Fish anatomy

Fish anatomy

Overview
Fish anatomy is primarily governed by the physical characteristics of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, which is much denser
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 than air, holds a relatively small amount of dissolved oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, and absorbs more light than air does.

Fish have a variety of different body plan
Body plan
A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...

s. Their body is divided into head, trunk, and tail, although the divisions are not always externally visible. The body is often fusiform
Fusiform
Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends.* Aneurysms can be classified as saccular or fusiform...

, a streamlined body plan often found in fast-moving fish.
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Encyclopedia
Fish anatomy is primarily governed by the physical characteristics of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, which is much denser
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 than air, holds a relatively small amount of dissolved oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, and absorbs more light than air does.

Body


Fish have a variety of different body plan
Body plan
A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...

s. Their body is divided into head, trunk, and tail, although the divisions are not always externally visible. The body is often fusiform
Fusiform
Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends.* Aneurysms can be classified as saccular or fusiform...

, a streamlined body plan often found in fast-moving fish. They may also be filiform (eel
Eel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...

-shaped) or vermiform (worm-shaped). Also, fish are often either laterally (thin) or dorsally (flat) compressed.

The caudal peduncle is the narrow part of the fish's body to which the caudal or tail fin is attached. The hypural joint is the joint between the caudal fin and the last of the vertebrae. The hypural is often fan-shaped.

Photophore
Photophore
A photophore is a light-emitting organ which appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors...

s
are light-emitting organs which appears as luminous spots on some fishes. The light can be produced from compounds during the digestion of prey, from specialized mitochondrial cells in the organism called photocytes, or associated with symbiotic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, and are used for attracting food or confusing predators.

The lateral line
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

is a sense organ used to detect movement and vibration in the surrounding water. In most species, it consists of a line of receptors running along each side of the fish.

The ampullae of Lorenzini
Ampullae of Lorenzini
The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled pores. They are mostly discussed as being found in cartilaginous fishes ; however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also been...

allow sharks to sense electrical discharges.

The genital papilla is a small, fleshy tube behind the anus in some fishes, from which the sperm or eggs are released; the sex of a fish often can be determined by the shape of its papilla.

Head



The head includes the snout, from the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

to the forwardmost point of the upper jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...

, the operculum
Operculum (fish)
The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body....

or gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 cover (absent in sharks and jawless fish
Agnatha
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....

), and the cheek
Cheek
Cheeks constitute the area of the face below the eyes and between the nose and the left or right ear. They may also be referred to as jowls. "Buccal" means relating to the cheek. In humans, the region is innervated by the buccal nerve...

, which extends from eye to preopercle. The operculum and preopercle may or may not have spines. In sharks and some primitive bony fish a spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...

, small extra gill opening, is found behind each eye.

The skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 is fishes is formed from a series of only loosely connected bones. Jawless fish and sharks only poses a cartilaginous endocranium
Endocranium
For internal cast of the cranium, see Endocast.The endocranium in comparative anatomy is a part of the skull base in vertebrates and represent the basal, inner part of the cranium. The term is also applied to the outer layer of the dura mater in human anatomy.-Basic structure:Structurally, the...

, with both the upper an lower jaws being separate elements.Bony fishes has additional dermal bone
Dermal bone
A dermal bone - bony structures derived from intramembranous ossification that form components of the vertebrate skeleton including the skull, jaws, gills, fins and exoskeleton. In contrast to endochondral bone, dermal bone does not form from cartilage first and then calcify...

, forming a more or less coherent skull roof
Skull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...

 in lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...

 and holost fish
Holostei
Holostei are bony fish that show primitive characteristics. There are eight species divided among two orders, the Amiiformes represented by a single living species, the bowfin , and the Lepisosteiformes, the gars. There are more species to be found in the fossil record.Holostei share with other...

. The lower jaw defines a chin
Chin
In the human anatomy, the chin is the lowermost part of the face.It is formed by the lower front of the mandible.People show a wide variety of chin structures. See Cleft chin....

.

In lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s, 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 a suckermouth
Suckermouth
In fishes, a suckermouth is a ventrally-oriented mouth adapted for grazing on algae and small organisms that grow on submerged objects....

adapted for clinging onto objects in fast-moving water.

