FACULTY OF AGRICULTURE

DEPARTMENT OF FISHERIES AND AQUACULTURE

LECTURE NOTE 3.

For any fish, feeding is one of the main concerns of day-to-day living. Although many a luckless
angler would swear otherwise, the fact is that a fish devotes more time and energy to eating, or
searching for food to eat, than to anything else. Throughout the long evolution of fishes, there
has been unrelenting selective pressure for those adaptations that enable a fish to come out on the
better end of the eat-or-beaten contest. Certainly, the most far-reaching single event was the
evolution of jaws. Their possessors were freed from a largely passive filter feeding existence and
could adopt a predatory mode of life. Improved means of capturing larger prey demanded
stronger muscles, more agile movement, better balance, and improved special senses. More than
any other aspect of its life habit, feeding behavior shapes the fish. Most fishes are carnivores that
prey on a myriad of animal foods from zooplankton and insect larvae to large vertebrates. Some
deep-sea fishes are capable of eating victims nearly twice their own size—an adaptation for life
in a world where meals are necessarily infrequent. Most advanced ray-finned fishes cannot
masticate their food as we can because doing so would block the current of water across the gills.
Some, however, such as the wolf eel, have molarlike teeth in the jaws for crushing their prey,
which may include hard-bodied crustaceans. Others that do grind their food use powerful
pharyngeal teeth in the throat. Most carnivorous fish almost invariably swallow their prey whole,
using sharp pointed teeth in the jaws and on the roof of the mouth to seize their prey. The
incompressibility of water makes the task even easier for many large mouthed predators. When
the mouth is suddenly opened, a negative pressure is created that sweeps the victim inside A
second group of fishes are herbivores that eat plants and algae. Although plant eaters are
relatively few in number, they are crucial intermediates in the food chain, especially in
freshwater rivers, lakes, and ponds that contain very little plankton.
Suspension-feeders that crop the abundant microorganisms of the sea form a third and diverse
group of fishes ranging from fish larvae to basking sharks. However, the most characteristic
group of plankton feeders are herring-like fishes (menhaden, herring, anchovies, capelin,
pilchards, and others), mostly pelagic (open-sea dwellers) fishes that travel in large schools. Both
phytoplankton and the smaller zooplankton are strained from the water with the sieve-like gill
rakers. Because plankton feeders are the most abundant of all marine fishes, they are important
food for numerous larger but less abundant carnivores. Many freshwater fishes also depend on
plankton for food.
A fourth group of fishes contains omnivores that feed on both plant and animal food. Finally,
there are scavengers that feed on organic debris (detritus) and parasites that suck the body fluids
of other fishes. Digestion in most fishes follows the vertebrate plan. Except in several fishes that
lack stomachs altogether, the food proceeds from stomach to tubular intestine, which tends to be
short in carnivores but may be extremely long and coiled in herbivorous forms. In the
herbivorous grass carp, for example, the intestine may be nine times the body length, an
adaptation for the lengthy digestion required for plant carbohydrates. In carnivores, some protein
digestion may be initiated in the acid medium of the stomach, but the principal function of the
stomach is to store the often large and in frequent meals while awaiting their reception by the
intestine. Digestion and absorption proceed simultaneously in the intestine. A curious feature of
ray-finned fishes, especially the teleosts, is the presence of numerous pyloric ceca found in no
other vertebrate group. Their primary function appears to be fat absorption, although all classes
of digestive enzymes (protein-, carbohydrate-, and fat-splitting) are secreted there.
Feeding Adaptation of Fishes
Different types of changes have been made in the digestive system of fish due to different diets.
This change in the body’s ability to take food is called feeding adaptation. The lips, mouth, teeth,
gills and digestive tract are modified and adapted based on the diet. These are described below:
Modification of Lips
Eating habits can cause structural changes in the lips of fish. Different types of lip structure are
described below:
f Fish Feed in Nigeria
1. Thin lip: Fish that eat large foods have very stiff jaws and thin, unaltered lips such as
Xenentodon.
2. Thick and Fleshy lip: The fishes that take food by sucking, they have thick and fleshy type
lip. Carp (Cyprinidae) have thick and fleshy lip so they can bite or suck.
3. Transformation into Hold fast Organ: Free-living Sisorid catfish in Asia, some South
American catfish (Loricaridae), Loach-like Gyrinocheilid (Gyrinochilus) of Southeast Asia have
sucktorial lips. Gyrinochilus has separate water entry and exit techniques in the operculum so
they do not have to breathe through their mouth while they are engaged in food sucking. In
parasitic lampreys (Petromyzonidae) and Hagfish (Myxinidae), the jawless sucking mouth also
acts as a holdfast organ for attachment to the host and a food-extracting organ from the host.
Moreover, the sucking mouth of the lamprey can remove stones.
