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|regnum=Animalia|phylum=Chordata|subphylum=Craniata or Vertebrata|class=Pisces|includes = :Hagfishes
Extinct jawless fishes
Sharks and rays
Ray-finned fishes
A fish is any aquatic vertebrate animal that is typically ectothermic (or cold-blooded) covered with scales and equipped with two sets of paired fins and several unpaired fins Fish are abundant in the sea and in fresh water with species being known from mountain streams (eg char and gudgeon) as well as in the deepest depths of the ocean (eg gulpers and anglerfish)
Food prepared from fish is also called fish and it is an important food source for humans They are harvested either from wild fisheries (see fishing) or farmed in much the same way as cattle or chickens (see aquaculture) They are also exploited by recreational fishers and fishkeepers and are exhibited in public aquaria Fish have had a role in many cultures through the ages ranging from deities and religious symbols to the subjects of books and popular movies

Diversity of fish

The term "fish" is most precisely used to describe any non-tetrapod craniate (ie an animal with a skull and in most cases a backbone) that has gills throughout life and has limbs if any in the shape of fins Unlike groupings such as birds or mammals fish are not a single clade but a paraphyletic collection of taxa including hagfishes lampreys sharks and rays ray-finned fishes coelacanths and lungfishesHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing p 3, 1997 ISBN 0-86542-256-7Tree of life web project - Chordates
A typical fish is ectothermic has a streamlined body that allows it to swim rapidly extracts oxygen from the water using gills or an accessory breathing organ to enable it 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 scales and lays eggs that are fertilized internally or externally
a close relative of the seahorse Their leaf-like appendages enable them to blend in with floating seaweed
To each of these there are exceptions Tuna swordfish and some species of sharks show some warm-blooded adaptations and are able to raise their body temperature significantly above that of the ambient water surrounding themHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing pp 83-86 1997 ISBN 0-86542-256-7 Streamlining and swimming performance varies from highly streamlined and rapid swimmers which are able to reach 10–20 body-lengths per second (such as tuna salmon and jacks) through to slow but more maneuverable species such as eels and rays that reach no more than 0.5 body-lengths per secondHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing p 103 1997 ISBN 0-86542-256-7 Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures Lungfish have paired lungs similar to those of tetrapods gouramis have a structure called the labyrinth organ that performs a similar function while many catfish such as Corydoras extract oxygen via the intestine or stomachHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing pp 53-57 1997 ISBN 0-86542-256-7 Body shape and the arrangement of the fins is highly variable covering such seemingly un-fishlike forms as seahorses pufferfish anglerfish and gulpers Similarly the surface of the skin may be naked (as in moray eels) or covered with scales of a variety of different types usually defined as placoid (typical of sharks and rays) cosmoid (fossil lungfishes and coelacanths) ganoid (various fossil fishes but also living gars and bichirs cycloid and ctenoid (these last two are found on most bony fishHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing pp 33-36 1997 ISBN 0-86542-256-7 There are even fishes that spend most of their time out of water Mudskippers feed and interact with one another on mudflats and are only underwater when hiding in their burrows The catfish Phreatobius cisternarum lives in underground phreatic habitats and a relative lives in waterlogged leaf litter
Fish range in size from the 16 m (51 ft) whale shark to the 8 mm (just over ¼ of an inch) long stout infantfish
Many types of aquatic animals commonly referred to as "fish" are not fish in the sense given above; examples include shellfish cuttlefish starfish crayfish and jellyfish In earlier times even biologists did not make a distinction - sixteenth century natural historians classified also seal whales amphibians crocodiles even hippopotamuses as well as a host of aquatic invertebrates as fishJrCleveland P Hickman Larry S. Roberts Allan L. Larson: Integrated Principles of Zoology McGraw-Hill Publishing Co, 2001 ISBN 0–07–290961–7 In some contexts especially in aquaculture the true fish are referred to as finfish (or fin fish) to distinguish them from these other animals