The head may have several fleshy structures known as barbel
Barbel (anatomy)
A barbel on a fish is a slender, whiskerlike tactile organ near the mouth. Fish that have barbels include the catfish, the carp, the goatfish, sturgeon, the zebrafish and some species of shark...

s
, which may be very long and resemble whiskers. Many fish species also have a variety of protrusions or spines on the head. The nostril
Nostril
A nostril is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation...

s or nares of almost all fishes do not connect to the oral cavity, but are pits of varying shape and depth.

Fins


The fins are the most distinctive features of a fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, composed of bony spines protruding from the body with skin covering them and joining them together, either in a webbed fashion, as seen in most bony fish, or more similar to a flipper
Flipper (anatomy)
A flipper is a typically flat limb evolved for movement through water. Various creatures have evolved flippers, for example penguins , cetaceans A flipper is a typically flat limb evolved for movement through water. Various creatures have evolved flippers, for example penguins (also called...

, as seen in shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s. These usually serve as a means for the fish to swim. Fins can also be used for gliding or crawling, as seen in the flying fish and frogfish
Frogfish
Frogfishes, family Antennariidae, are a type of anglerfish in the order Lophiiformes. They are known as anglerfishes in Australia, where 'frogfish' refers to a different type of fish...

. Fins located in different places on the fish serve different purposes, such as moving forward, turning, and keeping an upright position.

Spines and rays


In bony fish, most fins may have spines or rays. A fin may contain only spiny rays, only soft rays, or a combination of both. If both are present, the spiny rays are always anterior
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

. Spines are generally stiff and sharp. Rays are generally soft, flexible, segmented, and may be branched. This segmentation of rays is the main difference that separates them from spines; spines may be flexible in certain species, but they will never be segmented.

Spines have a variety of uses. In catfish
Catfish
Catfishes are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...

, they are used as a form of defense; many catfish have the ability to lock their spines outwards. Triggerfish
Triggerfish
Triggerfishes are about 40 species of often brightly colored fishes of the family Balistidae. Often marked by lines and spots, they inhabit tropical and subtropical oceans throughout the world, with the greatest species richness in the Indo-Pacific...

 also use spines to lock themselves in crevices to prevent them being pulled out.

Lepidotrichia are bony, bilaterally-paired, segmented fin rays found in bony fishes
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...

. They develop around actinotrichia as part of the dermal exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

. Lepidotrichia may have some cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 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 proteins actinodin 1 and actinodin 2.

Types of fin

  • Dorsal fin
    Dorsal fin
    A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

    s
    are located on the back. A fish can have up to three of them. The dorsal fins serve to protect the fish against rolling, and assists in sudden turns and stops.
    • In anglerfish
      Anglerfish
      Anglerfishes are members of the teleost order Lophiiformes . They are bony fishes named for their characteristic mode of predation, wherein a fleshy growth from the fish's head acts as a lure; this is considered analogous to angling.Some anglerfishes are pelagic , while others are benthic...

      , the anterior of the dorsal fin is modified into an illicium and esca, a biological equivalent to a fishing rod
      Fishing rod
      A fishing rod or a fishing pole is a tool used to catch fish, usually in conjunction with the pastime of angling, and can also be used in competition casting. . A length of fishing line is attached to a long, flexible rod or pole: one end terminates in a hook for catching the fish...

       and lure
      Fishing lure
      A fishing lure is an object attached to the end of a fishing line which is designed to resemble and move like the prey of a fish. The purpose of the lure is to use movement, vibration, and colour to catch the fish's attention so it bites the hook...

      .
    • The bones that support the dorsal fin are called Pterygiophore. There are two to three of them: "proximal", "middle", and "distal". In spinous fins the distal is often fused to the middle, or not present at all.
  • The caudal fin is the tail fin, located at the end of the caudal peduncle and is used for propulsion.
    • The tail can be heterocercal, which means that the vertebrae extend into a larger lobe of the tail or that the tail is asymmetrical
      • Epicercal means that the upper lobe is longer (as in sharks)
      • Hypocercal means that the lower lobe is longer (as in flying fish)
    • Protocercal means that the caudal fin extends around the vertebral column, present in embryonic fish and hagfish
      Hagfish
      Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...

