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Fish anatomy

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Fish anatomyis the study of the form ormorphologyoffish.It can be contrasted withfish physiology,which is the study of how the component parts of fish function together in the living fish.[1]In practice, fishanatomyand fishphysiologycomplement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, such as might be observed on the dissecting table or under the microscope, and the latter dealing with how those components function together in living fish.

Diagram showing the external anatomy of a Hector's lanternfish, a type of bony fish
External anatomy of abony fish(Hector's lanternfish): 1.operculum(gill cover), 2.lateral line,3.dorsal fin,4.adipose fin,5.caudal peduncle,6.caudal fin,7.anal fin,8.photophores,9.pelvic fins(paired), 10.pectoral fins(paired)
Diagram showing the internal anatomy of a generic bony fish
Internal anatomy of a bony fish

The anatomy of fish is often shaped by the physical characteristics of water, the medium in which fish live. Water is muchdenserthan fish, holds a relatively small amount of dissolved oxygen, and absorbs more light than air does. The body of a fish is divided into a head, trunk and tail, although the divisions between the three are not always externally visible. The skeleton, which forms the support structure inside the fish, is either made ofcartilage(cartilaginous fish) or bone (bony fish). The main skeletal element is thevertebral column,composed of articulatingvertebraewhich are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. The main external features of the fish, thefins,are composed of either bony or soft spines calledrayswhich, with the exception of thecaudal fins,have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk.[2] The heart has two chambers and pumps the blood through the respiratory surfaces of thegillsand then around the body in a single circulatory loop.[3]The eyes are adapted for seeing underwater and have only local vision.[definition needed]There is aninner earbut no external ormiddle ear.Low-frequency vibrations are detected by thelateral linesystem of sense organs that run along the length of the sides of fish, which responds to nearby movements and to changes in water pressure.[2]

Sharksandraysarebasalfish with numerousprimitiveanatomical features similar to those of ancient fish, including skeletons composed of cartilage. Their bodies tend to be dorso-ventrally flattened, and they usually have five pairs of gill slits and a large mouth set on the underside of the head. Thedermisis covered with separate dermalplacoid scales.They have acloacainto which the urinary and genital passages open, but not aswim bladder.Cartilaginous fish produce a small number of largeyolkyeggs. Some species areovoviviparous,having the young develop internally, but others areoviparousand the larvae develop externally inegg cases.[4]

The bony fish lineage shows morederivedanatomical traits, often with major evolutionary changes from the features of ancient fish. They have a bony skeleton, are generally laterally flattened, have five pairs of gills protected by anoperculum,and a mouth at or near the tip of the snout. The dermis is covered with overlappingscales.Bony fish have a swim bladder which helps them maintain a constant depth in thewater column,but not a cloaca. They mostlyspawna large number of small eggs with little yolk which they broadcast into the water column.[4]

Body

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Anatomical directionsand axes

In many respects, fish anatomy is different frommammaliananatomy. However, it still shares the same basicbody planfrom which allvertebrateshave evolved: anotochord,rudimentary vertebrae, and a well-defined head and tail.[5][6]

Fish have a variety of different body plans. At the broadest level, their body is divided into the head, trunk, and tail, although the divisions are not always externally visible. The body is oftenfusiform,a streamlined body plan often found in fast-moving fish. Some species may be filiform (eel-shaped) orvermiform(worm-shaped). Fish are often either compressed (laterallythin and tall) or depressed (dorso-ventrallyflattened).

Skeleton

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Skeleton of abony fish
See also:Fish bone

There are two different skeletal types: theexoskeleton,which is the stable outer shell of an organism, and theendoskeleton,which forms the support structure inside the body. The skeleton of the fish is made of either cartilage (cartilaginous fishes) or bone (bony fishes). The endoskeleton of the fish is made up of two main components: the axial skeleton consisting of the skull and vertebral column, and the appendicular skeleton supporting the fins.[7]The fins are made up of bony fin rays and, except for the caudal fin, have no direct connection with the spine. They are supported only by the muscles. The ribs attach to the spine.

Bones are rigidorgansthat form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produceredandwhite blood cellsand storeminerals.Bone tissue is a type of denseconnective tissue.Bones come in a variety of shapes and have a complex internal and external structure. They are lightweight, yet strong and hard, in addition to fulfilling their many otherbiological functions.

Vertebrae

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Skeletal structure of abassshowing the vertebral column running from the head to the tail
Skeletal structure of anAtlantic cod
TheX-ray tetra(Pristella maxillaris) has a visiblebackbone.Thespinal cordis housed within its backbone.
A vertebra (diameter 5 mm (0.20 in)) of a smallray-finned fish

Fish are vertebrates. All vertebrates are built along the basicchordatebody plan: a stiff rod running through the length of the animal (vertebral column or notochord),[8]with a hollow tube of nervous tissue (thespinal cord) above it and thegastrointestinal tractbelow. In all vertebrates, the mouth is found at, or right below, theanteriorend of the animal, while theanusopens to the exterior before the end of the body. The remaining part of the body beyond the anus forms a tail with vertebrae and the spinal cord, but no gut.[9]

The defining characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) found in all chordates has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs,derived embryonically and evolutionarily from the notochord). However, a few fish have secondarily[clarification needed]lost this anatomy, retaining the notochord into adulthood, such as thesturgeon.[10]

The vertebral column consists of acentrum(the central body or spine of the vertebra),vertebral archeswhich protrude from the top and bottom of the centrum, and variousprocesseswhich project from the centrum or arches. An arch extending from the top of the centrum is called aneural arch,while thehaemal archorchevronis found underneath the centrum in thecaudal vertebraeof fish. The centrum of a fish is usually concave at each end (amphicoelous), which limits the motion of the fish. In contrast, the centrum of amammalis flat at each end (acoelous), a shape that can support and distribute compressive forces.