Fish are a paraphyletic group: that is, any clade containing all fish also contains the tetrapods which are not fish For this reason groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications
Fish are classified into the following major groups:
  • Subclass Pteraspidomorphi (early jawless fish)
  • Class Thelodonti
  • Class Anaspida
  • (unranked) Cephalaspidomorphi (early jawless fish)
    • (unranked) Hyperoartia or Petromyzontida
      • Petromyzontidae (lampreys)
    • Class Galeaspida
    • Class Pituriaspida
    • Class Osteostraci
  • Infraphylum Gnathostomata (jawed vertebrates)
    • Class Placodermi (armoured fishes extinct)
    • Class Chondrichthyes (cartilaginous fish)
    • Class Acanthodii (spiny sharks extinct)
    • Superclass Osteichthyes (bony fish)
      • Class Actinopterygii (ray-finned fish)
        • Subclass Chondrostei
          • Order Acipenseriformes (sturgeons and paddlefishes)
          • Order Polypteriformes (reedfishes and bichirs)
        • Subclass Neopterygii
          • Infraclass Holostei (gars and bowfins)
          • Infraclass Teleostei (many orders of common fishes)
      • Class Sarcopterygii (lobe-finned fish)
        • Subclass Coelacanthimorpha (coelacanths)
        • Subclass Dipnoi (lungfish)

Some palaeontologists consider that Conodonta are chordates and so regard them as primitive fish For a fuller treatment of classification see the vertebrate article
The various fish groups taken together account for more than half of the known vertebrates There are almost 28000 known extant species of fish of which almost 27000 are bony fish with the remainder being about 970 sharks rays and chimeras and about 108 hagfishes and lampreysNelson J. S.: Fishes of the World John Wiley & Sons Inc p 4-5 2006 ISBN 0471250317 A third of all of these species are contained within the nine largest families; from largest to smallest these families are Cyprinidae Gobiidae Cichlidae Characidae Loricariidae Balitoridae Serranidae Labridae and Scorpaenidae On the other hand about 64 families are monotypic containing only one species It is predicted that the eventual number of total extant species will be at least 32500Nelson J. S.: Fishes of the World John Wiley & Sons Inc p 3, 2006 ISBN 0471250317


of Lampanyctodes hectoris
(1) - operculum (gill cover) (2) - lateral line (3) - dorsal fin (4) - fat fin (5) - caudal peduncle (6) - caudal fin (7) - anal fin (8) - photophores (9) - pelvic fins (paired) (10) - pectoral fins (paired)

Digestive system

The advent of jaws allowed fish to eat a much wider variety of food including plants and other organisms In fish food is ingested through the mouth and then broken down in the esophagus When it enters the stomach the food is further broken down and in many fish further processed in finger-like pouches called pyloric caeca The pyloric caeca secrete digestive enzymes and absorb nutrients from the digested food Organs such as the liver and pancreas add enzymes and various digestive chemicals as the food moves through the digestive tract The intestine completes the process of digestion and nutrient absorption

