

'Fish' is a [[paraphyletic]] term in [[cladistics]] because it lacks a [[monophyletic]] group of descendants. It does not include the land vertebrates or [[tetrapod]]s, which descended from fish.

== Types of fish ==
"Fish" is not a formal [[Taxonomy|taxonomic]] grouping in systematic biology. Amphibians, reptiles, birds and mammals all descended from [[lobe-finned fish]] (and not from fish as a whole). But the use of the term "fish" is so convenient that we go on using it.

Fish are the oldest vertebrate group. The term includes a huge range of types, from the Middle [[Ordovician]], about 490 million years ago, to the present day. These are the main groups:<ref>Moyle P.B. and Cech J.J. 2003. ''Fishes: an introduction to Ichthyology''. 5th ed, Cummings. {{ISBN|978-0-13-100847-2}}</ref><ref>Maisey J.G. 1996. ''Discovering fossil fishes''. Holt N.Y.</ref>

* Agnatha: the jawless fish. [[Cambrian]] to present day.<ref>Janvier, Philippe 2010. MicroRNAs revive old views about jawless vertebrate divergence and evolution. ''Proceedings of the National Academy of Sciences (USA)'' '''107''':19137-19138. [http://www.pnas.org/content/107/45/19137.full.pdf+html] {{Webarchive|url=https://web.archive.org/web/20150924154215/http://www.pnas.org/content/107/45/19137.full.pdf+html |date=2015-09-24 }}</ref>
** [[Pteraspid]]s: the head-shields
** [[Anaspid]]s: gills opened as holes. [[Silurian]] to end-Devonian.
*** [[Cephalaspid]]s: early jawless fish
*** [[Lamprey]]s: living ectoparasites
** [[Osteostraci]]: bony-armoured jawless fish.
* [[Gnathostomata]]: the jawed fish. Includes all types commonly called fish, except the lamprey.
** [[Placoderm]]s: heavily armoured fish
** Chondrichthyes: cartilaginous fish: [[shark]]s, [[ray]]s and [[Skate (fish)|skate]]s.
** [[Acanthodii]]: extinct spiny sharks
* [[Osteichthyes]]: bony fish.
** Actinopterygii: the [[ray-finned fish]].
*** [[Chondrostei]]: [[sturgeon]]s and some other early types.
*** [[Neopterygii]]: first seen in the later Permian, lighter and faster-moving than previous groups.
**** [[Holostei]]: the [[gar]]s and [[bowfin]]s
**** [[Teleost]]ei: the most successful group, [[Triassic]] to present day.
** Sarcopterygii: the lobe-finned fish
*** [[Dipnoi]]: the lungfish; eight [[genera]] survive.
*** [[Coelacanth]]s: two [[species]] survive. They were probably a [[sister-group]] to the tetrapods.

Certain animals that have the word ''fish'' in their name are not really fish: [[Crayfish]] are [[crustacean]]s, and [[jellyfish]] are [[Cnidaria]]ns. Some animals look like fish, but are not. [[Whale]]s and [[dolphin]]s are [[mammal]]s, for example.

== Anatomy ==
[[File:Lampanyctodes hectoris (Hector's lanternfish).svg|thumb|center|400px|The anatomy of ''Lampanyctodes hectoris'' <br /> 1. operculum (gill cover) 2 & 5. lateral line 3. dorsal fin 4 fat fin 6. caudal fin 7. anal fin 8. photophores 9. pelvic fins paired 10. pectoral fins (paired)]]

=== Bony and cartilagenous fish ===
Most kinds of fish have [[bone]]s. Some kinds of fish, such as sharks and rays, do not have real bones. Their [[skeleton]]s are made of [[cartilage]], and so they are known as [[cartilaginous fish]].

=== Fish scales ===
All fish are covered with overlapping [[Scale (zoology)|scales]], and each major group of fish has its own special type of scale. Teleosts ('modern' fish) have what are called ''leptoid'' scales. These grow in concentric circles and overlap in a head to tail direction like roof tiles. Sharks and other chondrichthyes have ''placoid'' scales made of denticles, like small versions of their teeth. These also overlap in a head to tail direction, producing a tough outer layer. Shark skin is available for purchase as [[shagreen]], a leather which as original is smooth in one direction, and rough in the other direction. It may be polished for use, but is always rough in texture and resistant to slipping.

The scales are usually covered with a layer of [[slime]] which improves passage through the water, and makes the fish more slippery to a [[predator]].

== Freshwater fish ==
41% of all fish live in [[freshwater]]. There are also some important fish which breed in rivers, and spend the rest of their life in the seas. Examples are [[salmon]], [[trout]], the sea lamprey,<ref>Silva S. ''et al'' 2014. The haematophagous feeding stage of anadromous populations of sea lamprey ''Petromyzon marinus'': low host selectivity and wide range of habitats. ''Hydrobiologia'', '''734'''(1), 187-199.</ref> and three-spined [[stickleback]]. Some other fish are born in salt water, but live most of their adult lives in fresh water: for example the [[eels]].
Species like these change their [[physiology]] to cope with the amount of salt in the water.