The vertebrae oflobe-finned fishesconsist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is broadly similar in form to that found in most other vertebrates. Just beneath the arch lies the small plate-like pleurocentrum, which protects the upper surface of the notochord. Below that, a larger arch-shaped intercentrum protects the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in primitivetetrapods,but in the evolutionary line that led toreptiles,mammals and birds, the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body.[11]

In mostray-finned fishes,including allteleosts,these two structures are fused with and embedded within a solid piece of bone superficially resembling the vertebral body of mammals. In livingamphibians,there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods.[11]

In cartilaginous fish such assharks,the vertebrae consist of two cartilaginous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilaginous structures filling in the gaps between the vertebrae, enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord and has a complex structure, often including multiple layers ofcalcification.[11]

Lampreyshave vertebral arches, but nothing resembling the vertebral bodies found in allhigher vertebrates.Even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region.Hagfisheslack a true vertebral column, but a few tiny neural arches are present in the tail.[11][12]Hagfishes do, however, possess acranium.For this reason, hagfishes have sometimes been excluded from Vertebrata in the past, and instead placed as a sister group of vertebrates within the taxon "Craniata".[13]Molecular analyses[specify]since 1992 have shown that hagfishes are the sister group of lampreys within the cladeCyclostomi,and therefore are vertebrates in aphylogeneticsense.[14]

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Skull bones as they appear in aseahorse
Positions of fish mouths: terminal (a), superior (b), and subterminal or inferior (c).
Illustration ofbarbelson the head of a fish

The head orskullincludes theskull roof(a set of bones covering the brain, eyes and nostrils), the snout (from the eye to the forward-most point of theupper jaw), the operculum orgill cover(absent in sharks andjawless fish), and thecheek,which extends from the eye to thepreopercle.The operculum and preopercle may or may not have spines. In sharks and some primitive bony fish thespiracle,a small extra gill opening, is found behind each eye.

The skull in fishes is formed from a series of only loosely connected bones. Jawless fish and sharks only possess a cartilaginousendocranium,with the upper and lower jaws of cartilaginous fish being separate elements not attached to the skull. Bony fishes have additionaldermal bone,forming a more or less coherent skull roof inlungfishandholost fish.Thelower jawdefines a chin.

In lampreys, the mouth is formed into an oral disk. In most jawed fish, however, there are three general configurations. The mouth may be on the forward end of the head (terminal), may be upturned (superior), or may be turned downwards or on the bottom of the fish (subterminal or inferior). The mouth may be modified into asuckermouthadapted for clinging onto objects in fast-moving water.

The simpler structure is found in jawless fish, in which the cranium is represented by a trough-like basket of cartilaginous elements only partially enclosing the brain and associated with the capsules for the inner ears and the single nostril. Distinctively, these fish have no jaws.[15]

Cartilaginous fish such as sharks also have simple, and presumably primitive, skull structures. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a largefontanelle.The most anterior part of the cranium includes a forward plate of cartilage, therostrum,and capsules to enclose theolfactoryorgans. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the inner ear. Finally, the skull tapers towards the rear, where theforamen magnumlies immediately above a singlecondyle,articulating with the first vertebra. Smallerforaminafor the cranial nerves can be found at various points throughout the cranium. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.[15]

In the ray-finned fishes, there has also been considerable modification from the primitive pattern. The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced; there is little cheek region behind the enlarged orbits, and little if any bone in between them. The upper jaw is often formed largely from thepremaxilla,with themaxillaitself located further back, and an additional bone, thesympletic,linking the jaw to the rest of the cranium.[15]

Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes. The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. The upper jaw is formed from thepterygoid bonesandvomersalone, all of which bear teeth. Much of the skull is formed from cartilage, and its overall structure is reduced.[15]

The head may have several fleshy structures known asbarbels,which may be very long and resemble whiskers. Many fish species also have a variety of protrusions or spines on the head. Thenostrilsornaresof almost all fishes do not connect to the oral cavity, but are pits of varying shape and depth.

External organs

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Jaw

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Moray eelshave two sets of jaws: the oral jaws that capture prey and the pharyngeal jaws that advance into the mouth and move prey from the oral jaws to theesophagusfor swallowing.
Main article:Fish jaw
See also:Shark tooth

The vertebrate jaw probably originally evolved in theSilurianperiod and appeared in thePlacoderm fishwhich further diversified in theDevonian.Jaws are thought to derive from thepharyngeal archesthat support the gills in fish. The two most anterior of these arches are thought to have become the jaw itself (seehyomandibula) and thehyoid arch,which braces the jaw against the braincase and increasesmechanical efficiency.While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible inextantjawed animals (thegnathostomes), which have seven arches, and primitive jawless vertebrates (theAgnatha), which have nine.[citation needed]

Jaws of agreat white shark
External videos
Video of a slingjaw wrasse catching prey by protruding its jaw
Video of a red bay snook catching prey by suction feeding

It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increaserespirationefficiency. The jaws were used in thebuccal pump(observable in modern fish and amphibians) that pumps water across the gills of fish or air into the lungs of amphibians. Over evolutionary time, the more familiar use of jaws in feeding was selected for and became a very important function in vertebrates.