Respiratory system

Most fish exchange gases by using gills that are located on either side of the pharynx Gills are made up of threadlike structures called filaments Each filament contains a network of capillaries that allow a large surface area for the exchange of oxygen and carbon dioxide Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gill filaments The blood in the capillaries flows in the opposite direction to the water causing counter current exchange They then push the oxygen-poor water out through openings in the sides of the pharynx Some fishes like sharks and lampreys possess multiple gill openings However most fishes have a single gill opening on each side of the body This opening is hidden beneath a protective bony cover called an operculum
Juvenile bichirs have external gills a very primitive feature that they hold in common with larval amphibians
)Many fish can breathe air The mechanisms for doing so are varied The skin of anguillid eels may be used to absorb oxygen The buccal cavity of the electric eel may be used to breathe air Catfishes of the families Loricariidae Callichthyidae and Scoloplacidae are able to absorb air through their digestive tracts Lungfish and bichirs have paired lungs similar to those of tetrapods and must rise to the surface of the water to gulp fresh air in through the mouth and pass spent air out through the gills Gar and bowfin have a vascularised swim bladder that is used in the same way Loaches trahiras and many catfish breathe by passing air through the gut Mudskippers breathe by absorbing oxygen across the skin (similar to what frogs do) A number of fishes have evolved so-called accessory breathing organs that are used to extract oxygen from the air Labyrinth fish (such as gouramis and bettas) have a labyrinth organ above the gills that performs this function A few other fish have structures more or less resembling labyrinth organs in form and function most notably snakeheads pikeheads and the Clariidae family of catfish
Being able to breathe air is primarily of use to fish that inhabit shallow seasonally variable waters where the oxygen concentration in the water may decline at certain times of the year At such times fishes dependent solely on the oxygen in the water such as perch and cichlids will quickly suffocate but air-breathing fish can survive for much longer in some cases in water that is little more than wet mud At the most extreme some of these air-breathing fish are able to survive in damp burrows for weeks after the water has otherwise completely dried up, entering a state of aestivation until the water returns
gills inside of the head The fish head is oriented snout-downwards with the view looking towards the mouthFish 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 will suffocate Facultative air breathers such as the catfish Hypostomus plecostomus will only breathe air if they need to and will otherwise rely solely on their gills for oxygen if conditions are favourable Most air breathing fish are not obligate air breathers as there is an energetic cost in rising to the surface and a fitness cost of being exposed to surface predators

Circulatory system

Fish have a closed circulatory system with a heart that pumps the blood in a single loop throughout the body The blood goes from the heart to gills from the gills to the rest of the body and then back to the heart In most fish the heart consists of four parts: the sinus venosus the atrium the ventricle and the bulbus arteriosus Despite consisting of four parts the fish heart is still a two-chambered heart The sinus venosus is a thin-walled sac that collects blood from the fish's veins before allowing it to flow to the atrium which is a large muscular chamber The atrium serves as a one-way compartment for blood to flow into the ventricle The ventricle is a thick-walled muscular chamber and it does the actual pumping for the heart It pumps blood to a large tube called the bulbus arteriosus At the front end the bulbus arteriosus connects to a large blood vessel called the aorta through which blood flows to the fish's gills

Excretory system

As with many aquatic animals most fish release their nitrogenous wastes as ammonia Some of the wastes diffuse through the gills into the surrounding water Others are removed by the kidneys excretory organs that filter wastes from the blood Kidneys help fishes control the amount of ammonia in their bodies Saltwater fish tend to lose water because of osmosis In salt-water fish the kidneys concentrate wastes and return as much water as possible back to the body The reverse happens in freshwater fish: they tend to gain water continuously The kidneys of freshwater fish are specially adapted to pump out large amounts of dilute urine Some fish have specially adapted kidneys that change their function allowing them to move from freshwater to salt-water


The scales of fish originate from the mesoderm (skin); they may be similar in structure to teeth

Sensory and nervous system

Dorsal view of the brain of the rainbow trout

Central nervous system

Fish typically have quite small brains relative to body size when compared with other vertebrates typically one-fifteenth the mass of the brain from a similarly sized bird or mammalHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing p 48-49 1997 ISBN 0-86542-256-7 However some fish have relatively large brains most notably mormyrids and sharks which have brains of about as massive relative to body weight as birds and marsupialsHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing p 191 1997 ISBN 0-86542-256-7
The brain is divided into several regions At the front are the olfactory lobes a pair of structure the receive and process signals from the nostrils via the two olfactory nerveshagfish sharks and catfish Behind the olfactory lobes is the two-lobed telencephalon the equivalent structure to the cerebrum in higher vertebrates In fishes the telencephalon is concerned mostly with olfactionforebrain
Connecting the forebrain to the midbrain is the diencephalon (in the adjacent diagram this structure is below the optic lobes and consequently not visible) The diencephalon performs a number of functions associated with hormones and homeostasisThe midbrain or mesencephalon contains the two optic lobes These are very large in species that hunt by sight such as rainbow trout and cichlidsThe hindbrain or metencephalon is particularly involved in swimming and balanceHagfish and lampreys have relatively small cerebellums but at the other extreme the cerebellums of mormyrids are massively developed and apparently involved in their electrical senseThe brain stem or myelencephalon is the most posterior part of the brainfish at least the brain stem is also concerned with respiration and osmoregulation

