Meristic features

Meristic refers to the countable features of fish that are useful in\identifying them. Different species of fish have varying numbersof fin rays, scales, barbells, vertebrae and other features that can be used for their identification. The meristics are discrete variables,which mean that there are either 4 or 5 dorsal fin spines not 4.5.Meristic traits are often described in a shorthand notation calleda meristic formula. Meristics are often combined with details onanatomy and morphometrics. Meristic characteristics of a fish canbe compared to those listed in key reference books, descriptionsand in dichotomous keys to help us work out which species we arelooking at.A meristic study is an often difficult task, as counting the featuresof a fish is not easy. Much of the meristic analyses are performedon dead fish that have been preserved in alcohol or formalin. These
traits are less easily observed on live fish, though it is possible. Onvery small fish, a microscope may be required.The meristic counts commonly used in fish identification andtaxonomic studies are fin rays count, lateral line scales count,vertebral count, branchiostegal rays count, pyloric caeca count,gill rackers count, pharyngeal teeth count, etc.
a. Counting fin rays
Counting fin rays is a shorthand method of describing the numberand way of arrangement of the fin rays of a bony fish fin, whichis comparable to the floral formula of flowers. As described inabove section, fin rays are of two main types: hard and soft rays.All of the hard and soft rays in all fins are counted separately(Figure 4.19), including the tiny ones on the inside of the pelvicfins, which may not easy to detect on a live fish. When makingcounts it is important to examine the base of each fin where eachfin rays inserts into the body. Since soft-rays are often branched,examining them at the tips would result in two or more counts fora single ray. The last ray of dorsal and anal fins in most fishes isusually divided to the base and may look like two rays, but it shouldbe counted as a single ray. As with most things in Ichthyology, finray counts are written in a strict formula known as fin formula.Different fish species have their own fin formula. Below is a guidefor expressing fin formula.
dorsal fin formula of this fin can be written as D: I, 8 or D 9 (1/8).

1. Different types of fins are abbreviated as:Dorsal - D; Pectoral - P or P1; Anal - A; Pelvic - V or P2; Caudal
- C. If a fish have two dorsal fins, it is denoted by D1 and D2.

2. Hard rays or spines are generally written using Roman numerals
and soft rays are written using Arabic numbers. For example,
D: V, 8 - is a fish that has 5 dorsal spines followed by 8 soft raysP : 7 - is a fish which has 7 pectoral rays and no spineSometimes an oblique stroke (/) is used to separate two types offin rays instead of Roman numerals. For example,D 16 (3/13) - is a fish that has 3 dorsal spines followed by 13 softraysA 7 (2/5) - is a fish that has 2 anal fin spines and 5 soft raysSometimes a vertical stroke (|) is used toseparates different fins,such as rayed dorsal from the adipose dorsal or the spinyfinfromthnonspiny one. For example, the fin formula for dorsal fin inthe Silurid fish may be D 1/7|1, i.e., it has 1 spine, 7 soft rays and1 adipose fin.3. A complete fin formula of a fish (e.g. Labeo rohita) is written as:D: III, 13; P: 17; V: 9; A: II, 5; C: 19orD 16 (3/13); P 17; V 9; A 7 (2/5); C 19This means theLabeorohitahas a dorsal fin with 3 spines and 13soft rays, pectoral fin with 17 soft rays, pelvic fin with 9 soft rays,
anal fin with 2 spines and 5 soft rays and caudal fin with 19 softrays.4. Since there areslightvariations in fin rays between members ofthe same species, the numbers of fin rays are generally provided inarange. For example, D: 10-13, A: 10-14, P: 12-15.

b. Counting lateral line scales
Lateral line is a row of perforated scales between the angle of gillopening and the base of caudal fin. Lateral line scales are countedalong the lateral line (distinguished by pores on scales) startingfrom the scale just behind the upper end of the gill opening to thebase of caudal fin and their number is shown after the abbreviationL1 (Figure 4.20). However, not all fish have a lateral line, and evenif present, it is often only partially complete. In these cases, we cansimply count the scales along an imaginary line where the lateral
line would be normally present.Other scale counts include counting the lateral line transverse
scales, counting the number of pre-dorsal or post-dorsal scalesand counting the number of scales around the narrowest part ofthe caudal peduncle. Lateral line transverse scales are generallydivided into two parts, scales above lateral line and scales belowlateral line. The former includes counting the scales from the
anterior base of the first dorsal fin to the lateral line and the laterincludes counting the scales below the lateral line to the anteriorbase of the pelvic fins (or anal fins) (Figure 4.20). Their number isshown after the abbreviation L.tr. The scales above and below thelateral line are generally separated by an oblique (/). For example,if a species have a lateral line scales count of 40, scales abovelateral line are 8 and scales below lateral line are 9, the lateral lineformula is written as L1 40; L. tr. 8/9. The total count of transverse
scales can be equal to “count of scales above lateral lines” + “countof scales below lateral line” + 1 (lateral line scale), i.e. 8+9+1 = 18.Thus, the lateral line formula can also be written as L1 40; L. tr. 18.Conducting counts of scales can be quite easy or very difficultdepending on the species being examined. Some fish have
relatively large and firmly attached scales, which make countingvery easy. Others have very small scales that may be covered with mucous, or have scales that are very deciduous. 
c. Counting vertebrae
For systematic studies, it is often necessary to collect data onvertebral counts, which can vary between and within species.These counts are often made from radiographs (X-rays), butcounts are sometimes made from cleared and stained specimensand dried skeletons. The typical hypural plate of most teleosts iscounted as a single vertebra. However, definite suture along thevertebral axis are regarded as separating vertebrae, even thoughthe sutures lie within the hypural complex.
d. Counting branchiostegal rays
Branchiostegals are the bony rays supporting the gill membranesbehind the lower jaw. They are mostly found in Elasmobranchii,but also in some teleosts such as salmonids. When makingcounts, it is often desirable to separate by a plus sign (+) thosebranchiostegal rays which lie on the outer side of the hyoid arch
from those that are inserted more interiorly and more ventrally onthe inner face of the arch. The anterior rays may be small and,especially in laterally compressed fishes, several of them may be
bound tightly together, necessitating the use of a sharp probeto locate each one.
e. Counting pyloric caeca
Pyloric caeca are the finger like projections located near the junction of the stomach and the intestine. In counting pyloric caeca, all tips are enumerated unless the condition of branching isspecifically described.
f. Counting gill rackers
Gill rackers are cartilaginous structures situated along the anterior side of the gill arch in the opposite direction of gill filaments, which is covered by operculum. Gill rackers are generally counted
on the first gill arch. The gill arch is divided into the upper and lower arms (hemibranch), and gill rackers are counted from top to bottom. There is frequently one gill racker at the junction between
the two arms; this gill racker is included in the count of gill rackers on the lower arm.

