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Body Parts Of Fish

Fish Body parts
Fish Body parts
Image Credit: Sandeep Raghuvanshi


Identification of fish is an important aspect of the hobby.
This is a series of 2 articles, the first article describes the various body parts which can be used to identify the fish.
The second article How a Fish is described describes the process by which these body parts are used to describe the fish.

A fish is identified by its External Morphology.
Morphology is a branch of science which deals with study of the form and features of any living organism.

The body of fish can be divided into two main parts

  • Head
  • Body.

This article covers only external features of fish.

Head Region contains following parts:

  • Snout.
  • Lips.
  • Mouth.
  • Jaws.
  • Teeth.
  • Barbells.
  • Nostrils.
  • Eyes.
  • Operculum/gills.
  • Median groove.
  • Pectoral girdle.
  • Occipital process.

Body Region consists of following parts:

  • Fins.
  • Lateral Line.
  • Skin.
  • Scales.


Snout is the anterior most part of the fish (forward end of head).
It is rounded or obtuse in most cases.
There can be many variations to the shape of snout.

  • Pointed and Sharp

    Electric Eel

    Electrophroys electricus:Electric Eel
    Image Credit: Wikipedia

  • With a groove across the top.
  • Tubular with jaws at tip.
  • Smooth in most cases.
  • Overhanging the mouth.


The bone of the upper and lower jaw are covered by lips.
Mostly they are thin smooth membranes but in some cases they may have pores, stripes or modified to form a sucker like disc in Garra species.
Depending upon the shape of mouth the lips may be terminal ( in front) or inferior (beneath the head).


Lips of Garra
Image Credit: Wikipedia


Mouth is the main organ which fish use while feeding.
The position and shape of the mouth depends the type of food a fish eats and the level at which it swims.

The mouth shapes can be :

  • Terminal as in many fishes ( Danios, Rasbora, Putnius.)
    These fish feed from water column on other fish or zooplankton.


    Terminal Fish Mouth
    Image Credit: Wikipedia

  • Sub terminal


    Sub-Terminal Fish Mouth
    Image Credit: Wikipedia

  • Superior


    Fish Mouth- Superior
    Image Credit: Wikipedia


The jaw consists of the upper jaw and lower jaw.
The upper jaw consists of bones called pre-maxillaries and maxillaries, while the lower jaw consists of Mandible bones.
These are connected by a joint which enables the fish to open and close the mouth.


Jaws of Megladon; An Extinct Shark
Image Credit: Wikipedia

Jaws contain teeth and frame the shape of the mouth.
In most fish both jaws are of same length but in some cases the upper or lower jaw can be longer then the other.

The family Clupeidae has a longer lower jaw,


Longer Lower Jaw;Family Cupeidae
Image Credit: Wikipedia

while the fish in family Engraulidae have a longer upper jaw.


Longer Upper Jaw;Family Engraulidae
Image Credit: Wikipedia

Depending upon the habitat and feeding pattern, there are many modifications to the jaw, it may protrude to form a beak as in pipefish, both jaws divided by a median suture with a cutting edges as in puffer fish.


Most fish have teeth on jaws and palate.
In addition to these teeth some fish have pharyngeal teeth also.
However not all fish have teeth like cyprinids.

There are many types of teeth :-

  • Canine


    Canine Teeth;Snappers
    Image Credit: Wikipedia

    Canine Teeth are large conical teeth frequently located at the corners of the mouth, for example, snappers.

  • Viliform


    Villiform Teeth;
    Image Credit: Wikipedia

    These are small, fine teeth, and are found in Needlefishes (Belonidae) and Lion fishes(Pterois).

  • Molariform

    Molar teeth

    Molar Teeth
    Image Credit: Wikipedia

    Molars are pavement like crushing teeth
    These type of teeth are found in cow nose rays (Rhinopterinae) in which they form plates, or as individual molars in fishes such as the wolffishes (Anarhichadidae).

  • Cardiform These are fine, pointed teeth arranged as in a wool card
    They are found in pickerels.

  • Incisors

    Incisor teeth

    Incisor Teeth;
    Image Credit: Wikipedia

    These are large teeth with flattened cutting surfaces adapted for feeding on mollusks and crustaceans; for example, chimaeras (Holocephali).

  • Pharyngeal teeth

    Pharyngeal teeth

    Pharyngeal Teeth
    Image Credit: Wikipedia

    Pharyngeal or throat teeth are found in fish like cichlids and Cyprinids.
    In these fish the fifth gill arch is modified and each has either 2-3 rows of teeth.

In most fish the teeth in lower jaws are in form of the band.
This band can be broad or narrow and separated in the middle.
The teeth in upper jaw are continuous.



Image Credit: Wikipedia

Barbells are slender, whisker like tactile organs near the mouth.
They are found in fish like Arowana,catfish, carps etc.
They house the taste buds are used by fish to find food.