Sense organs

Most fish possess highly developed sense organs Nearly all daylight fish have well-developed eyes that have color vision that is at least as good as a human's Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell Although they have ears in their heads many fish may not hear sounds very well However most fishes have sensitive receptors that form the lateral line system The lateral line system allows for many fish to detect gentle currents and vibrations as well as to sense the motion of other nearby fish and prey Some fish such as catfish and sharks have organs that detect low levels electric currentAlbert JS and WGR Crampton 2005 Electroreception and electrogenesis Pp. 431-472 in The Physiology of Fishes 3rd Edition DH Evans and JB Claiborne (eds) CRC Press Other fish like the electric eel can produce their own electricity
Fish orient themselves using landmarks and may use mental maps of geometric relationships based on multiple landmarks or symbols By studying fish in mazes it has been determined that fish routinely use spacial memory and visual discrimination

Capacity for pain

Experiments done by William Tavolga provide evidence that fish have pain and fear responses For instance in Tavolga’s experiments toadfish grunted when electrically shocked and over time they came to grunt at the mere sight of an electrodeDunayer Joan "Fish: Sensitivity Beyond the Captor's Grasp" The Animals' Agenda July/August 1991 pp. 12-18
In 2003 Scottish scientists at the University of Edinburgh performing research on rainbow trout concluded that fish exhibit behaviors often associated with pain At tests conducted at both the University of Edinburgh and the Roslin Institute bee venom and acetic acid were injected into the lips of rainbow trout resulted in fish rocking their bodies and rubbing their lips along the sides and floors of their tanks which the researchers believe were efforts to relieve themselves of pain similar to what mammals would also do Vantressa Brown “Fish Feel Pain British Researchers Say” Agence France-Presse 1 May 2003 Neurons in the brains of the fish fired in a pattern resembling that of humans when they experience painProfessor James D. Rose of the University of Wyoming critiqued the study claiming it was flawed mainly since it did not provide proof that fish possess "conscious awareness particularly a kind of awareness that is meaningfully like ours" Rose JD 2003 A Critique of the : "Do fish have nociceptors: Evidence for the evolution of a vertebrate sensory system" Rose argues that since the fish brain is rather different from ours fish are probably not conscious (in the manner humans are) whence reactions similar to human reactions to pain instead have other causes Rose had published his own opinion a year earlier arguing that fish cannot feel pain as their brains lack a neocortexJames D. Rose Do Fish Feel Pain? 2002 Retrieved September 27, 2007 However animal behaviorist Temple Grandin argues that fish could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions"Animal protection advocates have raised concerns about the possible suffering of fish caused by angling In light of recent research some countries like Germany have banned specific types of fishing and the British RSPCA now formally prosecutes individuals who are cruel to fish Leake J. “Anglers to Face RSPCA Check” The Sunday Times – Britain 14 March 2004

Muscular system

Most fish move by contracting paired sets of muscles on either side of the backbone alternately These contractions form S-shaped curves that move down the body of the fish As each curve reaches the back fin backward force is created This backward force in conjunction with the fins moves the fish forward The fish's fins are used like an airplane's stabilizers Fins also increase the surface area of the tail allowing for an extra boost in speed The streamlined body of the fish decreases the amount of friction as they move through water Since body tissue is denser than water fish must compensate for the difference or they will sink Many bony fishes have an internal organ called a swim bladder that adjusts their buoyancy through manipulation of gases