== Swimming ==
Fish swim by exerting force against the surrounding water. There are exceptions, but this is usually done by the fish contracting muscles on either side of its body in order to generate waves of flexion that travel the length of the body from nose to tail, generally getting larger as they go along. Most fishes generate thrust using lateral movements of their body & tail fin (caudal fin). However, there are also species which move mainly using their median and paired fins. The latter group profits from the gained manoeuvrability that is needed when living in coral reefs for example. But they can not swim as fast as fish using their bodies & caudal fins.<ref>Breder C.M. 2003. The locomotion of fishes. ''Zoologica'', '''4''', 159-256, (1926); ''Journal of Experimental Biology'' '''206''', 2749-2758 (2003)</ref>

=== Muscle ===
Fish can swim slowly for many hours using red muscle fibres. They also make short, fast bursts using white muscle. The two types of muscle have a fundamentally different physiology. The red fibres are usually alongside a much greater number of white fibres.

The white fibres get their energy by converting the [[carbohydrate]] [[glycogen]] to lactate ([[lactic acid]]). This is [[anaerobic metabolism]], that is, it does not need oxygen. They are used for fast, short bursts. Once the lactic acid builds up in the muscles, they stop working, and it takes time for the lactate to be removed, and the glycogen replaced.<ref name=McN>Alexander, R. McNeill. 1974. ''Functional design in fishes''. London: Hutchinson, p31. {{ISBN|0-09-104751-X}}</ref> Using their white fibres, fish can reach speeds of 10 lengths per second for short bursts.<ref name=McN/>

Swimming for long periods needs oxygen for the red fibres. The oxygen supply has to be constant because these fibres only operate [[aerobic]]ally. They are red because they have a rich blood supply, and they contain [[myoglobin]]. Myoglobin transports the oxygen to the oxidising systems. Red muscle gets its energy by oxidising [[fat]], which weight for weight has twice as much energy as carbohydrate or protein.<ref name=McN/> Using their red fibres, fish can keep up a speed of 3{{ndash}}5 lengths per second for long periods.<ref name=McN/>

=== Swimming in groups ===
{{main|Shoaling and schooling}}
Many fish swim in groups.  Schools of fish can swim together for long distances, and may be chased by predators which also swim in schools. Casual groups are called 'shoals'.

== Body shape ==
The shape of the body of a fish is important to its swimming. This is because [[wikt:streamline|streamlined]] body shapes makes the water drag less. Here are some common fish shapes:-

[[File:White shark.jpg|thumb|230px|right|alt=A smooth, rocket-shaped shark swims quickly through the water.|The rocket-shape of this shark makes it an efficient swimmer. It is fast over short distances.]]

The picture on the right shows a shark. This shark's shape is called ''fusiform'', and it is an ovoid shape where both ends of the fish are pointy. This is the best shape for going through water quickly.<ref>Nielsen, Knut Schmidt 1984. ''Scaling: why is animal size so important?'' Cambridge. Chapter 15</ref><ref>Baily, Jill 1997. ''How fish swim''. Benchmark, N.Y.</ref> Fishes with fusiform shapes can chase prey and escape predators quickly. Many live in the open ocean and swim constantly, like [[marlin]]s, [[swordfish]], and [[tuna]]. [[Ichthyosaur]]s, [[porpoise]]s, dolphins, [[killer whale]]s all have similar shapes. This is an example of [[convergent evolution]].

=== Eel-like ===
The long, ribbon-like shape of an [[eel]]'s body shows another shape.  This enables them to hide in cracks, springing out quickly to capture prey, then returning quickly to their hiding spot.

=== Flatfish ===
[[File:Flounder hawaii.jpg|thumb|230px|alt=a depressed flounder.|An adult flounder has both eyes on the upper side of its body.]]

[[Flatfish]] live on the bottom of the ocean or lake. Most use camouflage: they change colours to match the ocean floor. During their early lives, their eyes move to the upper side of their flat body.

[[Reef]] fish also have flat bodies, and their body is often highly coloured. Flat bodies can slip in and out among the [[coral]]s, [[sponge]]s, and rocks, avoiding predators. [[Marine angelfish|Angelfish]], [[surgeonfish]], and [[butterflyfish]] are examples.

== Fish as food ==
{{Main article|Fish as food}}
[[File:Goldfish2.cropped.jpg|240px|thumb|[[Koi]] (and [[goldfish]]) have been kept in decorative ponds for centuries in China and Japan]]
Some people eat many different kinds of fish. These include carp, [[cod]], [[herring]], [[perch]], [[sardine]]s, sturgeon, [[tilapia]], trout, tuna, and many others.  A person who buys and sells fish for eating is called a [[fishmonger]].

The word ''to fish'' is also used for the activity of catching fishes.  People catch fish with small [[Fishing net|nets]] from the side of the water or from small [[boat]]s, or with big nets from big boats.  People can also catch fish with [[fishing pole]]s and fishhooks with [[bait]].  This is often called [[angling]].  Anglers also different types of [[fishing lure]]s.

Because people are catching too many fish for food or other uses, there are less and less fish in the sea. This is a problem known as [[Overfishing]].

== Fish as pets ==
[[Selective breeding]] of carp made them into the [[domesticated]] koi in [[Japan]], and goldfish in [[China]]. This breeding began over 2,000 years ago. The [[Chinese people|Chinese]] brought their goldfish indoors during the [[Song Dynasty]]. They kept them in large [[ceramic]] vessels. That we now do in glass fish tanks.

== References ==
{{Reflist}}

== See also ==
* [http://en.citizendium.org/wiki/Fish Fish] -Citizendium


{{commonscat|Fish}}

{{vertebrates}}

[[Category:Basic English 850 words]]
[[Category:Fish|*]]