Morphometric features

Morphometric refers to measurable characters or length-based
measures of specific body parts of fish, such as total length, head
length, eye diameter, or ratio between such measurements. These
characters are usually measured in the millimeter scale. Although
the morphometric measures can be compared to each other
directly, their proportions give more accurate information.
The body of fish can be divided into three parts: head, trunk
and tail. There is no neck in fish as in the land animals. The gill
opening is the boundary between the head and the trunk (Figure
4.17 & 4.18). Similarly, the vent is the dividing line between the
trunk and the tail. The relative proportion of these parts varies
greatly in different fishes. Some morphometric measures used in
fish identification and taxonomic studies are described below:
i. Total length: The maximum distance from the tip of the snout
to the tip of the caudal fin. This includes long pointy beaks, as
in Xenentodon, and long trailing tail filaments, as in Bagarius.
ii. Standard length: The distance from the tip of the snout to the
base of the caudal fin, not including the tail.
iii. Body depth or height: The distance from the highest part of
the dorsal surface to the ventral surface in a straight, vertical
line.
iv. Head length: Head length is measured from the tip of the snout
or upper jaw to the most posterior (preferably bony) portion of
the operculum.
v. Head depth: Sometimes called height of head at occiput, this
measurement goes from the top of the head (occiput) to the
bottom of the head or breast.
vi. Snout length: It is taken from the most anterior point on the
snout or upper lip to the front margin of the orbit.
vii. Eye diameter: The distance between the margin to margin of
the bony orbit.
viii. Pre-orbital: Widely used on cichlid studies, this is a bone at
the front of the head.
ix. Post-orbital: The region behind the eye to the rear of the
Introductory Ichthyologyoperculum.
x. Pre-dorsal length: A straight measurement from the tip of the
snout to the first ray of the dorsal fin.
xi. Post-dorsal length: A straight line measurement from the base
of the posterior of the dorsal fin to base of the caudal fin or end
of vertebral column.
xii. Caudal peduncle depth: It is measured along the vertical line
at its narrowest part between posterior base of anal fin to the
base of the caudal fin.
xiii. Caudal peduncle length: It is measured from posterior base
of anal fin to the base of the caudal fin.

FINS

Structure and types

The fins constitute the major locomotory organs in fishes. These
are either folds of skin or projections from the body surface. The
fins are supported by fin rays. These supporting rays may be bony,
cartilaginous, fibrous or horny. There are mainly two types of fins
in fishes: (1) unpaired or median fins, and (2) paired fins
The unpaired fins include dorsal fin on the back, an anal
fin on the ventral side behind the vent and a caudal fin at the
end of the tail. In some species, an adipose fin is present between
the dorsal and caudal fins. The paired fins are the pectorals and
pelvics or ventrals corresponding to the fore and hind limb,
respectively of the terrestrial vertebrates. In some species, tiny fin
like structures, known as finlets, are present between the dorsal
and the caudal fins or between the anal fin and the caudal fin

Finlets are generally without fin rays and
non-retractable. The shape, size, color and position of the fins
are greatly varied among fishes. The diversity in the fin system in
fishes is due to their adaptive responsiveness.

Origin of fins
It has been believed that fins of fishes are evolved from the ancestors
that had no fins at all. Presumably, ancestral forms responded to
movement in water and developed a median ridge or keel on the
body, from near the head around one end of the tail to the anus.
These ridge, perhaps initially without supporting rays, gave rise to
median or unpaired fins. Similar ridges appearing on the sides and
then coalescing at the proper sites have been presumed further to
be the ancestral antecedents of the paired fins.

Origin of unpaired fins
The unpaired fins in fishes are held to be originated from a
continuous fold of tissues. This fold extends from the posterior
region of the head and continuous posteriorly around the tail and
forward up to the anus. The fold is supported by series of parallel
rods. In course of development, each supporting rods divides into
a lower piece, i.e., basal, embedded in the body wall and an upper
piece laying in the fin-fold, i.e., radial. From such continuous
fin-fold, the dorsal, caudal and anal fins have been evolved by
restriction of the radials at certain areas and the progressive
degeneration of the fold between them.

Origin of paired fins
Most of the Ichthyologists are convinced about the derivation of
the unpaired fins from the continuous fin-fold, but the origin of
paired fins has been much debated, and many theories have been
put forward in explanation. Two major conventional theories are:
(1) Gill-arch theory, and (2) Fin-fold theory.

Gill-arch theory
This theory was put forward by Carl Gegenbaur (1898), a German
Anatomist. According to this theory, the paired fins with their
girdles are derived from the gill arches. The two pairs of gill arches
transformed into the pectoral and pelvic girdles and their gill rays
gave rise to skeleton of the fins. Actually, this theory was proposed
on assumption that the primitive gnathostomes possessed gill
arches extending further behind than they are at present.
Soon on the basis of this theory the direction of fins has evidently been
dorso-ventral and both the pectoral and pelvic fins were very close
together behind the head. However, the present position of pelvic
fins is explained on the assumption that some of the posterior gillarches
have been shifted posteriorly. This hypothesis has found no
support from the embryological, morphological, placontological
and molecular studies. It has been confirmed that the endoskeletal
girdles of paired fins never exhibit characteristics of gill arches.

Fin-fold theory
According to this theory, the paired fins are derived from
longitudinal lateral folds of epidermis extending backward along
the body from just behind the gills to the anus (Figure 4.14). By
accentuation of the anterior and the posterior, and suppression
and reduction of the intermediate portions of the folds the
pectoral and the pelvic fins were formed. Into these folds, muscle
buds migrated from the ventral border of the adjoining myotomes,
giving rise to radial muscles. The muscle buds disclosed a
metameric arrangement and derived their nerve supply from
ventral roots of the spinal nerves. There are many placontological
and embryological evidences in favor of the origin of paired fins
from the lateral fin-folds.

Fin rays
The fins of fishes are supported by dermal rays or spines. The
dermal fin rays of elasmobranchs and bony fishes are known
collectively as dermatotrichia or lepidotrichia. These supporting
rays may be bony, cartilaginous, fibrous or horny. They may be
segmented or unsegmented, branched or unbranched, biserial
or uniserial. The movements of the fins are due to the action of
the muscles, these movements beings possible because of the
articulations and often flexibility of these rays. In teleosts, the fin
rays are of two types

1. Soft rays: These are thin, flexible, most often branched, Y-shaped,
segmented and always biserial (two lateral components paired
on the midline). These are called lepidotrichia.
2. Hard rays (spines): A number of soft rays united solidly to
compose hard rays which are stout, rigid, unsegmented,
uniserial and sometimes sharply pointed. These are called
actinotrichia.

Types of caudal fin
Of the unpaired fins, the caudal fin plays the most important role
in forward propulsion during swimming. The caudal fin differs
from the dorsal and anal fins in the nature of its supporting
skeleton. Four major types of caudal fins have been reported in
different fishes.
1. Protocercal or diphycercal
This type of caudal fin is regarded to be the most primitive type. The
posterior end of the vertebral column exrends up to the tip of the
tail and divides the caudal fin into two equal halves (Figure 4.16-
A). The dorsal half is called epichordal lobe and the ventral one
is known as hypochordal lobe. The epichordal and hypochordal
parts of the caudal fin are equal in size and symmetrical. It is
found in Amphioxus and Cyclostomata.
2. Heterocercal or epicercal
In this case, the posterior end of the vertebral column bends
upwards. As a consequence the caudal fin is divided into two
unequal halves. The vertebral column is bent upwards and
continues almost up to the tip of the fin (Figure 4.16-B). The
epichordal part is greatly reduced while the hypochordal lobe
is specially enlarged to make the caudal fin asymmetrical both
internally as well as externally. This type of caudal fin is found
in elasmobranches, extinct crossopterygian and primitive
actinopterygians.
3. Hypocercal
This type of caudel fin is seen in the fossil Agnatha. The tail is
asymmetrical, as the vertebral column is bent downwards so that
the epichordal lobe is larger than the lower hypochordal lobe
(Figure 4.16-C). It is also known as ‘reversed heterocercal’. This
type of caudal fin is not present in any modern fishes.
4. Homocercal
This type of caudal fin is the characteristic of the higher bony fishes.
The fin is symmetrical externally but internally it is asymmetrical.

The posterior end of the vertebral column is turned upwards and
becomes greatly reduced (Figure 4.16-D). The tip of the vertebral
column does not reach the posterior limit of the fins. There is no
apparent dorsal lobe but the ventral lobe is greatly enlarged and
divided into two equal superficial lobes.
In many teleosts belonging to Anguilliformes, Clupeiformes,
Osteoglossiformes and some others, the tail is tapering and
symmetrical. This is not a truly protocercal and is called isocercal or
leptocercal. During development, the hypochordal lobe is greatly
reduced, while the dorsal and the anal fins become elongated so as
to form a contineous fin.
The protocercal or diphycercal type of caudal fin is considered as
the most primitive type, the heterocercal as the intermediate stage
and the homocercal condition represents the advanced stage. In
most teleosts, the caudal fins start as diphyceral then becomes
heterocercal and finally becomes the homocercal condition. The
transition of three types of caudal fin in the development history
of fishes is significant from the phylogenetic point of view.