Linkage systemsare widely distributed in animals. The most thorough overview of the different types oflinkages in animalshas been provided by M. Muller,[16]who also designed a new classification system which is especially well suited for biological systems. Linkage mechanisms are especially frequent and various in the head of bony fishes, such aswrasses,which have evolved many specializedaquatic feeding mechanisms.Especially advanced are the linkage mechanisms ofjaw protrusion.Forsuction feedinga system of connected four-bar linkages is responsible for the coordinated opening of the mouth and 3-D expansion of the buccal cavity. Other linkages are responsible forprotrusionof the premaxilla.

Eyes

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Zenion hololepisis a small deep water fish with large eyes.
The deep seahalf-naked hatchetfishhas eyes which look overhead where it can see the silhouettes of prey.
See also:Vision in fish

Fish eyes are similar toterrestrialvertebrates likebirdsand mammals, but have a more sphericallens.Theirretinasgenerally have bothrod cellsandcone cells(forscotopicandphotopic vision), and most species havecolour vision.Some fish can seeultravioletand some can seepolarized light.Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots.[17]The ancestors of modern hagfish, thought to be protovertebrate,[18]were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators and where it is advantageous to have a convex eyespot, which gathers more light than a flat or concave one. Unlike humans, fish normally adjustfocusby moving the lens closer to or further from the retina.[19]

Gills

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Gill of arainbow trout
Main article:Fish gill
See also:Fish respiration

Skin

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The skin of the fish are a part of theintegumentary system,which contains two layers: the epidermis and the dermis layer. Theepidermisis derived from theectodermand becomes the most superficial layer that consists entirely of live cells, with only minimal quantities ofkeratin.It is generally permeable. Thedermisis derived from themesodermand resembles the littleconnective tissuewhich are composed of mostlycollagen fibersfound in bony fish. Some fish species have scales that emerge from the dermis, penetrate the thin layer of thebasement membranethat lies between the epidermis and dermis, and becomes externally visible and covers the epidermis layer.[20]

Generally, the skin also containssweat glandsandsebaceous glandsthat are both unique to mammals, but additional types of skin glands are found in fish. Found in the epidermis, fish typically have numerous individualmucus-secreting skin cells calledgoblet cellsthat produce a slimy substance to the surface of the skin. This aids in insulation and protection from bacterial infection.[21][22]The skin colour of many mammals are often due tomelaninfound in their epidermis. In fish, however, the colour of the skin are largely due tochromatophoresin the dermis, which, in addition to melanin, may containguanineorcarotenoidpigments. Many species, such asflounders,change the colour of their skin by adjusting the relative size of their chromatophores. Some fishes may also havevenomglands,photophores,or cells that produce a more wateryserous fluidin the dermis.[23][21][24]

Placoid scales from a shark: A. epidermis; B. bermis; C. pulp core; D. dentine; E. basal plate; F. enamel; G. spine

Scales

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Main article:Fish scale

Also part of the fish's integumentary system are the scales that cover the outer body of many jawed fish. The commonly known scales are the ones that originate from the dermis or mesoderm, and may be similar in structure to teeth. Some species are covered byscutesinstead. Others may have no scales covering the outer body.

  • Singular bowfin cycloid scale
  • Cycloid scales coveringrohu
  • Bowfin cycloid scales

There are four principal types of fish scales that originate from the dermis.[25][26]

  • Placoid scales,also called dermal denticles, are pointed scales. They are similar to the structure ofteeth,in which they are made ofdentinand covered byenamel.They are typical of cartilaginous fish (even thoughchimaerashave it on claspers only).
  • Ganoid scalesare flat, basal-looking scales. Derived from placoid scales, they have a thick coat of enamel, but without the underlying layer of dentin. These scales cover the fish's body with little overlapping. They are typical ofgarandbichirs.
  • Cycloid scalesare small, oval-shaped scales withgrowth ringslike the rings of a tree. They lack enamel, dentin, and a vascular bone layer.Bowfinandremorahave cycloid scales.
  • Ctenoid scalesare similar to cycloid scales, also having growth rings, lack enamel, dentin, and a vascular bone layer. They are distinguished by spines or projections along one edge.Halibuthave this type of scale.
  • Fish scales: 1. cycloid scale; 2. ctenoid scale; 3. placcoid scale; 4. ganoid scale
  • Cycloid scale
  • Fish scales: A. ganoid; B. cycloid; C. ctenoid

Lateral line

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Main article:Lateral line
The lateral line is clearly visible as a line of receptors running along the side of thisAtlantic cod.

The lateral line is asense organused to detect movement and vibration in the surrounding water. For example, fish can use their lateral line system to follow thevorticesproduced by fleeing prey. In most species, it consists of a line of receptors running along each side of the fish.