      . This is not to be confused with a caudal fin that has fused with the dorsal and anal fins to form a contiguous fin.
    • Diphycercal refers to the special, three-lobed caudal fin of the coelacanth
      Coelacanth
      Coelacanths are members of an order of fish that includes the oldest living lineage of Sarcopterygii known to date....

       and lungfish
      Lungfish
      Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...

       where the vertebrae extend all the way to the end of the tail.
    • Most fish have a homocercal tail, where the vertebrae do not extend into a lobe and the fin is more or less symmetrical. This can be expressed in a variety of shapes.
      • The tail fin may be rounded at the end.
      • The tail fin may be truncated, or end in a more-or-less vertical edge (such as in salmon).
      • The fin may be forked, or end in two prongs.
      • The tail fin may be emarginate, or with a slight inward curve.
      • The tail fin may be lunate, or shaped like a crescent moon.
  • The anal fin is located on the ventral
    Anatomical terms of location
    Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

     surface behind the anus
    Anus
    The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

    . This fin is used to stabilize the fish while swimming.
  • The paired pectoral fins are located on each side, usually just behind the operculum, and are homologous
    Homology (biology)
    Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

     to the forelimbs of tetrapod
    Tetrapod
    Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

    s.
    • A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish, such as shark
      Shark
      Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

      s, in maintaining depth and also enables the "flight" for flying fish.
    • In many fish, the pectoral fins aid in walking
      Walking fish
      Walking fish, sometimes called ambulatory fish, is a general term that refers to fish that are able to travel over land for extended periods of time. The term may also be used for some other cases of nonstandard fish locomotion, e.g., when describing fish "walking" along the sea floor.-Types of...

      , especially in the lobe-like fins of some anglerfish and in the mudskipper
      Mudskipper
      Mudskippers are members of the subfamily Oxudercinae , within the family Gobiidae . They are completely amphibious fish, fish that can use their pectoral fins to walk on land...

      .
    • Certain rays of the pectoral fins may be adapted into finger-like projections, such as in sea robin
      Sea robin
      Sea robins, also known as gurnard, are bottom-feeding scorpaeniform fishes in the family Triglidae. They get their name from their large pectoral fins, which, when swimming, open and close like a bird's wings in flight....

      s and flying gurnards.
      • The "horns" of manta ray
        Manta ray
        The manta ray is the largest species of the rays. The largest known specimen was more than across, with a weight of about . It ranges throughout waters of the world, typically around coral reefs...

        s and their relatives are called cephalic fins; this is actually a modification of the anterior portion of the pectoral fin.
  • The paired pelvic or ventral fins are located ventrally below the pectoral fins. They are homologous
    Homology (biology)
    Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

     to the hindlimbs of tetrapod
    Tetrapod
    Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

    s. The pelvic fin assists the fish in going up or down through the water, turning sharply, and stopping quickly.
    • In gobies
      Goby
      The gobies form the family Gobiidae, which is one of the largest families of fish, with more than 2,000 species in more than 200 genera. Most are relatively small, typically less than 10 cm in length...

      , the pelvic fins are often fused into a single sucker disk. This can be used to attach to objects.

  • The adipose fin is a soft, fleshy fin found on the back behind the dorsal fin and just forward of the caudal fin. It is absent in many fish families, but is found in Salmonidae
    Salmonidae
    Salmonidae is a family of ray-finned fish, the only living family currently placed in the order Salmoniformes. It includes salmon, trout, chars, freshwater whitefishes and graylings...

    , characins
    Characidae
    The Characidae, characids or characins are a family of freshwater subtropical and tropical fish, belonging to the order Characiformes. The name "characins" is the historical one, but scientists today tend to prefer "characids" to reflect their status as a by and large monophyletic group at family...

     and catfish
    Catfish
    Catfishes are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...

    es. 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. Additional released 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.
  • Some types of fast-swimming fish have a horizontal caudal keel
    Keel
    In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

     just forward of the tail fin. This is a lateral ridge on the caudal peduncle, usually composed of scutes (see below), that provides stability and support to the caudal fin. There may be a single paired keel, one on each side, or two pairs above and below.
  • Finlets are small fins, generally behind the dorsal and anal fins (in bichir
    Bichir
    The bichirs are a family, Polypteridae, of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes.All species occur in freshwater habitats in tropical Africa and the Nile River system, mainly swampy, shallow floodplains and estuaries.-Anatomy and appearance:Bichirs are...

    s, there are only finlets on the dorsal surface and no dorsal fin). In some fish such as tuna
    Tuna
    Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

     or sauries, they are rayless, non-retractable, and found between the last dorsal and/or anal fin and the caudal fin.