Functions
1. The principal function of the fins is regarded for locomotion in
the form of progression, steering or balancing.
2. The basic function of a rudder is fulfilled by the pectoral and
pelvic fins. These fins permit the fish to steer, stabilize, and stop.
3. The dorsal and anal fins do not perform the function of forward
movement of the fish. But they assist in upward and downward
turning. They also serve to stabilize the fish while it is swimming
and stop the fish from rolling over.
4. The caudal fin is used to propel the fish forwards. Fish that have
forked caudal fins are fast-swimmers. Fish that have rounded
caudal fins are fish capable of quick action like predators.
5. In some fishes (e.g., Colisa spp.), the pelvic fins prolonged into
trailing filaments serving as tactile organs.
6. In some fishes (e.g. Gambusia affinis), the anal fin of male is
modified into gonopodium to briefly hook into the vent of a
female fish to deposit sperm.
7. In some hill-stream fishes, pectoral and pelvic fins are modified
to form an adhesive surface for attaching them to the stones
and rocks of the river.
8. In some fishes, pectoral fins are modified as long, pointed and
serrated spines, containing poison glands, which act as defense
organs (e.g., Heteropneustes fossilis).
9. The climbing perch (Anabas spp.) can walk on land or even climb
up the trees with the help of its pectoral fins and operculum.
10. In the anglerfish (e.g., Bufoceratias wedli), the dorsal fin has
been modified into a fishing rod and lure.
11. The dorsal fins have been modified in the suckerfish (e.g.,
Remora spp.) into a sucking disc that allows them to cling to
sharks or other large fishes.
12. In some fishes, the spines present in the fin acts as a sound
producing organ by stridulating.
13. In some species, paired fins are used for nest building and to
hold the eggs during spawning.

Coloration

Fishes are among the world's most colorful inhabitants. A large
number of teleost fishes are brightly and brilliantly colored, while
some are of more uniform and sober shade. The coloration or
patterns of fish may reveal information about its life style and
ecosystem and has a functional significance. Generally, the fishes
are darker on their upper surfaces due to sunlight while the ventral
and lateral sides are relatively lighter in coloration.
Source of color
Coloration in fishes is attributed to the presence of two kinds of
special cells, chromatophores and iridocytes. The chromatophores
are branched connective tissue cells located in the dermis either
above or below the scales. Chromatophores contain pigments
of various kinds of pigment granules which may be carotenoids
(yellow - red), melanin (black), flavins (yellow), purines (white or
silvery), pterins, porphyrins and bile pigments. Based upon the
color of the pigments, the chromatophores are of the following
types:
a. Melanophores
The pigmentary material of melanophore is black colored, called
as melanin. It is formed from an amino acid tyrosine, which is
converted into melanin in the presence of copper containing
Introductory Ichthyology
enzymes tyrosine melanin, so formed, associates with the
cytoplasmic organ all called the melanosome. A brown in black
pigment called ‘eumelanin’ is also sometimes found within the
melanophores.
b. Iridophores
The pigmentary material in these cells is guanine. Guanine is
opaque, whitish or silvery. It is a waste product and is deposited
in the form of granules. These granules are opaque and posses
great reflecting power, so that the iridocytes may also be called
the ‘mirror cells’. Tiny crystal of this pigment produces a
white or silvery white apperance and against a background of
melanophores, imparts a blue color to the fish.
c. Xanthrophores and erythrophores
The pigmentary materials of these cells include carotenoids and
pterins. They are white, yellow, orange or red colored pigments,
which are abundantly found in plants but the fishes are incapable
to synthesize them, hence they depend upon the plants for a
dietary supply of the pigmentary material. The cellular organelle
containing pteridines are known as pterinosome and the ultramicroscope
vesicles of cells containing carotenoid are called the
carotenoid vesicles.

Mixed coloration in fish

Color patterns of a majority of a fishes are due to the combined
effects of chromatophores containing different kinds of pigmentary
granules. For example, the black and yellow chromatophores
together develop green color. Likewise, yellow and black or orange
and blue may combine separately to give brown appearance to the
fish. Similarly, yellow mixed with black gives brown or blackish
coloration in fishes.Color change in fish
Various species of fish change the pattern of their coloration in
accordance to their surroundings or in relation to the phases of
their behavior. These changes in appearance are under the control
of chromatophores.Significance of coloration
Coloration is important, not only for look, but for survival. The
coloration in fishes performes many adaptive functions and
is useful to the fish in a variety of ways such as camouflage,
concealment and disguise, aggressive purpose, courting patterns,
warning pattern, etc.

Integumentary system

The integumentary system of fish compries skin and its derivatives. The scales, coloration and some special structures like the electric organs, poisonous glands, and phosphorescent organs are the integumentary derivatives. The major function of the integumentary system is to protect the fish from external injuries and enemies.
 Structure of skin
The skin forms the external covering of the body and performs various important functions in fish. The skin of fish is quite firmly attached and is hard and rough, and composed of two layers. The outer layer is called epidermis and the inner layer is called dermis or corium (Figure 4.6). The two layers differ in origin, structure and function. A thick basement membrane is present between the epidermis and dermis. In most fish species, the epidermis is thinner than the dermis.

The epidermis is a thin, multilayered epithelium derived from the ectoderm of the embryo. It does not contain blood vessels. The upper portion of epidermis consists of several layers of flattened epithelial cells, called stratified epithelium. The inner most layer is made up of columnar cells, called stratum germinativum in which cells are always multiplying by mitotic division to replace the outer worn out cells. Different types of unicellular and multicellular glands are formed from the epidermis. Some of these are confined to the epidermis while others grow deep into the underlying tissue of the dermis. These include holocrine mucous cells (produce mucous), chromatophores (impart color), taste buds, sensory cells, ampullary organs and a variety of other secretory cells. The mucous cells are basically of two types, the goblet cells and the club cells. The goblet cells produce mucous, which makes skin slimy and frictionless as well as protects from pathogens. The goblet cells develop from the stratum germinativum and migrate to the surface. The club cells produce specific secretions and are named accordingly as alarm cells, poison cells, photophores, etc. Other secretory cells that the epidermis may include are sacciform cells. Sacciform cells of some fish species may secrete alarm pheromones, whereas sacciform cells of other species may secrete toxic or repellent substances for antipredator defense. The epidermis is a fragile layer which is constantly sloughed off and renewed.
The dermis is mesodermal in origin and is composed mainly of fibrous connective tissues with collagen proteins. It contains blood vessels, nerves, lymph vessels, scales cutaneous sense organs, chromatophores and adipose tissues. The dermis is composed of three layers. The thin upper layer of loose connective tissue, where the scales have their base, is called stratum spongiosum. The thicker dense inner layer is called stratum compactum, which is built out of strong fibrous connective tissue. The middle layer is a subcutaneous layer which contains sense organs. All these three layers are sharply demarked from each other. Immediately below the dermis, and separating the dermis from the underlying skeletal musculature, is a layer of well-vascularized loose connective tissues with chromatophores and lipid cells called the hypodermis or subcutis. Integumentary colors are primarily dependent on the presence of chromatophores, which mostly occur in the stratum spongiosum, in the hypodermis, or both. The scales of teleost fishes arise from the secretary activity of the dermis.