Photophores

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Main article:Photophores

Fins

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Main article:Fish fin
Thehaddock,a type ofcod,is ray-finned. It has three dorsal and two anal fins.
Types ofcaudal fin:heterocercal (A), protocercal (B), homocercal (C), and diphycercal (D)
Sharks possess aheterocercalcaudal fin. Thedorsalportion is usually larger than theventralportion.
The high performance[definition needed]bigeye tunais equipped with ahomocercalcaudal fin, finlets and keels.

Fins are the most distinctive features of fish. They are either composed of bony spines or rays protruding from the body with skin covering them and joining them together, either in a webbed fashion as seen in most bony fish, or similar to aflipperas seen in sharks. Apart from the tail or caudal fin, fins have no direct connection with the spine and are supported by muscles only. Their principal function is to help the fish swim. Fins can also be used for gliding or crawling, as seen in theflying fishandfrogfish.Fins located in different places on the fish serve different purposes, such as moving forward, turning, and keeping an upright position. For every fin, there are a number of fish species in which this particular fin has been lost during evolution.[citation needed]

Spines and rays

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Spines have a variety of uses. Incatfish,they are used as a form of defense; many catfish have the ability to lock their spines outwards.Triggerfishalso use spines to lock themselves in crevices to prevent them being pulled out.

Lepidotrichiaare bony, bilaterally-paired, segmented fin rays found in bony fishes. They develop aroundactinotrichiaas part of the dermal exoskeleton. Lepidotrichia may have some cartilage or bone in them as well. They are actually segmented and appear as a series of disks stacked one on top of another. The genetic basis for the formation of the fin rays is thought to be genes coding for the proteinsactinodin 1andactinodin 2.[27]

Types of fin

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  • Dorsal fins:Located on the back of the fish, dorsal fins serve to prevent the fish from rolling and assist in sudden turns and stops. Most fishes have one dorsal fin, but some fishes have two or three. Inanglerfish,the anterior of the dorsal fin is modified into anilliciumandesca,a biological equivalent to afishing rodandlure.The two to three bones that support the dorsal fin are called the proximal, middle, anddistalpterygiophores.In spinous fins, the distal pterygiophore is often fused to the middle or not present at all.
  • Caudal/Tail fins: Also called the tail fins, caudal fins are attached to the end of the caudal peduncle and used for propulsion. The caudal peduncle is the narrow part of the fish's body. The hypural joint is the joint between the caudal fin and the last of the vertebrae. The hypural is often fan-shaped. The tail may beheterocercal,reversed heterocercal,protocercal,diphycercal,orhomocercal.
    • Heterocercal: vertebrae extend into the upper lobe of the tail, making it longer (as in sharks)
    • Reversed heterocercal: vertebrae extend into the lower lobe of the tail, making it longer (as in theAnaspida)
    • Protocercal: vertebrae extend to the tip of the tail; the tail is symmetrical but not expanded (as incyclostomatans,the ancestral vertebrates andlancelets).
    • Diphycercal: vertebrae extend to the tip of the tail; the tail is symmetrical and expanded (as in the bichir, lungfish, lamprey andcoelacanth). MostPalaeozoicfishes had a diphycercal heterocercal tail.[28]
    • Homocercal: vertebrae extend a very short distance into the upper lobe of the tail; tail still appears superficially symmetric. Most fish have a homocercal tail, but it can be expressed in a variety of shapes. The tail fin can be rounded at the end, truncated (almost vertical edge, as in salmon), forked (ending in two prongs), emarginate (with a slight inward curve), or continuous (dorsal, caudal, and anal fins attached, as in eels).
  • Anal fins:Located on the ventral surface behind the anus, this fin is used to stabilize the fish while swimming.
  • Pectoral fins:Found in pairs on each side, usually just behind the operculum. Pectoral fins arehomologousto the forelimbs of tetrapods, and aidwalkingin several fish species such as some anglerfish and themudskipper.A peculiar function of pectoral fins, highly developed in some fish, is the creation of thedynamic liftingforce that assists some fish such as sharks in maintaining depth and also enables the "flight" for flying fish. Certain rays of the pectoral fins may be adapted into finger-like projections, such as insea robinsandflying gurnards.
    • "Cephalic fins": The "horns" ofmanta raysand their relatives, sometimes called cephalic fins, are actually a modification of the anterior portion of the pectoral fin.
  • Pelvic/Ventral fins:Found in pairs on each side ventrally below the pectoral fins, pelvic fins are homologous to the hindlimbs of tetrapods. They assist the fish in going up or down through the water, turning sharply, and stopping quickly. Ingobies,the pelvic fins are often fused into a single sucker disk that can be used to attach to objects.
  • Adipose fin:A soft, fleshy fin found on the back behind the dorsal fin and just in front of the caudal fin. It is absent in many fish families, but is found inSalmonidae,characinsand catfishes. Its function has remained a mystery, and is frequently clipped off to mark hatchery-raised fish, though data from 2005 showed that trout with their adipose fin removed have an 8% higher tailbeat frequency.[29]Additional research published in 2011 has suggested that the fin may be vital for the detection of and response to stimuli such as touch, sound and changes in pressure. Canadian researchers identified a neural network in the fin, indicating that it likely has a sensory function, but are still not sure exactly what the consequences of removing it are.[30]

Internal organs

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Internal organs of a maleyellow perch:A.gill,B.heart atrium,C.heart ventricle,D.liver(cut), E.stomach,F.pyloric caeca,G.swim bladder,H.intestine,I.testis,J.urinary bladder
Internal organsof a femaleAtlantic cod:1.liver,2.gas bladder,3.ovary,4.pyloric caeca,5.stomach,6.intestine
Theblack swalloweris a species ofdeep sea fishwith an extensible stomach which allows it to swallow fish larger than itself.