Nostrils are pair of apertures or slits on the snout of the fish.
They are openings for the smell organs leading to the nasal canal on the skull.
They are small to medium and are sunk in snout, in some fish like catfish they are covered with mucus.


Image Credit: Wikipedia

A pair of nasal barbells is often seen.
Position and type of nostrils is often used as for identifying a fish.


Eyes are mainly used by fish for seeing, food, enemies and predators.
They are placed dorso -laterally (upper part- on the side) in most fish.


Eyes Placement Normal
Image Credit: Wikipedia

However the placement depends upon the habitat of fish.
They can be placed at the top or bottom of the skull.


Eyes Placement on top of head
Image Credit: Wikipedia

Deepwater swimming fish have large eyes as they receive less light at that depth.
On the other hand eyes of hill stream fish are small as they live near the surface and receive a large amount of light.
Some fish like Astyanax jordani which live in underground caves do not have any eyes.

blind fish

Blind Fish: Astyanax jordani
Image Credit: Wikipedia

Operculum and gills

` Operculum along with gills form breathing apparatus for the fish.
On each side of fish there are slits called gills.
The gills are composed of comb-like filaments called "gill lamellae", which help increase their surface area for oxygen exchange.
In bony fish, the gills lie in a branchial chamber covered by a bony operculum.


Image Credit: Wikipedia

A fish breathes by taking water in through its mouth and forcing it out from gills.
There are tissue linings in the gills which absorb oxygen.
Co2 is also expelled from the gills.
The majority of fish have 5 pairs of gills.
Gills also perform vital function of electrolyte balance of fish as excess electrolytes are expelled from the gills.

Median groove

There are two longitudinal externally visible long depressions on the head and covered by skin in catfish.
They may be single or double.

Pectoral girdle

These are paired bony structure on the either side of the fish in the head region, they are inserted laterally in most cases.
They bear the pectoral fin and pectoral spine in some fish.

Occipital process

Occipital Process is an arrow like conical bone with a broad base extending from the supra-occipital to the basal bone of the dorsal fin.


Fins are thin appendages on the body of fish.
In case of bony fish or Osteichthyes they are made of bony spines protruding from the body of fish with skin covering the spines and joining them.

spiny fins

Fins of a Bony Fish
Image Credit: Wikipedia

In case of cartilaginous fish they are present as flippers.

cartelegious fins

Fins of a Cartilaginous Fish
Image Credit: Wikipedia

Fins are foil shaped and are primary means of locomotion for the fish.
Some generate thrust when moved, others are used for stabilizing and treeing.

In bony fish (Osteichthyes), 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 (in front).

combined fins

Spiny rays alonght with Soft rays
Image Credit: Wikipedia

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.

Fins occur in both pairs and single, they may be covered with scales or without scales (naked).

Dorsal Fin

dorsal fins

Dorsal Fin
Image Credit: Wikipedia

Dorsal Fin is a single fin present on the top of the body(viewed from top) also known as dorsal side.
The dorsal fin serves to protect the fish against rolling, and assists in sudden turns and stops.
When the top of rays is connected with membrane they are called soft, else they are called hard spines.

There are following types of Dorsal fins

  • Single Dorsal Fin:- In many fish it is single and concave in shape.
    First spine is longest and last spine shortest.

    dorsalSingle fins

    Single Dorsal Fin
    Image Credit: Wikipedia

  • Double Dorsal Fin

    In perches there are two dorsal fins, one after another.
    The first fin is separated by either a short or long gap, or it may be combined.

    dorsaldouble fins

    Double Dorsal Fin
    Image Credit: Wikipedia

    dorsaldoubles fins

    Double Dorsal Fin Combined
    Image Credit: Wikipedia

    Normally the first dorsal fin is shorter then the other one, but there are exceptions also.
    Dorsal fin normally appears at the center of dorsal side, but in many cases it may be placed far posterior (back) above the anal fin.
    It may be free or even blended with Caudal (tail) fin.

    Caudal Fin

    Caudal or tail fin is located at the end Caudal peduncle of the fish.
    The caudal peduncle is the narrow part of the fish's body to which the caudal or tail fin is attached.
    It is always a single fin and acts as rudder for the fish.

    They normally have following shapes:

    • Round

      Caudal Fin Round

      Round Caudal Fin
      Image Credit: Wikipedia

    • Indented

      Caudal Fin Indented

      Indented Caudal Fin
      Image Credit: Wikipedia

    • Luncate

      Caudal Fin Luncate

      Luncate Caudal Fin
      Image Credit: Wikipedia

    • Pointed

      Caudal Fin Pointed

      Pointed Caudal Fin
      Image Credit: Wikipedia

    • Truncate

      Caudal Fin Truncate

      Truncate Caudal Fin
      Image Credit: Wikipedia

    • Forked

      Caudal Fin Forked

      Forked Caudal Fin
      Image Credit: Wikipedia

    Anal Fin

    Anal Fin

    Anal Fin
    Image Credit: Wikipedia

    The anal fin is located on the ventral (lower) surface behind the anus.
    This fin is used to stabilize the fish while swimming.
    Anal fin is usually a single fin, but can be paired also.
    It is mostly free but can be joined with the tail fin in some cases.