Mucous/Slime
The goblet or mucoid cells are single-celled glands in the epidermis, producing a glycoprotein called mucin. This secretion is fibrous and swells up in water to form a thick layer of viscous mucous. Not all fish species have these cells, and the amount of mucous produced varies from enormous amounts (e.g., a bucketful by the hagfish, Myxine glutinosa) to none. Mucous has a protective function against infections and parasites, reduces the danger of damage to the skin through collisions and helps the fish to escape the grasp of predators. Mucous seals the body and prevents the exchange of ions and water. There are many more special functions described for a variety of species, varying from the protective nightgown of parrotfishes to a foodstuff for young cichlids.

Functions of skin
1.    Lubricating function: Mucous secreted by the skin glands lubricates the fish and reduces body friction in water while swimming, thus enabling the fish to move with a greater speed.
2.    Protective function: Mucous present in skin protects the body from external injuries, parasites, fungus, bacteria and other microorganisms.
3.    Osmoregulatory function: Skin helps in regulating the osmotic exchanges of water and ions between the body fluids and surrounding medium.
4.    Excretory function: Excretes metabolic wastes from the body.
5.    Repairing and maintenance function: The skin performs an important function in healing the surface wounds.
6.    Respiratory function: The skin is an important respiratory organ in certain species like Anguilla spp. (Eel), Anabas spp. (Climbing perch), Monopterus spp. (Mud eel), etc.
7.    Offensive or defensive function: The club cells of the epidermis are modified to form poison gland in certain fishes (in many elasmobranches and few teleosts) used for offence as well as defense.
8.    Impart color or Tint: The chromatophores of various kinds present in the epidermis and dermis of the fish give beautiful color patterns to the body making it conspicuous or inconspicuous (e.g., carotenoids – yellow-red, melaninsblack, flavins – yellow, erythrophores - red, purines – white, bile pigments- bluish green).
9.    Metabolic function: The skin absorbs the dissolved nutrients from the surrounding water.
10.    Heat regulation: The skin contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss, while constricted vessels greatly reduce cutaneous blood flow and conserve heat.
11.    In some species, special structures like the electric organs,  and phosphorescent organs are also integumentary derivatives.
 Scales
Types and Structure
The body of all fishes (except the members of the Order- Siluriformes and some bottom-dwellers) is covered by an exoskeleton in the form of scales. In a few teleosts, scales are entirely absent (e.g., Silurids); or they exist only as microscopic vestiges hidden in the skin (e.g., Eels) and in some fishes, they become reduced to mare papillae of the dermis (e.g., Silurids). The body of some fishes is also covered by scutes, a bony plate (e.g., Sturgeons). The number of scales covering the body remains constant throughout life, and in general, scale growth is proportional to the fish growth. According to the mode of origin, there are two types of scales, placoid and non-placoid. The placoid scale is formed due to the secretary activities of both epidermis and dermis, while the non-placoid scale is derived from the dermis only. The scale of teleosts is of non-placoid type. Structurally, the non-placoid scales are classified as cosmoid, ganoid or rhomboid, cycloid, and ctenoid. Cycloid and ctenoid scales are also called bony ridge scales. 

a. Placoid scale
The placoid scales are found in the Elasmobranchii and Holocephali. It consists of two parts: a flattened, disk like basal plate (embedded in the dermis) and a spine or cusp (projecting out through epidermis) (Figure 4.7). In structure, a placoid scale resembles that of a tooth. The spine has an external covering of enamel-like, hard, transparent material called vitrodentine. This is followed by a layer of dentine enclosing a pulp cavity from which several branching dental tubules radiate in different directions. The centre of the basal plate is perforated by an aperature known as opening of pulp cavity through which blood vessels and nerve from dermis supplied to the scale. These scales are closely set but do not overlap each other except where they protect the lateral line canal. The protruding spines of placoid scales give skin its characteristic rough texture. Placoid scales do not exhibit contineous, indefinite growth; they may be replaced when old, worn out, or lost.
b. Cosmoid scale
The cosmoid scales were found in the extinct Crossopterygii and Dipnoi. They are not present in the living fish. In the living Crossopterygii (Latimeria) and Dipnoi, they have become thin and modified to look like cycloid scales. The external layer of the cosmoid scale is thin and enamel-like and is called the vitrodentine (Figure 4.8). The middle layer is made up of hard, non-cellular, dentine-like material called the cosmine, and contains a large number of branching tubules and chambers. The inner layer is made up of vascularised bony substance, isopedine. These scales grew by the addition of new isopedine material from below, along the edges.
c. Ganoid scale
The ganoid scales are found in the primitive Actinopterygii and are of various forms and structures. Ganoid scales are usually rhomboidal or diamond-shaped. These scales are heavy and have an outer layer of hard, inorganic, enamel-like material called ganoine. The middle layer is cosmine containing numerous branching tubules. The innermost layer is thickest and is made up of lamellar bone, isopedine. These scales grow by the addition of new layers to lower as well as upper surface. They may slightly overlap and articulate one another by peg-and-sucket joints at the margin.
 Ctenoid scales
•    The ctenoid scales are also roughly rounded in shape as in cycloid scales (Figure 4.10-B).
•    The anterior margin has more or less serrated edges.
•    Several spines (ctenii) are present on the surface of the posterior area of the scale which give a rough texture to spiny-rayed fish. The word 'ctenoid' comes from the Greek word cteno, meaning comb or spine.
•    Found in teleosts having spiny-rayed fins. E.g., Channa spp., Anabas spp., and other fishes of Perciformes order.

Both cycloid and ctenoid scales may be present on the same individual of certain fish species, such as Glossogobius giuris. In G. giuris, cycloid and cteoid scales are found on the body and head regions, respectively. Cyloid and ctenoid scales are of considerable help in calculating the age and growth rate of fishes. Many species undergo seasonal growth, which is apparent form, the lines of growth on the scale. In some species such as Salmo, spawning marks can be seen on the scales, so that it is possible to find how many times a fish has spawned.

Functions of scale
1.    The scales of fishes are in the form of exoskeleton, which provides protection to the body.
2.    It protects the body from external injury, parasites, fungus, bacteria and other microorganisms.
3.    In some species (e.g. Puffers), the scales are elongated to form spines for protection.
4.    In most teleosts, the scales along the lateral line become perforated to communicate sensory canal with the exterior.
5.    The scales are of considerable help in calculating the age and growth rate of fishes.
6.    In some fishes, scales exhibit remarkable coloration, which helps the fish to match with the surroundings.
7.    The scales of many fishes show spawning rings and marks (e.g. Salmo spp.), which are the result of the cessation of feeding and exhaustion during the spawning period.

MORPHOLOGY OF FISH

External features of fish

Shape and Size
There is great diversity and ecological modifications in the
shape and size of fishes. In general, the body of fish is fusiform
and streamlined but they may be laterally compressed (e.g.,
flounders), depressed (e.g., rays), serpentine (e.g., eels), filliform
(e.g. snipe eel), taeniform (e.g. gunnels), sagittiform (e.g. pikes),
or globiform (e.g., puffers) (Figure 4.1). Compared to other body
shapes, fusiform shape is quite energy efficient for swimming. In
spite of many variations in shape, the ground plan of the body is
bilaterally symmetrical with prominent lateral line system. Body
shape is a good indicator of how a fish moves and where it lives.