Intestines

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As with other vertebrates, theintestinesof fish consist of two segments, thesmall intestineand thelarge intestine.In most higher vertebrates, the small intestine is further divided into theduodenumand other parts. In fish, the divisions of the small intestine are not as clear, and the termsanterior intestineorproximal intestinemay be used instead of duodenum.[31] In bony fish, the intestine is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number ofpyloric caeca,small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is noileocaecal valvein teleosts, with the boundary between the small intestine and therectumbeing marked only by the end of the digestiveepithelium.[24]There is no small intestine as such in non-teleost fish, such as sharks, sturgeons, and lungfish. Instead, the digestive part of the gut forms aspiral intestine,connecting thestomachto the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This fold creates a valve-like structure that greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.[24]In lampreys, thespiral valveis extremely small, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions.[24]

Pyloric caeca

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Many fish have a number of small outpocketings, called pyloric caeca, along their intestine. The purpose of the caeca is to increase the overall surface area of the intestines, thereby increasing the absorption of nutrients.[32][33]

The number of pyloric caeca varies widely between species, and in some species of fish no caeca are present at all. Species with few or no caeca compensate for their lack by having longer intestines, or by have taller or more convoluted intestinal villi, thereby achieving similar levels of absorptive surface area.[32][33]

Lungfish also have a pouch located at the beginning of their intestine, which is also called a pyloriccaecum,but it has a different structure and function that the pyloric caeca of other fish species. The lungfish caecum is homologous (due to common descent) with the caecum present in mostamniotes(tetrapod vertebrates that include all mammals, reptiles, and birds).[32]In most herbivores the caecum receives partially digested food from the small intestine, and serves as a fermentation chamber to break down cellulose (such as grass or leaves) in the diet. In carnivores the caecum is often greatly reduced or missing.

Stomach

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As with other vertebrates, the relative positions of theesophagealandduodenalopenings to the stomach remain relatively constant. As a result, the stomach always curves somewhat to the left before curving back to meet thepyloric sphincter.However, lampreys, hagfishes,chimaeras,lungfishes, and some teleost fish have no stomach at all, with theesophagusopening directly into the intestine. These fish consume diets that either require little storage of food, no pre-digestion with gastric juices, or both.[34]

Kidneys

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Thekidneysof fish are typically narrow, elongated organs, occupying a significant portion of the trunk. They are similar to themesonephrosof higher vertebrates (reptiles, birds, and mammals). The kidneys contain clusters ofnephrons,serviced by collecting ducts which usually drain into amesonephric duct.However, the situation is not always so simple. In cartilaginous fish, there is also a shorter duct which drains the posterior (metanephric) parts of the kidney, and joins with the mesonephric duct at thebladderor cloaca. Indeed, in many cartilaginous fish, the anterior portion of the kidney may degenerate or cease to function altogether in the adult.[35]Hagfish and lamprey kidneys are unusually simple. They consist of a row of nephrons, each emptying directly into the mesonephric duct.[35] Like the Nile tilapia, the kidney of some fish shows its three parts; head, trunk, and tail kidneys.[36] Fish do not have a discrete adrenal gland with distinct cortex and medulla, similar to those found in mammals. The interrenal and chromaffin cells are located within the head kidney.[36]

Urinary bladder

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This section is an excerpt fromBladder § Fish.[edit]

The gills of mostteleostfish help to eliminate ammonia from the body, and fish live surrounded by water, but most still have a distinct bladder for storing waste fluid. The urinary bladder ofteleostsis permeable to water, though this is less true for freshwater dwelling species than saltwater species.[37]: p. 219 In freshwater fish the bladder is a key site of absorption for many major ions[38]in marine fish urine is held in the bladder for extended periods to maximise water absorption.[38]The urinary bladders of fish andtetrapodsare thought to be analogous while the former's swim-bladders and latter'slungsare considered homologous.