A 3 to 4 m great white shark
off Isla GuadalupeAlthough most fish are exclusively aquatic and ectothermic there are exceptions to both cases
Fish from a number of different groups have evolved the capacity to live out of the water for extended periods of time Of these amphibious fish some such as the mudskipper can live and move about on land for up to several days
Also certain species of fish maintain elevated body temperatures to varying degrees Endothermic teleosts (bony fishes) are all in the suborder Scombroidei and include the billfishes tunas and one species of "primitive" mackerel (Gasterochisma melampus) All sharks in the family Lamnidae – shortfin mako long fin mako white porbeagle and salmon shark – are known to have the capacity for endothermy and evidence suggests the trait exists in family Alopiidae (thresher sharks) The degree of endothermy varies from the billfish which warm only their eyes and brain to bluefin tuna and porbeagle sharks who maintain body temperatures elevated in excess of 20 °C above ambient water temperatures See also gigantothermy Endothermy though metabolically costly is thought to provide advantages such as increased contractile force of muscles higher rates of central nervous system processing and higher rates of digestion

Reproductive system


: 1. Liver 2. Gas bladder 3. Roe 4. Pyloric caeca 5. Stomach 6. IntestineFish reproductive organs include testes and ovaries In most fish species gonads are paired organs of similar size which can be partially or totally fusedIn terms of spermatogonia distribution the structure of teleosts testes has two types: in the most common spermatogonia occur all along the seminiferous tubules while in Atherinomorph fishes they are confined to the distal portion of these structures Fishes can present cystic or semi-cystic spermatogenesis in relation to the phase of release of germ cells in cysts to the seminiferous tubules lumen
Fish ovaries may be of three types: gymnovarian secondary gymnovarian or cystovarian In the first type the oocytes are released directly into the coelomic cavity and then enter the ostium then through the oviduct and are eliminated Secondary gymnovarian ovaries shed ova into the coelom and then they go directly into the oviduct In the third type the oocytes are conveyed to the exterior through the oviduct Gymnovaries are the primitive condition found in lungfishes sturgeons and bowfins Cystovaries are the condition that characterizes most of the teleosts where the ovary lumen has continuity with the oviductother 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 nucleus ooplasm and the surrounding layers characterize the oocyte maturation processPostovulatory follicles are structures formed after oocyte release; they do not have endocrine function present a wide irregular lumen and are rapidly reabosrbed in a process involving the apoptosis of follicular cells A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned This process can also occur but less frequently in oocytes in other development stagesSome fish are hermaphrodites having testes and ovaries either at different phases in their life cycle or, like hamlets can be simultaneously male and female