    Pectoral Fins

    Pectoral Fin

    Pectoral Fin
    Image Credit: Wikipedia

    The pectoral fins occur in pair and are located on each side.
    They usually just behind the operculum (gill cover), and are similar to the forelimbs of tetra pods.
    There are many adaptions to these fins in some cases they create a dynamic lifting force that assists some fish, such as sharks, in maintaining depth and also enables the "flight" for flying fish.
    In many fish, the pectoral fins aid in walking, especially in the lobe-like fins of some anglerfish and in the mudskipper.

    Pectoral Fin Mudskipper

    Pectoral Fin of Mud skipper
    Image Credit: Wikipedia

    Pelvic Fins

    Pelvic Fin

    Pelvic Fin
    Image Credit: Sandeep Raghuvanshi

    Pelvic fins occur in pairs and are found on the ventral (lower) side of the fish below the pectoral fins.
    They assist the fish in going up and down in water, turning and stopping.
    In some fish like gobies they are joined into a single sucker like disc which is used to attach to objects.

    Pelvic Fin Goby

    Modified Pelvic Fin of Goby
    Image Credit: Wikipedia

    Adipose Fins

    Adipose Fin

    Adipose Fin
    Image Credit: Wikipedia

    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, characins and catfishes.
    The function of adipose fin is not clearly understood, however it has a network of nerves, so might be used as a sense organ.

    Caudal Keel and Fin lets

    Caludal Keel

    Caudal Keel
    Image Credit: Wikipedia

    Many fast swimming fish have a horizontal keel just in front of tail fin.
    it is present as ridge in Caudal penciled.
    Caudal Keel provides stability and support to the tail fin.
    They are always present as either a single pair or double pair, one of each side.

    Fin lets are small fins, generally behind the dorsal and anal fins.
    In bichirs, they are the only fin on dorsal side as dorsal fin itself is absent.

    Lateral Line

    Total Length

    Lateral Line
    Image Credit: Wikipedia

    The lateral line 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.
    It can be continuous or broken. In some fish families like cichlids it is broken, the two parts of lateral line are separated and may not be in a single line.


    Fish Skin

    Structure of Fish Skin
    Image Credit: Wikipedia

    The skin of fish consists of live cells, there is very superficial amount of keratin in outermost layer.
    The skin is generally permeable,i.e. fluids can pass through very easily.
    Fish typically have numerous individual mucus-secreting skin cells that aid in insulation and protection.
    They may also have poison glands, photospheres, or cells that produce a more watery, serous fluid.

    Fish Skin color

    Colors in Fish Skin
    Image Credit: Wikipedia

    The color in skin is mostly provided by melanin, however often the skin is colorless.
    The colour is fish is provided by largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments.
    Many species, such as flounders, change the color of their skin by adjusting the relative size of their chromatophores.


    Fish Scales

    Scales of Fish
    Image Credit: Wikipedia

    The outer body of many fish is covered with scales.
    The scales originate from the mesoderm (skin), and may be similar in structure to teeth.
    Some species are covered instead by scutes.

    Fish ScaleScutes

    Scutes instead of Scale in Fish
    Image Credit: Wikipedia

    Others have no outer covering on the skin.
    Most fish are covered in a protective layer of slime.

    There are four principal types of fish scales.

    • Placoid Scales

      Fish ScalePlacoid
      Fish ScalePlacoid

      Placoid Scales in Fish
      Image Credit: Wikipedia

      Placoid Scales also called dermal denticles and are similar to teeth.
      Just like teeth they are made of dentin covered by enamel.
      They are typically found in sharks and rays.

    • Ganoid scales

      Fish ScaleGanoid
      Fish ScaleGanoid2

      Ganoid Scales in Fish
      Image Credit: Wikipedia

      Ganoid scales are flat, basal-looking scales that cover a fish body with little overlapping.
      They are typical of gar and bichirs.

    • Cycloid scales

      Fish ScaleCycloid

      Cycloid Scales in Fish
      Image Credit: Wikipedia

      Cycloid scales are small oval-shaped scales with growth rings.
      Bowfin and remora have cycloid scales.

    • Ctenoid scales

      Fish Ctenoid

      Ctenoid Scales in Fish
      Image Credit: Wikipedia

      Ctenoid scales are similar to the cycloid scales, with growth rings.
      They are distinguished by spines that cover one edge.
      Halibut have this type of scale.

    • Scutes


      Scutes in Fish
      Image Credit: Wikipedia

      Some fish have type of scale called scute.
      This 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.
      These are usually found in the lateral line, or on the caudal peduncle forming caudal keels, or along the ventral profile.
      Some fish, such as pinecone fish, are completely or partially covered in scutes.