Most fish also have an organ called aswim-bladderwhich is unrelated to the urinary bladder except in its membranous nature. Theloaches,pilchards,andherringsare among the few types of fish in which a urinary bladder is poorly developed. It is largest in those fish which lack an air bladder, and is situated in front of theoviductsand behind therectum.[39]

Spleen

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Thespleenis found in nearly all vertebrates. It is a non-vital organ, similar in structure to a largelymph node.It acts primarily as a blood filter, and plays important roles in regards to red blood cells and theimmune system.[40]In cartilaginous and bony fish it consists primarily of red pulp and is normally a somewhat elongated organ as it actually lies inside theserosallining of the intestine.[41]The only vertebrates lacking a spleen are the lampreys and hagfishes. Even in these animals, there is a diffuse layer ofhaematopoietictissue within the gut wall, which has a similar structure tored pulp,and is presumed to be homologous to the spleen of higher vertebrates.[41]

Liver

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The liver is a largevital organpresent in all fish. It has a wide range of functions, includingdetoxification,protein synthesis,and production of biochemicals necessary for digestion. It is very susceptible to contamination by organic and inorganic compounds because they can accumulate over time and cause potentially life-threatening conditions. Because of the liver's capacity for detoxification and storage of harmful components, it is often used as an environmentalbiomarker.[42]

Heart

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Blood flow through the heart: sinus venosus, atrium, ventricle, and outflow tract
Cardiovascular cycle in a fish

Fish have what is often described as a two-chambered heart,[43]consisting of oneatriumto receive blood and oneventricleto pump it,[44]in contrast to three chambers (two atria, one ventricle) of amphibian and most reptile hearts and four chambers (two atria, two ventricles) of mammal and bird hearts.[43]However, the fish heart has entry and exit compartments that may be called chambers, so it is also sometimes described as three-chambered,[44]or four-chambered,[45]depending on what is counted as a chamber. The atrium and ventricle are sometimes considered "true chambers", while the others are considered "accessory chambers".[46]

The four compartments are arranged sequentially:

  1. Sinus venosus:A thin-walled sac or reservoir with some cardiac muscle that collects deoxygenated blood through the incominghepaticandcardinal veins.[verification needed][44]
  2. Atrium: A thicker-walled, muscular chamber that sends blood to the ventricle.[44]
  3. Ventricle: A thick-walled, muscular chamber that pumps the blood to the fourth part, the outflow tract.[44]The shape of the ventricle varies considerably, usually tubular in fish with elongated bodies, pyramidal with a triangular base in others, or sometimes sac-like in some marine fish.[45]
  4. Outflow tract (OFT): Goes to the ventral aorta and consists of the tubularconus arteriosus,bulbus arteriosus,or both.[45]The conus arteriosus, typically found in more primitive species of fish, contracts to assist blood flow to the aorta, while the bulbus anteriosus does not.[46][47]

Ostial valves, consisting of flap-like connective tissues, prevent blood from flowing backward through the compartments.[45]The ostial valve between the sinus venosus and atrium is called the sino-atrial valve, which closes during ventricular contraction.[45]Between the atrium and ventricle is an ostial valve called theatrioventricular valve,and between the bulbus arteriosus and ventricle is an ostial valve called the bulbo-ventricular valve.[45]The conus arteriosus has a variable number ofsemilunar valves.[46]

The ventral aorta delivers blood to the gills where it is oxygenated and flows, through thedorsal aorta,into the rest of the body. (In tetrapods, the ventral aorta is divided in two; one half forms theascending aorta,while the other forms thepulmonary artery).[41]

The circulatory systems of all vertebrates areclosed.Fish have the simplest circulatory system, consisting of only one circuit, with the blood being pumped through the capillaries of the gills and on to thecapillariesof the body tissues. This is known assingle cyclecirculation.[48]

In the adult fish, the four compartments are not arranged in a straight row, instead forming an S-shape with the latter two compartments lying above the former two. This relatively simpler pattern is found in cartilaginous fish and in the ray-finned fish. In teleosts, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper. The conus arteriosus is not present in any amniotes, presumably having been absorbed into the ventricles over the course of evolution. Similarly, while the sinus venosus is present as a vestigial structure in some reptiles and birds, it is otherwise absorbed into theright atriumand is no longer distinguishable.[41]

Swim bladder

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Main article:Swim bladder
The swim bladder of arudd

Weberian apparatus

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Fishes of thesuperorderOstariophysipossess a structure called theWeberian apparatus,a modification which allows them to hear better. This ability may explain the marked success of ostariophysian fishes.[49]The apparatus is made up of a set of bones known asWeberian ossicles,a chain of small bones that connect the auditory system to the swim bladder of fishes.[50]Theossiclesconnect the gas bladder wall with Y-shapedlymph sinusthat is next to thelymph-filled transverse canal joining thesacculesof the right and left ears. This allows the transmission of vibrations to the inner ear. A fully functioning Weberian apparatus consists of the swim bladder, the Weberian ossicles, a portion of the anterior vertebral column, and some muscles and ligaments.[50]

Reproductive organs

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Main article:Fish reproduction
See also:Reproductive processes in fish
Testicles or ovaries (7)

Fish reproductive organs includetesticlesandovaries.In most species,gonadsare paired organs of similar size, which can be partially or totally fused.[51]There may also be a range of secondary organs that increasereproductive fitness.Thegenital papillais a small, fleshy tube behind the anus inteleostfishes from which the sperm or eggs are released; the sex of a fish often can be determined by the shape of itspapilla.[citation needed]Sex determination in fish, which is dependent on intrinsic genetic factors, is followed by sex differentiation through gene expression of feedback mechanisms that ensure the stability of the levels of particular hormones and cellular profile. However, thehermaphroditicspecies are an exception in which they are able to alter the course of sex differentiation in order to maximize their fitness. There are various determination mechanisms for gonadal sex in fish and processes that aid development of the gonadal function. Gonadal sex is influenced by a number of factors, includingcell-autonomous genetic mechanisms,endocrine,paracrine,behavioral, or environmental signals. This results in theprimordial germ cells (PGCs)to be able to interpret internal or external stimuli to develop intospermatogoniaoroogonia.[52]Spermatogenesisin testes is a process in which spermatogonia differentiates intospermatocytesthroughmitosisandmeiosis,which halves the number ofchromosomes,creatinghaploidspermatids.Duringspermiogenesis,the last stage of spermatogenesis, the haploid spermatids develop intospermatozoa.[53]In the ovaries,oogoniaalso undergo mitosis and meiosis duringoogenesis,and this gives rise to primary oocytes and then eventually theovum.The primary oocyte divides and produces the secondary oocyte as well as apolar body,before the secondary oocyte develops into the haploidootid.[54]