Reproductive method

Over 97% of all known fishes are oviparousPeter Scott: Livebearing Fishes p. 13. Tetra Press 1997 ISBN 1-5646-5193-2 that is, the eggs develop outside the mother's body Examples of oviparous fishes include salmon goldfish cichlids tuna and eels In the majority of these species fertilisation takes place outside the mother's body with the male and female fish shedding their gametes into the surrounding water However a few oviparous fishes practise internal fertilisation 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 horn shark and oviparous rays such as skates In these cases the male is equipped with a pair of modified pelvic fins known as claspers
Marine fish can produce high numbers of eggs which are often released into the open water column The eggs have an average diameter of 1mmFile:Oeufs002b57.png|Egg of lampreyFile:Oeufs002b54.png|Egg of catshark (mermaids' purses)File:Oeufs002b55.png|Egg of shark (?)File:Oeufs002b56.png|Egg of chimaera
The newly-hatched young of oviparous fish are called larvae They are usually poorly formed carry a large yolk sac (from which they gain their nutrition) and are very different in appearance to juvenile and adult specimens of their species The larval period in oviparous fish is relatively short however (usually only several weeks) and larvae rapidly grow and change appearance and structure (a process termed metamorphosis) to resemble juveniles of their species During this transition larvae use up their yolk sac and must switch from yolk sac nutrition to feeding on zooplankton prey a process which is dependent on zooplankton prey densities and causes many mortalities in larvae
Ovoviviparous fish are ones in which the eggs develop inside the mother's body after internal fertilization but receive little or no nutrition from the mother depending instead on the yolk Each embryo develops in its own egg Familiar examples of ovoviviparous fishes include guppies angel sharks and coelacanths
Some species of fish are viviparous In such species the mother retains the eggs as in ovoviviparous fishes but the embryos receive nutrition from the mother in a variety of different ways Typically viviparous fishes have a structure analogous to the placenta seen in mammals connecting the mother's blood supply with the that of the embryo Examples of viviparous fishes of this type include the surf-perches splitfins and lemon shark The embryos of some viviparous fishes exhibit a behaviour known as oophagy where the developing embryos eat eggs produced by the mother This has been observed primarily among sharks such as the shortfin mako and porbeagle but is known for a few bony fish as well such as the halfbeak Nomorhamphus ebrardtiiMeisner A & Burns J: Viviparity in the Halfbeak Genera Dermogenys and Nomorhamphus (Teleostei: Hemiramphidae) Journal of Morphology 234 pp 295-317 1997 Intrauterine cannibalism is an even more unusual mode of vivipary where the largest embryos in the uterus will eat their weaker and smaller siblings This behaviour is also most commonly found among sharks such as the grey nurse shark but has also been reported for Nomorhamphus ebrardtiiAquarists commonly refer to ovoviviparous and viviparous fishes as livebearers

Immune system

Types of immune organs vary between different types of fishAG 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 TNakanishi) New York Academic Press 1996 pages 1-55In the jawless fish (lampreys and hagfishes) true lymphoid organs are absent Instead these fish rely on regions of lymphoid tissue within other organs to produce their 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) resemble primitive bone marrow in hagfishCartilaginous fish (sharks and rays) have a more advanced immune system than the jawless fish They have three specialized organs that are unique to chondrichthyes; the epigonal organs (lymphoid tissue similar to bone marrow of mammals) that surround the gonads the Leydig's organ within the walls of their esophagus and a spiral valve in their intestine All these organs house typical immune cells (granulocytes lymphocytes and plasma cells) They also possess an identifiable thymus and a well-developed spleen (their most important immune organ) where various lymphocytes plasma cells and macrophages develop and are storedChondrostean fish (sturgeons paddlefish and birchirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system) and their heart is frequently covered with tissue that contains lymphocytes reticular cells and a small number of macrophages The chondrostean kidney is an important hemopoietic organ; where erythrocytes granulocytes lymphocytes and macrophages developLike chondrostean fish the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney) where many different immune cells are housedDP Anderson Fish Immunology (SF Snieszko and HR Axelrod eds) : TFH Publications Inc Ltd 1977 In addition teleost fish possess a thymus spleen and scattered immune areas within mucosal tissues (eg 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 thymusS Chilmonczyk The thymus in : development and possible function in the immune response Annual Review of Fish Diseases Volume 2, 1992 pages 181-200JD Hansen and AG Zapata Lymphocyte development in fish and amphibians Immunological Reviews Volume 166 1998 pages 199-220 Recently a lymphatic system similar to that described in mammals was described in one species of teleost fish the zebrafish Although not confirmed as yet this system presumably will be where naive (unstimulated) T cells will accumulate while waiting to encounter an antigenKucher et al Development of the zebrafish lymphatic system requires VegFc signalling Current Biology Volume 16, 2006 pages 1244-1248