As the lips of the fish have changed due to feeding habits, so has the mouth. Such changes are
seen in Gazers and Suctorial Feeders. The following are the descriptions of different types of
mouth of fish:
1. Predatory Mouth: The mouth is large; the jaw is stiff and both jaws have sharp teeth. The
mouth expands and swallows the prey. In freshwater shark (Wallago attu) and deep-sea
predatory fish such as viper fish (Chauliodus), this type of mouth is Xenentodon cancila seen.
2. Sucking Mouth: This type of mouth is round, funnel shaped. There is no jaw in the
downward mouth. The tongue has numerous small teeth. Example- Petomyzon marinus.
3. Absorbing mouth: This type of fish’s mouth is like a tube. Such a mouth does not have teeth.
The mouth is extending outwards. They eat small animals at the bottom of reservoirs as food
such as pipe fish– Syngnathus.
4. Grinding mouth: This type of mlouth looks like the lips of an aquatic bird. The jaw contains
hard and sharp teeth. Such fish can eat by breaking food or in pieces.
Example-
5. Plankton Feeder Mouth: This type of mouth is suitable for eating plankton. The mouth is
large or medium type. Jaws are soft with no teeth in mouth. Their gill raker is densely embedded
to form a sieve-like structure. Example- Tenualosa ilisha, Hypophthalmicthyes molitrix, Catla
catla.
Special type of Mouth: Special type of mouths are of three types, viz
1. Half beak: In such a mouth, the lower jaw is long and the upper jaw is short. The teeth are
small which are suitable for taking small foods on the surface of the water. Example:
Hyporhamphus gaimerdi
2. Paddle like Mouth: The upper jaw of this type of fish is extended but the lower jaw is short.
It moves the mud from the bottom with a spoon-shaped beak and finds small creatures and eats
them. Example: Paddle fish-Polyodon spathula.
3. Protrusive Mouth: The pre-maxillary bones of some fish are attached to other bones in front
of the skull in such a way that they can project at will. Such an enlarged mouth increases the
space inside the mouth compared to other fish. This type of fish with a mouth is predatory in
nature. Example: Nandus nandus.
Position by Mouth
Position by mouth is divided into the following distinct parts, e.g.
1. Superior Mouth: In such a mouth, the lower jaw is larger and the upper jaw is smaller. The
mouth is located diagonally upwards. Such mouth can be seen on plankton-eating fish living on
the surface of the water. Example: Catla-Catla catla, Silvercarp-Hypophpthalmicthyes molitrix,
Pabda catfish-Ompok pabda
2.Terminal mouth: In this case, the position of the mouth is almost parallel to the body. Such
mouth-fed fish eat surface and mid-surface food. Example: Puffer fish-Tetradon cutcutia.
3. Sub-terminal Mouth: In this case, the mouth opening is located below the upward mouth
opening and slightly above the lateral mouth opening. Example: Rohu- Labeo rohita.
4. Inferior Mouth: In this case, the upper jaw is large and the lower jaw is very small. Such fish
eat the food from the bottom. Example: Mrigel fish – Cirhinnus cirrhosus
5. Ventral Mouth: The mouth of such is located on the ventral surface. Such mouth can be seen
in many teleosts and elasmobranchs. Example: Wallago attu, Scoliodon laticaudus.
6. Semi-ventral Mouth: Such a mouth is not located in a completely ventral position. As a
result of the development of fleshy lips, it has taken a ventral position. Example- Conta conta.
Modification in the Teeth
There have been special changes in the teeth of fish for eating. The positional and morphological
changes of the teeth are significant in this case. In chondrichthyes, placoid scales are transformed
into teeth. Depending on where it is found, teeth of bony fish can be divided into the following
three types:
(1) jaw,
(2) mouth and
(3) pharynx.
1. Jaw Teeth
These teeth are located in the maxillary, pre-maxillary and dentine bones of fish. Such toothed
fish are predatory in nature. Example: Channa striatus
Also, the jaw teeth are divided into five types based on the shape, viz
(1) Cardiform: They are many in number, short and small but pointed. There is a pad of
teeth on a bone. Such fish eat plankton, soft plants and small animals. Such teeth are
found in North American catfish (Ictaluridae), Perch (Percidae) and seabass (Serranidae).

(2) Villiform teeth: Such teeth look like intestinal villi but are a little longer which helps to hold
food. Such toothed fish eat zooplankton, plants and small fish. Examples: Xenentodon cancila,
Pterois miles.
(3) Canine teeth: These teeth are like dog teeth. In most cases, they are look a venom tooth of
snake. These are elongated, conical, curved, and pierced. Predatory fish have such teeth. They
take small animals and small fish as food. Example- Whitespotted moray (Gymnothorax
punctatus), Deep-sea viper (Chauliodus solani).