1) Sagittal view of the anus and urogenital opening. 2) Ventral view of the anus and urogenital opening.

Testes

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Maturity stages of (A) ovaries and (B) testicles of the cichlidCrenicichla menezesi:[55](a) immature; (b) maturing; (c) mature; (d) partially spent

Most male fish have two testes of similar size. In the case of sharks, the testis on the right side is usually larger. The primitive jawless fish have only a single testis located in the midline of the body, although even this forms from the fusion of paired structures in the embryo.[41]

Under a tough membranous shell, thetunica albuginea,the testis of some teleost fish, contains very fine coiled tubes calledseminiferous tubules.The tubules are lined with a layer of cells (germ cells) that frompubertyinto old age, develop intospermcells (also known asspermatozoaor malegametes). The developing sperm travel through the seminiferous tubules to therete testislocated in themediastinum testis,to theefferent ducts,and then to theepididymides(depending on the species) where newly created sperm cells mature (seespermatogenesis). The sperm move into thevasa deferentia,and are eventually expelled through theurethraand out of the urethral orifice through muscular contractions.

However, most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures calledsperm ampullae.These are seasonal structures, releasing their contents during the breeding season and then being reabsorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebrates, including the same range of cell types.[56]

In terms ofspermatogoniadistribution, the structure of teleost testes have two types: in the most common, spermatogonia occur all along the seminiferous tubules, while inAtherinomorpha,they are confined to the distal portion of these structures. Fish can present cystic or semi-cystic spermatogenesis[definition needed]in relation to the release phase of germ cells in cysts to thelumenof the seminiferous tubules.[51]

Ovaries

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Many of the features found in ovaries are common to all vertebrates, including the presence offollicular cellsandtunica albugineaThere may be hundreds or even millions of fertile eggs present in the ovary of a fish at any given time. Fresh eggs may be developing from thegerminal epitheliumthroughout life.Corpora luteaare found only in mammals, and in someelasmobranchfish; in other species, the remnants of the follicle are quickly resorbed by the ovary.[56]The ovary of teleosts is often contains a hollow, lymph-filled space which opens into theoviduct,and into which the eggs are shed.[56]Most normal female fish have two ovaries. In some elasmobranchs, only the right ovary develops fully. In the primitive jawless fish and some teleosts, there is only one ovary, formed by the fusion of the paired organs in the embryo.[56]

Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, theoocytesare released directly into thecoelomiccavity and then enter theostium,then through the oviduct and are eliminated. Secondary gymnovarian ovaries shedovainto the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct.[57]Gymnovaries are the primitive condition found in lungfish, sturgeon, andbowfin.Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.[51]Secondary gymnovaries are found insalmonidsand a few other teleosts.

Nervous system

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Dorsal view of the brain of therainbow trout
See also:Brain § Vertebrates

Central nervous system

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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.[58]However, some fish have relatively large brains, most notablymormyridsand sharks, which have brains about as massive relative to body weight as birds andmarsupials.[59]

Fish brains are divided into several regions. At the front are theolfactory lobes,a pair of structures that receive and process signals from the nostrils via the twoolfactory nerves.[58]Similar to the way humans smell chemicals in the air, fish smell chemicals in the water by tasting them. 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-lobedtelencephalon,the structural equivalent to thecerebrumin higher vertebrates. In fish the telencephalon is concerned mostly witholfaction.[58]Together these structures form theforebrain.

The forebrain is connected to themidbrainvia thediencephalon(in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated withhormonesandhomeostasis.[58]Thepineal bodylies just above the diencephalon. This structure detects light, maintainscircadianrhythms, and controls colour changes.[58]The midbrain ormesencephaloncontains the twooptic lobes.These are very large in species that hunt by sight, such asrainbow troutandcichlids.[58]

Thehindbrainormetencephalonis particularly involved in swimming and balance.[58]Thecerebellumis a single-lobed structure that is typically the biggest part of the brain.[58]Hagfish and lampreys have relatively small cerebella, while the mormyrid cerebellum is massive and apparently involved in theirelectrical sense.[58]

Thebrain stemormyelencephalonis the brain's posterior.[58]As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs respiration andosmoregulation.[58]