Like other animals fish can suffer from a wide variety of diseases and parasites To prevent disease they have a variety of non-specific defences and specific defences Non-specific defences include the skin and scales as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth Should pathogens breach these defences fish can develop an inflammatory response that increases the flow of blood to the infected region and delivers the white blood cells that will attempt to destroy the pathogens Specific defences are specialised responses to particular pathogens recognised by the fish's body in other words an immune responseHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing pp 95-96 1997 ISBN 0-86542-256-7 In recent years vaccines have become widely used in aquaculture and also with ornamental fish for example the vaccines for furunculosis in farmed salmon and koi herpes virus in koiR C. Cipriano (2001) Furunculosis And Other Diseases Caused By Aeromonas salmonicida Fish Disease Leaflet 66. US Department of the Interior[1]K H Hartman et al. (2004) Koi Herpes Virus (KHV) Disease Fact Sheet VM-149 University of Florida Institute of Food and Agricultural Sciences[2]
Some fish will also take advantage of cleaner fish for removal of external parasites The best known of these are the Bluestreak wrasse]s of the genus Labroides found on coral reefs in the Indian Ocean and Pacific Ocean These small fish maintain so-called "cleaning stations" where other fish known as hosts will congregate and perform specific movements to attract the attention of the cleaner fishHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing p 380 1997 ISBN 0-86542-256-7 Cleaning behaviours have been observed in a number of other fish groups including an interesting case between two cichlids of the same genus Etroplus maculatus the cleaner fish and the much larger Etroplus suratensis the hostRichard L. Wyman and Jack A. Ward (1972) A Cleaning Symbiosis between the Cichlid Fishes Etroplus maculatus and Etroplus suratensis I. Description and Possible Evolution Copeia Vol 1972 No. 4, pp. 834-838


forms between sponge and man was a gigantic 10 meter (33 ft) long prehistoric fish Monster fish crushed opposition with strongest bite ever smhcomauThe early fossil record on fish is not very clear It became a dominant form of sea life and eventually branched to create land vertebrates
The proliferation was apparently due to the formation of the hinged jaw because jawless fish left very few descendants Classification of the Chordates Evolution ecology and biodiversity 05-1116-3 University of Winnipeg Retrieved 2007-04-07 Lampreys may be a rough representative of pre-jawed fish The first jaws are found in Placodermi fossils It is unclear if the advantage of a hinged jaw is greater biting force respiratory-related or a combination
Some speculate that fish may have evolved from a creature similar to a coral-like Sea squirt whose larvae resemble primitive fish in some key ways The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today) although perhaps the reverse of this is the case Candidates for early fish include Agnatha such as Haikouichthys Myllokunmingia and Conodonts

Importance to humans

Economic importance



A Whale shark
the world's largest fish is classified as VulnerableAs of 2006 the IUCN Red List describes 1173 species of fish as being threatened with extinction Included on this list are species such as Atlantic cod Devil's Hole pupfish coelacanths and great white sharks Because fish live underwater they are much 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 areas For example the Devil's Hole pupfish occupies only a single 3 m by 6 m poolHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing pp. 449-450 1997 ISBN 0-86542-256-7


In the case of edible fishes such as cod and tuna a major threat is overfishing Where overfishing persists it eventually causes the collapse of the fish population (known as stock) because the population cannot breed fast enough to replace the individuals removed by fishing One well-studied example of the collapse of a fishery is the Pacific sardine
Sadinops sagax caerulues fishery off the coast of California From a peak in 1937 of 790000 tonnes the amount of fish landed steadily declined to a mere 24000 tonnes in 1968 at which point the fishery stopped as no longer economically viable Such 'commercial extinction does not mean that the fish itself goes extinct merely that it can no longer sustain a profitable fisheryHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing p 462 1997 ISBN 0-86542-256-7 The main tension between fisheries science and the fishing industry is the need to balance conservation with preserving the livelihoods of fishermen In places such as Scotland Newfoundland and Alaska the fishing industry is a major employer so governments have a vested interest in finding a balance between conserving fish stocks while maintaining an economic level of commercial fishing On the other hand scientists and conservations push for increasingly stringent protection for fish stocks warning that many stocks could be wiped out within fifty years

Habitat destruction

A key stress on both freshwater and marine ecosystems is habitat degradation including water pollution the building of dams removal of water for use by humans and the introduction of exotic speciesHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing p 463 1997 ISBN 0-86542-256-7 An example of a fish that has become endangered because of habitat change is the pallid sturgeon a North American freshwater fish that living in rivers that have all been changed by human activity in a variety of different ways