External organs
a. Mouth
Mouth of fish is generally situated anteriorly in the head; however,
its location varies depends on species and feeding habit. The
location, type and size of the mouth is a good indicator of feeding
habit and habitat of that fish. There are three general locations of
the mouth in fish:
1. Upturned or Superior mouth: Lower jaw is longer than upper
jaw. The fish is generally a surface feeder.
2. Terminal or Sub-terminal mouth: Upper and lower jaws are
almost equal. The fish is generally a column feeder.
3. Underslung or Inferior mouth: Upper jaw is longer than lower
jaw. The fish is generally a bottom feeder.
Some fishes have tubular or elongated mouth (Figure 4.2).
Predatory fishes usually have a wide mouth, while herbivorous
and omnivorous fishes have smaller mouths. A fish may have
teeth along the jaws and in the pharynx as well as on its tongue.
The tongue is generally very simple, thick and immovable pad
located in the lower jaw.

             
b. Snout
It is that portion of the head situated in front of the eyes. The snout
has a pair of nostrils or nares on each side.
c. Nostrils
Generally, there are two nostrils on either side of the head (two
pairs) in fishes. They serve as olfactory organs that feel taste and
smell. Water flowing through the nostrils circulates to the smelling
organ (olfactory bulb) and nerves transmit signals to the brain. It
is important to note that although fish have nostrils they play no
role in respiration because they are not connected to the throat
or gills. In some fishes, nostrils are modified for aquatic vision as
shown by the presence of spherical lens.
d. Eyes
Fish has a pair of eyes situated on the lateral sides of the head
and each having its own vision zone. Eye size and position vary
depending on the habitat and behavior of the species. Their lenses
are spherical, a design that enables a fish to focus in water. Focus is
accomplished by moving the lens toward or away from the retina
instead of stretching the lens, as is the case in human eyes. Fish
donot have eye-lid (except in some sharks) because they live in water
which makes their eyes always moist. Fish do not sleep, but some
fish spend a good portion of their time on resting. Fish eyes are
not too good at judging distances, but colors are perceived well,
and many fish use colors for sexual advertisement and species
recognition.

e. Barbells
The barbells are elongated tactile organs located close to the
mouth, quite similar to the whiskers on a cat. The barbells are
used to screen the environment and increase awareness of its
surroundings. The barbells are equipped with taste buds and the
fish use them when catching fish in dark and cloudy waters where
the visibility is low. The number and size of barbells varies greatly
with species. For example, Heteropneustes fossilis and Clarias
batrachus have four pairs of long barbells whereas Labeo rohita
has only one pair of small barbells, and Ctenopharyngodon idella
has no barbell. The location and number of barbells are of great
helpful in identifying fishes, to some extent it reflects on their
feeding habit as well as habitat. Based on their origin, barbells are
named as

i. Maxillary barbell: arise from the upper jaw, usually at the angle
of the two jaws.
ii. Mandibular barbell: arise from the lower jaw.
iii. Rostral barbell: arise from the region of snout.
iv. Nasal barbell: arise from the region of nostrils.

f. Operculum
The operculum is the bony flap covering and protecting the
gills. It opens and closes to allow water to pass over the gills.
The operculum is composed of four fused bones; the opercle,
preopercle, interopercle, and subopercle. Elasmobranches do not
have an operculum but there are separate gill slits for each gill.
g. Lateral line
The lateral line is a sense organ that consists of a series of perforated
scales that most fish have along their sides, extending from their
head to tail. Under these scales is a system of fluid-filled tube
or canal and specialized cells which transmit vibrations to the
brain of the fish. The lateral line helps fish to detect movement
and vibration in the surrounding water including predators and
prey. Some Ichthyologists tell that the lateral line gives fishes the
sense of “distant touch”. It also helps the fish to feel pressure and
temperature changes in the water around it. Many fish species can
navigate without vision in darkness or muddy water. For example,
55
Introductory Ichthyology
Fig 4.5. Internal structure of a section of lateral line.
How the lateral line works ?
There are receptors in the line, called neuromasts. Each
neuromast consists of a group of hair cells, which are surrounded
by a protruding cupula (an organ that gives an animal a sense of
balance). Neuromasts may occur singly, in small groups called
pit organs, or in rows within grooves or canals, when they are
referred to as the lateral line system. The lateral line system runs
along the sides of the body onto the head, where it divides into
three branches, two to the snout and one to the lower jaw.
The lateral line system is sensitive to differences in water pressure.
These differences are thought to be due to changes in depth or
to the current like waves caused by approaching objects. When
pressure waves cause the gelatinous caps of the neuromasts to
move, bending the enclosed hairs, the frequency of the nerve
impulses is either increased or decreased, depending on the
direction of bending. A swimming fish sets up a pressure wave
in the water that is detectable by the lateral line systems of other
fishes. It also sets up a bow wave in front of itself, the pressure of
which is higher than that of the wave flow along its sides. These
the Blind Cave fish (Astyanax mexicanus) relies entirely on its
lateral line system. The lateral line may be complete, incomplete
or interrupted

How the lateral line works ?
There are receptors in the line, called neuromasts. Each
neuromast consists of a group of hair cells, which are surrounded
by a protruding cupula (an organ that gives an animal a sense of
balance). Neuromasts may occur singly, in small groups called
pit organs, or in rows within grooves or canals, when they are
referred to as the lateral line system. The lateral line system runs
along the sides of the body onto the head, where it divides into
three branches, two to the snout and one to the lower jaw.
The lateral line system is sensitive to differences in water pressure.
These differences are thought to be due to changes in depth or
to the current like waves caused by approaching objects. When
pressure waves cause the gelatinous caps of the neuromasts to
move, bending the enclosed hairs, the frequency of the nerve
impulses is either increased or decreased, depending on the
direction of bending. A swimming fish sets up a pressure wave
in the water that is detectable by the lateral line systems of other
fishes. It also sets up a bow wave in front of itself, the pressure of
which is higher than that of the wave flow along its sides
. Thesenear-field differences are registered by its own lateral line system.
As the fish approaches an object, such as a rock or the glass wall
of an aquarium, the pressure waves around its body are distorted,
and these changes are quickly detected by the lateral line system,
enabling the fish to turn or to take other actions. Because sound
waves are waves of pressure, the lateral line system is also able to
detect very low-frequency sounds of about 100 Hz or less.
h. Fins
The fins are the major locomotory organs in fishes. There are two
types of fins in fishes: (1) unpaired or median fins, and (2) paired
fins (Figure 4.12). The unpaired fins include dorsal fin on the
back, an anal fin on the ventral side behind the vent and a caudal
fin at the end of the tail. An adipose fin may also present behind
the dorsal fin of some fishes. The paired fins include pectorals and
pelvics corresponding to the fore and hind limb, respectively of the
terrestrial vertebrates. Fins give stability and control the direction
of movement during swimming, as explained later in Chapter 4.3.
i. Vent
The vent is the cumulative term for the external openings of
digestive, urinary and reproductive tracts. It is generally situated
on the posterior ventral side, just in front of the anal fin. There
is a separate opening for digestive tract, called anus. However,
the openings for urinery and reproductive tracts are generally
combined in male and separate in female. The functions of the
vent are to remove waste and extra water, and to expel eggs or milt
during spawning. The vent is the dividing line between the trunk
and tail of fish.
4.2. Integumentary system
The integumentary system of fish compries skin and its
derivatives. The scales, coloration and some special structures like
the electric organs, poisonous glands, and phosphorescent organs
are the integumentary derivatives. The major function of the
integumentary system is to protect the fish from external injuries
and enemies.
57
Introductory Ichthyology
4.2.1. Skin
Structure of skin
The skin forms the external covering of the body and performs
various important functions in fish. The skin of fish is quite firmly
attached and is hard and rough, and composed of two layers. The
outer layer is called epidermis and the inner layer is called dermis
or corium (Figure 4.6). The two layers differ in origin, structure
and function. A thick basement membrane is present between
the epidermis and dermis. In most fish species, the epidermis is
thinner than the dermis.
The epidermis is a thin, multilayered epithelium derived from the
ectoderm of the embryo. It does not contain blood vessels. The
upper portion of epidermis consists of several layers of flattened
epithelial cells, called stratified epithelium. The inner most layer is
made up of columnar cells, called stratum germinativum in which
cells are always multiplying by mitotic division to replace the outer
worn out cells. Different types of unicellular and multicellular
glands are formed from the epidermis. Some of these are confined
to the epidermis while others grow deep into the underlying tissue
of the dermis. These include holocrine mucous cells (produce
mucous), chromatophores (impart color), taste buds, sensory
cells, ampullary organs and a variety of other secretory cells. The
mucous cells are basically of two types, the goblet cells and the
club cells. The goblet cells produce mucous, which makes skin
slimy and frictionless as well as protects from pathogens. The
goblet cells develop from the stratum germinativum and migrate
to the surface. The club cells produce specific secretions and are
named accordingly as alarm cells, poison cells, photophores,
etc. Other secretory cells that the epidermis may include are
sacciform cells. Sacciform cells of some fish species may secrete
alarm pheromones, whereas sacciform cells of other species may
secrete toxic or repellent substances for antipredator defense. The
epidermis is a fragile layer which is constantly sloughed off and
renewed.
The dermis is mesodermal in origin and is composed mainly of
58
Introductory Ichthyology
fibrous connective tissues with collagen proteins. It contains blood
vessels, nerves, lymph vessels, scales cutaneous sense organs,
chromatophores and adipose tissues. The dermis is composed
of three layers. The thin upper layer of loose connective tissue,
where the scales have their base, is called stratum spongiosum.
The thicker dense inner layer is called stratum compactum, which
is built out of strong fibrous connective tissue. The middle layer is
a subcutaneous layer which contains sense organs. All these three
layers are sharply demarked from each other. Immediately below
the dermis, and separating the dermis from the underlying skeletal
musculature, is a layer of well-vascularized loose connective
tissues with chromatophores and lipid cells called the hypodermis
or subcutis. Integumentary colors are primarily dependent on the
presence of chromatophores, which mostly occur in the stratum
spongiosum, in the hypodermis, or both. The scales of teleost
fishes arise from the secretary activity of the dermis.