For every fin, there are a number of fish species in which this particular fin has been lost during evolution.

Internal fertilization


In many species of fish, fins have been modified to allow internal fertilization.

A gonopodium is an anal fin that is modified into an intromittent organ
Intromittent organ
An intromittent organ is a general term for an external organ of a male organism that is specialized to deliver sperm during copulation. Intromittent organs are found most often in terrestrial species, as most aquatic species fertilize their eggs externally, although there are...

 in males of certain species of live-bearing fish in the families Anablepidae
Anablepidae
Anablepidae is a family of freshwater and brackish water fishes living on river estuaries from southern Mexico to southern South America. There are three genera with sixteen species: the four-eyed fishes , the onesided livebearers and the white-eye, Oxyzygonectes dovii...

 and Poeciliidae
Poeciliidae
Poeciliidae is a family of fresh-water fish which are live-bearing aquarium fish . They belong to the order Cyprinodontiformes, tooth-carps, and include well-known aquarium fish such as the guppy, molly, platy, and swordtail...

. It is movable and used to impregnate females during mating. The male's anal fin’s 3rd, 4th and 5th rays are formed into a tube like structure in which the sperm of the fish is ejected. In some species, the gonopodium may be as much as 50% of the total body length. Occasionally the fin is too long to be used, as in the "lyretail" breeds of Xiphophorus helleri. Hormone treated females may develop gonopodia. These are useless for breeding. One finds similar organs having the same characteristics in other types of fish, for example the andropodium in the Hemirhamphodon
Hemirhamphodon
Hemirhamphodon is a genus of halfbeak found in peaty and lowland forest streams in Southeast Asia. Six species are known, all relatively small, the largest species being about 10 cm in length...

or in the Goodeidae
Goodeidae
Splitfins, are a family, Goodeidae, of teleost fish endemic to Mexico and some areas of the United States. This family contains 40 species within 18 genera. The family is named after ichthyologist George Brown Goode.-Range and Geographic Distribution:...

.

When ready for mating, the gonopodium becomes “erect” and points forward, towards the female. The male shortly inserts the organ into the sex opening of the female, with hook-like adaptations that allow the fish to grip onto the female to ensure impregnation. If a female remains stationary and her partner contacts her vent with his gonopodium, she is fertilized. The sperm is preserved in the female's oviduct. This allows females to, at any time, fertilize themselves without further assistance of males.

Male cartilaginous fish have claspers modified from pelvic fins. These are intromittent organs, used to channel semen into the female's cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...

 during copulation.

Skin



The outer body of many fish is covered with scale
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

s
. Some species are covered instead by scute
Scute
A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, the feet of some birds or the anterior portion of the mesonotum in insects.-Properties:...

s
. Others have no outer covering on the skin; these are called naked fish. Most fish are covered in a protective layer of slime (mucus).

There are four types of fish scales.
  1. Placoid scales, also called dermal denticles, are similar to teeth in that they are made of dentin
    Dentin
    Dentine is a calcified tissue of the body, and along with enamel, cementum, and pulp is one of the four major components of teeth. Usually, it is covered by enamel on the crown and cementum on the root and surrounds the entire pulp...

     covered by enamel
    Tooth enamel
    Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...

    . They are typical of shark
    Shark
    Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

    s and rays
    Batoidea
    Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

    .
  2. Ganoid scales are flat, basal-looking scales that cover a fish body with little overlapping. They are typical of gar
    Gar
    In American English the name gar is strictly applied to members of the Lepisosteidae, a family including seven living species of fish in two genera that inhabit fresh, brackish, and occasionally marine, waters of eastern North America, Central America, and the Caribbean islands.-Etymology:In...

     and bichir
    Bichir
    The bichirs are a family, Polypteridae, of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes.All species occur in freshwater habitats in tropical Africa and the Nile River system, mainly swampy, shallow floodplains and estuaries.-Anatomy and appearance:Bichirs are...

    s.
  3. Cycloid scales are small oval-shaped scales with growth rings. Bowfin
    Bowfin
    The Bowfin, Amia calva, is the last surviving member of the order Amiiformes , and of the family Amiidae...

     and remora
    Remora
    The remora , sometimes called a suckerfish or sharksucker, is an elongated, brown fish in the order Perciformes and family Echeneidae...

     have cycloid scales.
  4. Ctenoid scales are similar to the cycloid scales, with growth rings. They are distinguished by spines that cover one edge. Halibut
    Halibut
    Halibut is a flatfish, genus Hippoglossus, from the family of the right-eye flounders . Other flatfish are also called halibut. The name is derived from haly and butt , for its popularity on Catholic holy days...

     have this type of scale.