(4) Incisor teeth: These are cutting teeth with sharp edges. In some fishes, these are human-like
teeth. Such toothed fish are usually accustomed to crushing aquatic weeds, crustaceans and
molluscs. E.g. Rainbow parrot fish (Sarus guacamaia).
(5) Molariform teeth: These teeth are suitable for grinding and crushing. These are flat and
wide. Such teeth can be seen in the fish living at the bottom. Examples: Skates (Dasyatis),
Chimaera (Holocephali).
2. Mouth Teeth
This type of tooth is seen in parasitic fish. It is funnel type mouth which does not have a jaw.
There are small teeth inside the mouth. Example- Petromyzon marinus.
3. Pharyngeal Teeth
Different fish have a few sets of teeth of different shapes in the pharyngeal region. Such teeth are
used for grinding. It is again of four types, viz:
(1) Grasping type: Such teeth are used to grasp food such as Puntius has such teeth;
(2) Tearing type: This type of tooth helps in breaking food. It is seen in rudd (Scardinius);
(3) Grinding type: This type of tooth is used for grinding food. It is found in Cyprinus carpio.
(4) Comb like– This type of tooth is used to filter food. It is found in golden redhorse - Fish
Jaws
Fish have two types of jaws; the upper jaw is called the maxilla and the lower jaw is called the
mandible. Fish jaws can be divided into three types based on the presence of teeth, viz:
(1) Teeth jaw: The jaws of such fish are hard and sharp-toothed. These fish eat plankton or
zooplankton and small fish, such as Xenentodon cancila.
(2) Cut edged jaw: The jaws of this type of fish have no teeth but the edges are sharp. This type
of fish eats plant by cuttings, such as grass carp – Ctenopharyngodon idellus.
(3) Toothless jaw: This type of fish has no tooth on jaws and they eat phytoplankton,
zooplankton and aquatic plants, such as Labeo rohita.
Dentition
There is a deep connection between eating habits and the food consumed. In predatory fish such
as pike (Esox), gar (Lepisosteus) and deep-sea gulper, they have sharp pointed teeth which help
to catch prey. The toothless jawed fish eats plankton and periphyton (Swallowers-
Saccopharyngoidae). Skates (Rajidae) and drums (Sciaenidae) have molariform teeth in the oral
or pharyngeal cavity that help to eat snails, clams, oysters, crustaceans.
Razor-like incisors teeth are found in predatory fish, especially piranhas (Serrasalmus) and
warm-sea barracuda (Sphyraena). Some herbivorous fish (Parrot fish-Scaridae) have teeth in
their jaws and gills. The teeth of the jaw come together to break the food into pieces, and the
crushed teeth of the pharynx turn the pieces of food into fine particles.
Plankton eaters have no teeth in their mouths. However, some omnivores do not have teeth. The
number of bones is gradually increasing in predatory fish. Teeth are seen in the premaxillary
bone in most fish with teeth. Similar conditions are observed in Bofin (Amia), Gar (Lepisosteus),
Salmon and Trout (Salmoninae) and Perciformes. Fish with soft fins have pre-maxillary and
maxillary teeth. Spiny ray-finned fish do not have maxillary teeth.
Modification of gill rakers
The primary function of the gill rakers is to protect the gill filaments from injury. However, they
also help in eating fish. Carnivorous fish (Wallago attu, Chitala chitala (Notopterus chitala),
Channa marulias, C. striatus, Herpodon nehereus, Mystus seenghala) have usually long hard
teeth and they form rasping organs. However, in some fish such as Trichiurus, these are reduced
to form small granular structures.
Muraenesox does not contain any gill rakers. In some omnivorous species such as Tor tor,
Puntius sarana, P. ticto, etc., the gill rakers are short and thick. In herbivorous fishes such as
Rohu (Labeo rohita), Mrigel (Cirrhina cirrhosus), Catla (Catla catla), the gill raker forms a
wide sieve-like organ. The mouth cavity selects food particles by filtrating water into the
pharyngeal region. This condition is more developed in plankton-eating fish such as Tenualosa
ilisha, Gudusia chapra, Engraulis japonicum. The gill rakers of these fish are long and they
select zooplankton and phytoplankton as food in the mouth cavity region.
However, some fish, such as Boleopthalmus and Syngnathidae, are plankton-eating, despite the
presence of decaying gill rakers. In this way, the gill raker picks up the food particles and
arranges the food through the gill pouches so that the food does not go away from the mouth.
This filtration efficiency increases in omnivores from carnivores and is highest in herbivores.
Different fish vary in number and size of gill rakers. However, due to different ecological niches,
in the fish of the same species, the gill rakers are also different.
Types of gill rakers
Depending on the position, gill rakers can be divided into three categories, viz.
(1) Short gill raker: This type of gill raker is usually found in predatory fish;
(2) Medium sized gill raker: Such gill racker exists in omnivorous fish;
(3) Large and comb like gill raker: The fishes that take food by sieving, they have this kind of
gill raker.