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.
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.
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