Mucous/Slime
The goblet or mucoid cells are single-celled glands in the epidermis,
producing a glycoprotein called mucin. This secretion is fibrous
and swells up in water to form a thick layer of viscous mucous.
Not all fish species have these cells, and the amount of mucous
produced varies from enormous amounts (e.g., a bucketful by
the hagfish, Myxine glutinosa) to none. Mucous has a protective
function against infections and parasites, reduces the danger of
damage to the skin through collisions and helps the fish to escape
the grasp of predators. Mucous seals the body and prevents the
exchange of ions and water. There are many more special functions
described for a variety of species, varying from the protective
nightgown of parrotfishes to a foodstuff for young cichlids.
Functions of skin
1. Lubricating function: Mucous secreted by the skin glands
lubricates the fish and reduces body friction in water while
swimming, thus enabling the fish to move with a greater speed.
2. Protective function: Mucous present in skin protects the
body from external injuries, parasites, fungus, bacteria and
other microorganisms.
3. Osmoregulatory function: Skin helps in regulating the
osmotic exchanges of water and ions between the body fluids
and surrounding medium.
4. Excretory function: Excretes metabolic wastes from the body.
5. Repairing and maintenance function: The skin performs an
important function in healing the surface wounds.
6. Respiratory function: The skin is an important respiratory
organ in certain species like Anguilla spp. (Eel), Anabas spp.
(Climbing perch), Monopterus spp. (Mud eel), etc.
7. Offensive or defensive function: The club cells of the
epidermis are modified to form poison gland in certain fishes
(in many elasmobranches and few teleosts) used for offence as
well as defense.
8. Impart color or Tint: The chromatophores of various
kinds present in the epidermis and dermis of the fish give
beautiful color patterns to the body making it conspicuous
or inconspicuous (e.g., carotenoids – yellow-red, melanins –
60
Introductory Ichthyology
black, flavins – yellow, erythrophores - red, purines – white,
bile pigments- bluish green).
9. Metabolic function: The skin absorbs the dissolved nutrients
from the surrounding water.
10. Heat regulation: The skin contains a blood supply far greater
than its requirements which allows precise control of energy
loss by radiation, convection and conduction. Dilated blood
vessels increase perfusion and heat loss, while constricted
vessels greatly reduce cutaneous blood flow and conserve heat.
11. In some species, special structures like the electric organs, and
phosphorescent organs are also integumentary derivatives.

CLASH OF FISH part-2

Order: Anguilliformes
• Cylindrical, elongated and serpentine body.
• Minute scales are embedded in skin.
• Dorsal and anal fins are long and confluent behind.
• Dorsal fin is without spines.
• Origin of dorsal fin is far behind the pectoral origin.
• Gill opening in the pharynx as narrow or wide slits.
• Ovaries are without oviduct.
• It is represented by 2 families and 3 fish species in Nepal.
Family: Anguillidae E.g. Anguilla bengalensis (Raj Bam – Nepali)
Family: Moringuidae E.g. Moringua raitaborua (Purple Spaghetti Eel)

                                                       Figure 2.11. Anguilla bengalensis

Order: Beloniformes
• The body is elongated and sub-cylindrical.
• Both jaws are well prolonged into a long beak armed with rows
of sharp teeth.
• Pectoral fins are short and wing like, pelvic fins are abdominal.
• The dorsal fin placed far posterior of the body and is without
spines.
• The dorsal and anal fins are of looks origin.
• It is represented by 2 families and 2 fish species in Nepal.
Family: Belonidae E.g. Xenentodon cancila (Chuche
Bam Kauwa/Kabali – Nepali)
Family: Hemiramphidae E.g. Hyporhamphus limbatus
(Congaturi Half B

                                                            Figure 2.12. Xenentodon cancila

                                                   Figure 2.13. Beak of Xenentodon cancila.

                                                   Figure 2.14. Beak of Hyporhamphus limbatus.

Order: Clupeiformes
• The body is laterally compressed.
• Abdomen is generally keeled and serrated.
• The abdomen has scutes (bony plates).
• The lateral line is absent.
• Barbells are always present.
• It is represented by 2 families and 3 fish species in Nepal.
Family: Clupeidae E.g. Gudusia chapra (Suiya – Nepali)
Family: Engraulididae E.g. Setipinna phasa (Gnakabau/Phasi -
Nepali)

                                                           Figure 2.15. Setipinna phasa

Order: Cypriniformes
• The body is oblong and compressed with small to large scales.
• The head is without scales.
• All the fishes of this order have round abdomen.
• The mouth is usually protractile and always toothless.
• The lateral line is present.
• The pelvic fins are generally abdominal in location.
• Dorsal fin is single rayed.
• Air bladder constricted to two chambers.
• It is represented by 4 families, 36 genera and 115 fish species in
Nepal.
Family: Cyprinidae E.g. Labeo rohita (Rohu –
Nepali), Cirrhinus mrigala (Naini
or Mrigal – Nepali), Catla catla
(Bhakur or Catla – Nepali),
Tor putitora (Sahar or Mahseer –
Nepali), Garra gotyla (Buduna –
Nepali), etc.
Family: Psilorhynchidae E.g. Psilorhynchus balitora (Tite
Machha – Nepali)
Introductory Ichthyology
Family: Balitoridae E.g. Balitora brucei (Pathertata – Nepali
Family: Cobitidae E.g. Botia almorhae (Baghi – Nepali)
B. lohachata (Getu or Baghe – Nepali)

                                                                Figure 2.16. Tor putitora

                                                                   Figure 2.17. Labeo rohita

Order: Cyprinodontiformes
• The body is short and compressed with scales.
• The head is also with scales.
• The upper jaw or lower jaw well produced.
• Lateral line is absent or chiefly on the anterior part of the
body.
• The pectoral fin is elongated and wing like.
• It is represented by 2 families and 2 fish species in Nepal.
Family: Aplocheilidae E.g. Aplocheilus panchax (Tikuli – Nepali)
Family: Poeciliidae E.g. Gambusia affinis (Machar Machha - Nepali)