Vertebrates are the only chordate group to exhibit a proper brain. A slight swelling of the anterior end of thedorsal nerve cordis found in the lancelet, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towardscephalisation.[9]Thecentral nervous systemis based on a hollow nerve tube running along the length of the animal, from which theperipheral nervous systembranches out toinnervatethe various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of thecentral canal of spinal cordinto three primary brain vesicles; theprosencephalon(forebrain), mesencephalon (midbrain) andrhombencephalon(hindbrain) then further differentiated in the various vertebrate groups.[60]Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss.[61][62]The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and somesalamanders.Vesicles of the forebrain are usually paired, giving rise to hemispheres like thecerebral hemispheresin mammals.[60]The resulting anatomy of the central nervous system, with a single, hollowventral nerve cordtopped by a series of (often paired) vesicles is unique to vertebrates.[9]

Cross-section of the brain of aporbeagle sharkwith the cerebellum highlighted

Cerebellum

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The circuits in the cerebellum are similar across allclassesof vertebrates, including fish, reptiles, birds, and mammals.[63]There is also an analogous brain structure incephalopodswith well-developed brains, such asoctopuses.[64]This has been taken as evidence that the cerebellum performs functions important to all animal species with a brain.

There is considerable variation in the size and shape of the cerebellum in different vertebrate species. In amphibians, lampreys, and hagfish, the cerebellum is little developed; in the latter two groups, it is barely distinguishable from the brain-stem. Although thespinocerebellumis present in these groups, the primary structures are small paired nuclei corresponding to thevestibulocerebellum.[56]

The cerebellum of cartilaginous and bony fishes is extraordinarily large and complex. In at least one important respect, it differs in internal structure from the mammalian cerebellum: The fish cerebellum does not contain discretedeep cerebellar nuclei.Instead, the primary targets ofPurkinje cellsare a distinct type of cell distributed across the cerebellar cortex, a type not seen in mammals. In mormyrids (a family of weakly electrosensitive freshwater fish), the cerebellum is considerably larger than the rest of the brain put together. The largest part of it is a special structure called thevalvula,which has an unusually regular architecture and receives much of its input from the electrosensory system.[65]

Most species of fish and amphibians possess a lateral line system that sensespressure wavesin water. One of the brain areas that receives primary input from the lateral line organ, the medial octavolateral nucleus, has a cerebellum-like structure, with granule cells and parallel fibers. In electrosensitive fish, the input from the electrosensory system goes to the dorsal octavolateral nucleus, which also has a cerebellum-like structure. In ray-finned fishes (by far the largest group), theoptic tectumhas a layer—the marginal layer—that is cerebellum-like.[63]

Identified neurons

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A neuron is "identified" if it has properties that distinguish it from every other neuron in the same animal—properties such as location,neurotransmitter,gene expressionpattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties.[66]In vertebrate nervous systems, very few neurons are "identified" in this sense (in humans, there are believed to be none). In simpler nervous systems, some or all neurons may be thus unique.[67]

In vertebrates, the best known identified neurons are the giganticMauthner cellsof fish.[68]Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has anaxonthat crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. Thesynapsesgenerated by a Mauthner cell are so powerful that a singleaction potentialgives rise to a major behavioral response: within milliseconds the fish curves its body into aC-shape,then straightens, thereby propelling itself rapidly forward. Functionally, this is afast escape response,triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior, other types of cells usually contribute to shaping the amplitude and direction of the response.

Mauthner cells have been described ascommand neurons.A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior all by itself.[69]Such neurons appear most commonly in the fast escape systems of various species—thesquid giant axonandsquid giant synapse,used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances.[70]

Immune system

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See also:Fish diseases and parasites

Immune organs vary by type of fish.[71]In the jawless fish (lampreys and hagfish), true lymphoid organs are absent. These fish rely on regions oflymphoid tissuewithin other organs to produce immune cells. For example,erythrocytes,macrophagesandplasma cellsare produced in the anterior kidney (orpronephros) and some areas of the gut (wheregranulocytesmature). They resemble primitivebone marrowin hagfish.

Cartilaginous fish (sharks and rays) have a more advanced immune system. They have three specialized organs that are unique tochondrichthyes;the epigonal organs (lymphoid tissues similar to mammalian bone) that surround the gonads, theLeydig's organwithin the walls of their esophagus, and a spiral valve in their intestine. These organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiablethymusand a well-developed spleen (their most important immune organ) where variouslymphocytes,plasma cells and macrophages develop and are stored.

Chondrosteanfish (sturgeons, paddlefish and bichirs) possess a major site for the production of granulocytes within a mass that is associated with themeninges,the membranes surrounding the central nervous system. Their heart is frequently covered with tissue that contains lymphocytes,reticular cellsand a small number of macrophages. The chondrostean kidney is an importanthemopoieticorgan; it is where erythrocytes, granulocytes, lymphocytes and macrophages develop.

Like chondrostean fish, the major immune tissues of bony fish (teleostei) include the kidney (especially the anterior kidney), which houses many different immune cells.[72]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,neutrophilsand granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.[73][74]In 2006, a lymphatic system similar to that in mammals was described in one species of teleost fish, thezebrafish.Although not confirmed as yet, this system presumably will be where unstimulatednaive T cellsaccumulate while waiting to encounter anantigen.[75]

See also

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Works cited

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  • Helfman, G.; Collette, B.; Facey, D. (1997).The Diversity of Fishes(1st ed.). Wiley-Blackwell.ISBN978-0-86542-256-8.
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