Exotic species

Introduction of exotic species has occurred in a variety of places and for many different reasons One of the best studied (and most severe) examples was the introduction of Nile perch into Lake Victoria Since the 1960s the Nile perch gradually exterminated the 500 species of cichlid fishes found only in this lake and nowhere else Some species survive now only in captive breeding programmes but others are probably extinct Carp snakeheads tilapia European perch brown trout rainbow trout and sea lampreys are other examples of fish that have caused problems by being introduced into alien environments

Aquarium collecting


of Vishnu as a MermanIn the Book of Jonah a "great fish" swallowed Jonah the Prophet Legends of half-human half-fish mermaids have featured in stories like those of Hans Christian Andersen and movies like Splash (See Merman Mermaid)
Among the deities said to take the form of a fish are Ika-Roa of the Polynesians Dagon of various ancient Semitic peoples and Matsya of the Dravidas of India The astrological symbol Pisces is based on a constellation of the same name but there is also a second fish constellation in the night sky Piscis Austrinus
Fish have been used figuratively in many different ways for example the ichthys used by early Christians to identify themselves through to the fish as a symbol of fertility among BengalisJaffrey M.: A Taste of India Atheneum p 148 1988 ISBN 0-689-70726-6
Italy EstoniaFish have also featured prominently in art and literature as 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 Snakehead Terror and Piranha
The golden fish (Sanskrit: Matsya) represents in the semiotic of Ashtamangala(buddhist symbolism) the state of fearless suspension in samsara thus perceived as the harmless ocean referred to as 'buddha-eyes' or ' rigpa-sight] '. The fishes symbolises the auspiciousness of all living beings in a state of fearlessness without danger of drowning in the Samsaric Ocean of Suffering and migrating from teaching to teaching freely and spontaneously just as fish swim
In the following quotation the two golden fishes are linked with the Ganges and Yamuna and nadi prana and carp:
The two fishes originally represented the two main sacred rivers of India - the Ganges and Yamuna These rivers are associated with the lunar and solar channels which originate in the nostrils and carry the alternating rhythms of breath & prana
Flags.jpg|thumb|right|300px|Fish riders is a 1920s poster of the Republic of ChinaThey have religious significance in Hindu Jain and Buddhist traditions but also in Christianity who is first signified by the sign of the fish and especially referring to feeding the multitude in the desert In the dhamma of Buddha the fish symbolize happiness as they have complete freedom of movement in the water They represent fertility and abundance Often drawn in the form of carp which are regarded in the Orient as sacred on account of their elegant beauty size and life-span[3] coat of arms
The name of the Canadian city of Coquitlam British Columbia is derived from Kwikwetlem which is said to be derived from a Coast Salish term meaning "little red fish"Kwikwetlem First Nation: History & Culture Retrieved on 5 March 2009


Fish or fishes

Though often used interchangeably these words actually mean different things Fish is used either as singular noun or to describe a group of specimens from a single species Fishes describes a group containing more than one species
  • My aquarium contains three different fishes: guppies platies and swordtails
  • The North Atlantic stock of Gadus morhua is estimated to contain several million fish

  • Shoal or school

    right|thumb|250px|These fusilier are schooling because their swimming is synchronised]A random assemblage of fishes 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 advantagesHelfman G., Collette B., & Facey D.: The Diversity of Fishes Blackwell Publishing p 375 1997 ISBN 0-86542-256-7
    • Cichlids congregating at lekking sites form an aggregation
    • Many minnows and characins form shoals
    • Anchovies herrings and silversides are classic examples of schooling fishes

    While school and shoal have different meanings within biology they are often treated as synonyms by non-specialists with speakers of British English using "shoal" to describe any grouping of fish while speakers of American English often using "school" just as loosely

    See also

    For a topical guide to sharks see Outline of sharks


    External links