                                                               Figure 2.18. Gambusia affinis

Order: Osteoglossiformes
• The body is broad and strongly compressed with fine scales on
head as well.
• The pelvic fin is rudimentary and the dorsal fin is very small.
• The anal fin is long and confluent with small caudal fin.
• Lateral line is complete.
• It is represented by 1 family and 2 fish species in Nepal.
Family: Notopteridae E.g. Chitala chitala (Moi or Patara – Nepali)
Notopterus notopterus (Golhai – Nepali)

                                                      Figure 2.19. Notopterus notopterus

Order: Perciformes
• This is the largest and most diversified of all fish orders.
• Scales are present both on the head and body.
• Jaws are generally provided with teeth.
• The dorsal fin is very long (E.g., Channa) or occurs in two
parts (E.g., Chanda): one spinous and the other with soft rays,
continuous or separate.
• The caudal fin is long or forked.
• It is represented by 10 families and 26 fish species in Nepal.
Family: Ambassidae Chanda nama (Chanerbijua – Nepali)
Family: Sciaenidae Johnius coiter (Bhola – Nepali)
Family: Nandidae Nandus nandus (Dalahai or Dhoke– Nepali)
Family: Cichlidae Oreochromis niloticus (Nile tilapia)
Family: Mugilidae Rhinomugil corsula (Karsul – Nepali),
Sicamugil cascasia (Yellow-tail Mullet)
Family: Gobioididae Brachyamblyopus burmanicus
(Burmese Eel Goby)
Family: Gobiidae Glossogobius giuris (Bulla – Nepali)
Family: Anabantidae Anabas testudineus (Kabai – Nepali)
Family: Belontiidae Colisa faciatus (Kotari – Nepali),
C. lalius (Dwarf Gourami)
Family: Channidae Channa marulius (Bhaura – Nepali),
C. punctatus (Garai or Bhoti – Nepali)
C. striatus (Hile or Saura – Nepali)

                                                       Figure 2.20. Anabas testudineus

                                                            Figure 2.21. Channa striatus

Order: Salmoniformes
• The body is long and cylindrical.
• Adipose fin is present.
• Mouth is large with jaw teeth.
• It is represented by 1 family and 3 fish species in Nepal.
Family: Salmonidae E.g. Oncorhynchus mykiss (Rainbow trout)
O. rhodurus (Amago trout)
Salmo trutta (Brown trout)

                                                         Figure 2.22. Oncorhynchus mykiss

Order: Siluriformes
• The body is naked or with scutes, never with true scales.
• The maxillae are usually much reduced serving as bases of
maxillary barbells.
• Nearly one to four pairs of barbells are present.
• The first ray of pectoral and last ray of dorsal fin is modified as
hard pungent spines or thick rays.
• The adipose fin may present or absent.
• The lateral line may be ramified with or without short tubular
ossicles.
• It is represented by 10 families and 71 fish species in Nepal.
Family: Bagridae Mystus tengara (Tenger – Nepali)
Rita rita (Rita or Belaunda – Nepali)
Family: Siluridae Wallago attu (Buhari – Nepali)
Family: Schilbeidae Ailia coila (Patsi – Nepali)
Family: Pangasidae Pangasius pangasius (Jalkapoor – Nepali)
Family: Amblycipitidae Amblyceps mangois (Bokshi
Macho – Nepali)
Family: Sisoridae Bagarius bagarius (Gonch – Nepali)
B. yarrellii (Gounch – Nepali)
Hara hara (Tinkana or Datari – Nepali)
Family: Clariidae Clarias batrachus (Mangur – Nepali)
C. gariepinus (African Mangur – Nepali)
Introductory Ichthyology
Family: Heteropneustidae Heteropneustes fossilis (Singhi –
Nepali)
Family: Chacidae Chaca chaca (Kurkuree –
Nepali)
Family: Olyridae Olyra longicaudata (Himalayan
Olyra)

                                                           Figure 2.23. Clarias batrachus

Order: Synbranchiformes
• Body is cylindrical, elongated and eel-shaped.
• The gill openings are confluent as a single slit on ventral
surface.
• The dorsal and anal fins are vestigial or absent.
• The pectoral and pelvic fins are present or absent.
• The scales (when present) are small, oval and confined to
caudal region.
• The eyes are small or degenerate.
• It is represented by 2 families and 5 fish species in Nepal.
Family: Synbranchidae E.g. Monopterus cuchia (Andha Bam
or Anahi – Nepali)
Family : Mastacembelidae E.g.M acrognathus aral (Gainchi – Nepali)
Mastacembelus armatus (Dhare Bam –
Nepali)

                                                          Figure 2.24. Monopterus cuchia

                                                          Figure 2.25. Macrognathus aral

Order: Tetraodontiformes
• The body is short, rounded and globular in inflated condition.
• True scales are absent.
• Body is completely or incompletely covered with scutes and
fine dorsal spines.
• Pelvic fins are absent.
• Gill openings are restricted to lateral slits.
• The teeth are fused into a beak like dental plate.
• It is represented by 1 family and 1 fish species in Nepal.
Family: Tetraodontidae E.g. Tetraodon cutcutia (Puffer fish or
Pokcha – Nepali)

(A)

(B)

                                         Figure 2.26. Tetraodon cutcutia, (A) Normal swimming

                                                        condition, (B) Inflatted condition

CLASS OF FISH

Class: Placodermi
•    The placoderms dominated the waters of the late Devonian period and died out completely in the late Carboniferous period (about 350 million years ago).
•    Body was typically fish-like with bony armour.
•    Jaws were present, but not properly formed.
•    Paired and unpaired fins were present.
•    Cranium was partially ossified.
•    Vertebral column had neural and haemal arches.
•    Notochord persisted throughout life.
•    Example: Dunkleosteus spp. (Figure 2.2).

Figure 2.2. A. Dunkleosteus skull present at Sam Noble Museum, Oklahoma, USA. 

B. An imaginary drawing of Dunkleosteus.

Class: Elasmobranchii
•    Mostly marine and includes largest fishes that ever lived.
•    Endoskeleton is cartilaginous which is partially calcified.
•    There are 5-7 pairs of gills without operculum.
•    There is no air bladder and lungs.
•    Spiracle is generally present.
•    Skin is naked or covered by small placoid scales, composing toothlike denticles that give the skin an abrasive texture.
•    Mouth is generally ventral.
•    Dorsal fins are rigid.
•    Fins are without rays. Tail is heterocercal.
•    The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm (Figure 2.3).
•    Intestine has a characteristic spiral valve.
•    A cloaca is present.
•    Fertilization is generally internal.
•    Examples: Carcharodon carcharias (White shark) (Figure 2.4), Rhincodon typus (Whale shark), Scoliodon laticaudus (Spadenose shark), Manta birostris  (Ray) (Figure 2.5), Raja erinacea (Skate).

                       Figure 2.3. Male and female sharks, (A) Male with claspers, (B) female without claspers.

                                                      Figure 2.4. Carcharodon carcharias

                                                          Figure 2.5. Manta birostris

Class: Holocephali
•    This class includes deep sea and carnivorous fishes.
•    Endoskeleton is cartilaginous.
•    There are 4 pairs of gills with operculum.
•    Spiracle is absent.
•    There is no air bladder.
•    Skin is naked or covered by small placoid scales.
•    Mouth is generally ventral.
•    There is no cloaca.
•    In male, a pair of claspers is present posterior to the pelvic fins as in Elasmobranchii. Besides these, a frontal clasper is present on the dorsal surface of the head of some species known as tenaculum.
•    Examples: Chimaera spp. (Ratfish), Harriotta spp. (Spookfish) (Figure 2.6).

