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Morphology of fish and Identification-1
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Taxonomy and Classification of fish
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Morphology of fish and Identification-1
Morphology of fish and Identification-II








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 part of the article 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 and Body.

This article covers only external features of fish.

Head Region contains following parts

  1. Snout.
  2. Lips.
  3. Mouth.
  4. Jaws.
  5. Teeth.
  6. Barbells.
  7. Nostrils.
  8. Eyes.
  9. Operculum, gills.
  10. Median groove.
  11. Pectoral girdle.
  12. Occipital process.

Body parts consist of

  1. Fins.
  2. Lateral Line.
  3. Skin.
  4. 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 (Eels)
  • With a groove across the top.
  • Tubular with jaws at tip.
  • Smooth in most cases.
  • Overhanging the mouth.

2. Lips:-

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).

3. Mouth:-

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.


Sub terminal




4. Jaws:-

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

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.

5. Teeth:-

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 : large conical teeth frequently located at the corners of the mouth, for example, snappers.

  • Viliform : small, fine teeth.

  • Molariform : pavement like crushing teeth, as in cow nose rays (Rhinopterinae) in which they form plates, or as individual molars in fishes such as the wolffishes (Anarhichadidae).

  • Cardiform : Fine, pointed teeth arranged as in a wool card; for example, the pharyngeal teeth in pickerels (Esox).

  • Incisor : Large teeth with flattened cutting surfaces adapted for feeding on mollusks and crustaceans; for example, chimaeras (Holocephali).

In most fish the teeth in lower jaws are in form of the band which can be a broad or narrow, separated in the middle, while those in the upper jaw are continuous.

6. Barbels:-

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

7. Nostrils :-

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.

A pair of nasal barbels is often seen.

Position and type of nostrils is often used as for identifying a fish.

8. Eyes :-

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. However the placement depends upon the habitat of fish.

They can be placed at the top or bottom of the skull. 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 Horaglanis krishnai which live in underground caves do not have any eyes.

9.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, the 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.

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.

10.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.

11. 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.

12. Occipital process :-

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



Body Region

13. Fins:-

Fins are thin appendages on the body of fish.
They are made of bony spines protruding from the body of fish with skin covering them and joining them together in case of bony fish or Osteichthyes.
In case of cartilaginous fish they are present as flippers

They 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).
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 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.

In many fish it is single and concave in shape with first spine being largest and last spine shortest.

Single 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.

Separate Dorsal Combined Dorsal

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



Anal Fin :-

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:-

The pectoral fins occur in pair and are located on each side, usually just behind the operculum (gill cover), and are homologous 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.

Pelvic Fins :-

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.

Adipose Fins :-

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 :-

Many fast swimming fish have a horizontal keel just in front of tail fin. it is present as ridge in Caudal penciled.
It 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:-

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.


14. Lateral Line :-

The lateral line is a sense organ used to detect movement and vibration in the surrounding water. For example, fish can use their lateral line system to follow the vortices produced by fleeing prey.
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 seperated and may not be in a single line.


15. Skin:-

The skin of fish consists of live cells, there is very superficial amount of keratin in outermost layer.
It is generally permeable.

Fish typically have numerous individual mucus-secreting skin cells that aid in insulation and protection, but may also have poison glands, photospheres, or cells that produce a more watery, serous fluid.

The co lour in skin is mostly provided by melanin, however often the skin is colorless. The co lour 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 co lour of their skin by adjusting the relative size of their chromatophores.

16. Scales:-

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 cutes.
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.

1. Placoid Scales:-

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


2. Ganoid scales :-

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


3. Cycloid scales:-

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


4. Ctenoid scales :-

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.

Some fish have type of scale known as 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.
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.






  Sandeep Raghuvanshi
Picture Credits   Wikipdeia and other sources which have released under GNU, unless specifically credited
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