Another, less common, type of scale is the scute, which is:
  • an external shield-like bony plate, or
  • a modified, thickened scale that often is keeled or spiny, or
  • a projecting, modified (rough and strongly ridged) scale, usually associated with the lateral line, or on the caudal peduncle forming caudal keels, or along the ventral profile. Some fish, such as pineconefish, are completely or partially covered in scutes.

Vertebrae


The vertebrae of lobe-finned fishes
Sarcopterygii
The Sarcopterygii or lobe-finned fishes – sometimes considered synonymous with Crossopterygii constitute a clade of the bony fishes, though a strict classification would include the terrestrial vertebrates...

 consist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is of broadly similar form to that found in most other vertebrates. Just beneath the arch lies a small plate-like pleurocentrum, which protects the upper surface of the notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...

, and below that, a larger arch-shaped intercentrum to protect the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in primitive tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

s, but, in the evolutionary line that led to reptiles (and hence, also to mammals and birds), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body.

In most ray-finned fishes
Actinopterygii
The Actinopterygii or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines , as opposed to the fleshy, lobed fins that characterize...

, including all teleosts, these two structures are fused with, and embedded within, a solid piece of bone superficially resembling the vertebral body of mammals. In living amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods.

In cartilagenous fish
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

, such as shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, the vertebrae consist of two cartilagenous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilagenous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification
Calcification
Calcification is the process in which calcium salts build up in soft tissue, causing it to harden. Calcifications may be classified on whether there is mineral balance or not, and the location of the calcification.-Causes:...

.

Lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s have vertebral arches, but nothing resembling the vertebral bodies found in all higher 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. Hagfish
Hagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...

es lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in the tail.

The jaw





Linkage systems are widely distributed in animals. The most thorough overview of the different types of linkages in animals has been provided by M. Muller, who also designed a new classification system, which is especially well suited for biological systems.

Linkage mechanisms
Linkage (mechanical)
A mechanical linkage is an assembly of bodies connected together to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid. The connections between links are modeled as providing ideal movement, pure rotation or sliding for...

 are especially frequent and manifold in the head of bony fishes, such as wrasses, which have evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 many specialized feeding mechanisms
Aquatic predation
Aquatic predation presents a special difficulty as compared to predation on land, because the density of water is about the same as that of the prey, so that the prey tends to be pushed away. This problem was first identified by R.N. Alexander...

. Especially advanced are the linkage mechanisms of jaw protrusion. For suction feeding a system of linked four-bar linkages is responsible for the coordinated opening of the mouth and 3-D expansion of the buccal cavity. Other linkages are responsible for protrusion of the premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

.

The vertebrate jaw probably originally evolved in the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 period and appeared in the Placoderm
Placodermi
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were...

 fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 which further diversified in the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

. Jaws are thought to derive from the pharyngeal arches that support the gills in fish. The two most anterior of these arches are thought to have become the jaw itself (see hyomandibula
Hyomandibula
The hyomandibula, commonly referred to as hyomandibular [bone] , and Latin: mandibula, "jawbone") is a set of bones that is found in the hyoid region in most fishes. It usually plays a role in suspending the jaws and/or operculum...

) and the hyoid arch, which braces the jaw against the braincase and increases mechanical 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 in extant jawed (the Gnathostomes
Gnathostomata
Gnathostomata is the group of vertebrates with jaws. The term derives from Greek γνάθος "jaw" + στόμα "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates...

), which have seven arches, and primitive jawless vertebrates (the Agnatha
Agnatha
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....

), which have nine.

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

Internal organs


  • The gas bladder
    Gas bladder
    The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming...