                                          Figure 2.6. A. Chimaera spp., B. Harriotta haeckeli

Class: Dipnoi
•    Mostly found in freshwater and commonly known as lungfish.
•    The air bladder is modified into lung which serves for aerial respiration. Gills are also present.
•    Notochord is persistent and unconstricted. Skeleton is largely cartilaginous.
•    Skin is covered by cycloid scales.
•    Operculum is present and there is only one external branchial aperture.
•    Paired fins are lobate, cylindrical, with joined median axis.
•    Internal nostrils are present.
•    Intestine has a spiral valve.
•    A cloaca is present.
•    Examples: Protopterus spp. (African lungfish) (Figure 2.7), Lepidosiren spp. (American lungfish).

                                                          Figure 2.7. Protopterus spp.






Class: Teleostomi
•    The endoskeleton is bony, and are known as bony fishes.
•    There are 4 pairs of gills; the 5th pair of gill arch is modified into tooth bearing lower pharyngeal.
•    Operculum is present.
•    Spiracle is absent.
•    Air bladder is usually present.
•    Skin is generally covered by cycloid or ctenoid scales.
•    Branchial lamellae are supported by a double row of branchial rays.
•    Fins are with fin rays. Tail is homocercal or diphycercal.
•    No spiral valve in the intestine.
•    There is no cloaca.
•    Fertilization is generally external.
•    Examples: All fishes found in Nepal.

The class Teleostomi is divided into two sub-classes, Crossopterygii and Actinopterygii.

 Sub-class: Crossopterygii
•    This is the rare (almost extinct) sub-class, which is characterized by the presence of lobed paired fins covered with scales, and are known as lobed-fin fishes.
•    There are two dorsal fins.
•    The inner nostrils are present.
•    The air bladder is modified into lung.•    The scales are cosmoid type.
•    This sub-class is not represented in Nepal, and includes some rare marine fishes.
•    Examples: Latimeria (Figure 2.8), Coelocanthus.

                                                                      Figure 2.8. Latimeria


Sub-class: Actinopterygii
•    This sub-class contains most of the bony fish species that exist today.
•    The paired fins are unlobed and without scales.
•    There are generally single dorsal fin.
•    The inner nostrils are completely absent.
•    The external nostrils are present.
•    The scales are cycloid or ctenoid type.
•    This sub-class is represented by all fish species in Nepal.
•    Examples: Labeo rohita, Tor putitora, Catla catla (Figure 2.9), Oreochromis niloticus (Figure 2.10) and all other fish species found in Nepal.

                                                              Figure 2.9. Catla catla

                                                        Figure 2.10. Oreochromis niloticus

DISTINGUISHING CHARACTERISTICS OF EACH LEVEL

   Kingdom: Animalia
•    Absence of cell wall.
•    Heterotrophic.
•    Plastids and vacuoles are generally absent and centrioles and lysosomes are present.
•    Reserve food is in the form of glycogen.

    Phylum: Chordata
•    Presence of notochord.
•    Presence of gill slits.
•    Presence of dorsal tubular nerve cord.

    

     Sub-Phylum: Craniata
•    Presence of brain box (cranium).
•    Vertebrae surrounding nerve cord.

    Division: Gnathostomata
•    Presence of true jaw.
•    Presence of paired nostrils.

    Super-class: Pisces
•    Aquatic habitat.
•    Streamlined body.
•    Respiration by means of pharyangeal gills.
•    Locomotion by means of fins.
•    Body is generally covered by scales.
•    Presence of internal ear.

PARTR 2:- TAXONOMY OF FISHES OF NEPAL

The word taxonomy is derived from two Greek words, ‘taxis’  means arrangement and ‘nomia’ means method. Thus, the taxonomy can be simply defined as the science of classifying biological organisms. Fish taxonomy is concerned with the identification and classification of fishes to the level of species or subspecies, following the conventional principles of basic taxonomy.

The Swedish Botanist Carolus Linnaeus (1707-1778) is regarded as the father of modern taxonomy, as he developed a system known as Linnaean classification for categorization of organisms and binomial nomenclature for naming organisms. However, the word taxonomy was first coined by a French Professor A. P. de Candolle as ‘Taxonomie’ in 1813. Taxonomy has three stages; (i) alpha taxonomy, which deals with the description of new species and its arrangement in comprehensive genera, (ii) beta taxonomy, which works out the relationships to the level of species, and (iii) gamma taxonomy, which emphasis intraspecific variations and their evolutionary relationships. The term ‘taxon’ (plural taxa) refers to the taxonomic group of any rank which is sufficiently distinct to be worthy of being assigned to a definite category.

2.1. Nomenclature of fishes
Nomenclature is the scientific naming of fish or any organisms. Common names frequently vary so they are not reliable for identification. Every living organism has only one unique and internationally recognized scientific name. For example, common carp is called zoologically Cyprinus carpio all over the world. The nomenclature of zoological species is regulated by the International Code of Zoological Nomenclature (ICZN).

Whenever a species or genus is newly described, certain rules of zoological nomenclature have to be taken into consideration. In binomial nomenclature, the name of a species consists of two words, the first word denoting the genus and the second word denoting the species (e.g., Cyprinus carpio). Sometimes a species also has a subspecies, in which the full scientific name becomes trinomial. In case of trinomial nomenclature, the name of a species consists of three words, the first word denoting the genus, the second word denoting the species and the third word denoting the subspecies (e.g., Cyprinus carpio specularis). The genus name is always a noun and written with an initial capital letter while species and subspecies name are generally an adjective and are always written in small letters. The genus, species and subspecies name should be either underlined separately or written in Italic font. When a subgenous name is used, it is placed within parentheses between the geneus and species names [e.g., Channa (Ophiocephalus) striatus]. To indicate many species within a genus, the abbreviation ‘spp.’ is sometimes used and should not be italicized.

In taxonomic classification of fishes, the name of the order generally ends with ‘formes’, of the family with ‘idae’, of the subfamily with ‘inae’ of the tribe with ‘ini’. When the species is described for the first time, its name should be written, for example, as Neoanguilla nepalensis sp. nov., while a new genus should be expressed, for example, as Neoanguilla gen. nov. When the said species is cited by another author, it should be written as Neoanguilla nepalensis (Shrestha) to indicate the author of the species.

2.2. Classification and identification of fishes of Nepal
Fishes have been classified in different ways from time to time. The earliest classification was proposed by J. Muller (1844) who proposed seven sub-classes under the class ‘Pisces’. Later, Berg (1940) and Romer (1959) have given detailed classification of fishes, which have generally been accepted worldwide. In this book, we follow the classification given by Berg (1940), who divided the Super-class (Series) Pisces into seven classes: (1) Acanthodii, (2) Coccostei, (3) Pterichthys, (4) Elasmobranchii, (5) Holocephali, (6) Dipnoi, and (7) Teleostomi (Figure 2.1). Of these, the first three classes are completely extinct and are collectively known as Placodermi (meaning plate-skinned). Thus, the modern fishes are represented only by four major classes, i.e., Elasmobranchii, Holocephali, Dipnoi and Teleostomi. Out of these, the fishes of Nepal belong to the sub-class Actinopterygii of the class Teleostomi. Berg has divided the sub-class Actinopterygii into a series of 59 orders. Out of these, only 11 orders are represented in freshwater ecosystem of Nepal.

The total number of fish species represented in Nepal has been controvercial and is a subject of study. Different fish biologists have reported different number of fish species present in Nepal, which ranged from 228-232. According to Shrestha (2008), a total of 232 species belonging to 114 genera under 36 families and 11 orders existing in culture or natural water bodies of Nepal. Recently, Shrestha and Pandit (2012) have described one more exotic species, Pangasius hypophthalmus (Sauvage), which has been commercially cultured in the terai region of Nepal since few years. This indicated that the total number of fish species present in Nepal might be more than 232.