    , or swim bladder, is an internal organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth, ascend, or descend without having to waste energy in swimming. The bladder is only found in the bony fishes. In the more primitive groups like some minnows
    Leuciscinae
    Leuciscinae, commonly known as true minnows, are a subfamily of small fish belonging to the family Cyprinidae.However, the term minnow is also used as an unspecific term for tiny freshwater and saltwater fish, especially those used as fishing bait....

    , bichir
    Bichir
    The bichirs are a family, Polypteridae, of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes.All species occur in freshwater habitats in tropical Africa and the Nile River system, mainly swampy, shallow floodplains and estuaries.-Anatomy and appearance:Bichirs are...

    s and lungfish
    Lungfish
    Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...

    , the bladder is open to the esophagus
    Esophagus
    The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

     and double as a lung
    Lung
    The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

    . It is often absent in fast swimming fishes such as the tuna and mackerel families. The condition of a bladder open to the esophagus is called physostome
    Physostome
    Physostomes are fishes that have a pneumatic duct connecting the gas bladder to the alimentary canal. This allows the gas bladder to be filled or emptied via the mouth. In contrast, fishes without any connection to their gas bladder are called Physoclisti. The physostome fish encompass the...

    , the closed condition physoclist
    Physoclisti
    Physoclisti are fishes that lack a connection between the gas bladder and the alimentary canal. Addition and removal of the gasses from the gas bladder in these fishes occurs through specialised structures called the gas gland and ovale respectively...

    . In the latter, the gas content of the bladder is controlled through a rete mirabilis, a network of blood vessels effecting gas exchange between the bladder an the blood.
  • Certain groups of fish have modifications to allow them to hear, such as the Weberian apparatus
    Weberian apparatus
    The Weberian apparatus is an anatomical structure that connects the swim bladder to the auditory system in fishes belonging to the Superorder Ostariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes collectively referred to as the Weberian ossicles...

     of Ostariophysi
    Ostariophysi
    Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on all major continents except Antarctica...

    ans.
  • The gills, located under the operculum
    Operculum (fish)
    The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body....

    , are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. They are not usually visible, but can be seen in some species, such as the frilled shark.
  • The labyrinth organ of Anabantoidei
    Anabantoidei
    The Anabantoidei is a suborder of perciform ray-finned freshwater fish distinguished by their possession of a lung-like labyrinth organ, which enables them to breathe air. The fish in the Anabantoidei suborder are known as anabantoids or labyrinth fish...

     and Clariidae is used to allow the fish to extract oxygen from the air.
  • Gill rakers are bony or cartilaginous, finger-like projections off the gill arch which function in filter-feeders in retaining prey.
  • Electric fish
    Electric fish
    An electric fish is a fish that can generate electric fields. It is said to be electrogenic; a fish that has the ability to detect electric fields is said to be electroreceptive. Most electrogenic fish are also electroreceptive. Electric fish species can be found both in the sea and in freshwater...

     are able to produce electric fields by modified muscles in their body.
  • Many fish species are hermaphrodite
    Hermaphrodite
    In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...

    s. Synchronous hermaphrodites possess both ovaries
    Ovary
    The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

     and testes
    Testicle
    The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system...

     at the same time. Sequential hermaphrodites have both types of tissue in their gonad
    Gonad
    The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

    s, with one type being predominant while the fish belongs to the corresponding gender.
  • The blood circulation of fishes is called "single circuit circulatory system."

See also


  • Anatomical terms of location
    Anatomical terms of location
    Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

  • Digital Fish Library
    Digital Fish Library
    The is a University of California at San Diego project funded by the of the National Science Foundation . The DFL creates 2D and 3D visualizations of the internal and external anatomy of fish obtained with magnetic resonance imaging methods and makes these publicly available over the web.The...

  • Fish development
    Fish development
    The development of fishes is unique in some specific aspects compared to the development of other animals.-Cleavage:Cleavage, or initial cell division, of the fish zygote is meroblastic, meaning the early cell divisions are not complete...

  • Fish measurement
    Fish measurement
    Fish measurement refers to the measuring of the length of individual fish and of various parts of their anatomy. These data are used in many areas of ichthyology, including taxonomy and fisheries biology.-Overall length:...

  • Ichthyology terms
    Ichthyology terms
    Ichthyology uses several terms that are unique to the science.Contents: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z- A :* Abdomen: belly* Abdominal: pertaining to the belly* Actinosts: a series of bones at the base of the pectoral rays....

  • Panderichthys digits

External links