Tilapia

Habitat: Tilapia is a freshwater teleost or bony fish that lives in ponds, rivers, streams, lakes and lagoons.

Structure: The body is streamlined and laterally flattened. This enables the fish to move or cut through the water easily or smoothly with little or no resistance. The body consists of head, trunk and tail. There is no neck.

The head is pointed and extends from the tip of the body (i.e. the mouth) to the edges of the opercula (singular: operculum) or gill covers.

The apercula are bony plates which form the sides of the head and protect the gills of the fish. The mouth in terminal and wide gaped This enables the fish to swallow its food or prey whole. The mouth bears small. sharp, homodont teeth on the upper and lower laws. The teeth are used to hold onto the prey to prevent it from escaping from the mouth. The head bears one large, round and bulging simple eye on either side. The eyes lack eyelids but are protected by a layer of transparent membrane. The head also bears a pair of small nostrils which are used for smelling or detecting the presence of chemical substances. The nostrils are above the mouth and anterior to the eyes. They are not used for breathing.

The trunk extends from the edges of the opercula to the tall or caudal peduncle. The tail terminates with the extended, fan-shaped or homocereal tail fin or caudal fin. The trunk and tall are completely covered with overlapping and backward pointing round, bony-plates called scales. Under the microscope, the Scales are seen to have concentric circles, which give an indication about the fluctuating feeding habit of the fish. The concentric circles can also be used to determine the age of the fish.

On either side of the body are two lateral lines, which extend from the edge of the gill covers or opercula to the caudal peduncle. The lateral lines are sensitive to pressure variations or changes, vibrations and movements in water.

Projecting from the body are extensions of the skin called fins. The fins are supported by bony rods called fin rays. There are two types of fins: median or unpaired fins and pair fins. Located on the dorsal surface and on the median line is the dorsal fin, which is used for balancing. At the end of the tail is the tail fin or caudal fin. This beats from side to side to propel the fish very fast forward. 0n the ventral surface and posterior to the anal opening is the anal or ventral fin. which is also used tor balancing. The paired fins are pectoral and pelvic has The pectoral fins are on either Sides at the trunk just behind the edges of the opercula 0n ventral surface and below the mm! llns are the pelvic fins. Both of the paired lins ale used for steering. braking and slow nmvements. when the tail fin is not in use.

lateral view of Tilapia

LIFE PROCESSES 0F TILAPIA

Movement: tilapia moves in water by swimming. Fast smmming is brought about by the side to side beating of the tail or caudal fin. The beating of the tail fin propels the fish very fast toward The heating of the tail fin is brought about by the antagonistic muscle blocks on either sides of the vertebral column in the caudal peduncle. Contraction of the left muscle block causes the tail or caudal fin to bend towards the left. When the right muscle block contracts the tail fin bends towards the right.



The pectoral and pelvic fins are used for slow, forward and backward swimming movements when the tail fin is not in use or is at rest. They are also used for steering and balancing in water. Braking is effected when the paired fins are held at right angles to the body surface. The dorsal and anal tins prevent the body from wobbling or rolling over sideways. They therefore, keep the fish stabilized. Pitching is also controlled by the paired tins. Pitching is the tendency to rise and fall or upward and downward movements in water.

The streamlined body shape and the backward pointing and overlapping scales enable the fish to move through the water smoothly or easily with little or no resistance.

Water moving backwardly out of the body through the opercula openings propels the fish forward.

Teleosts or bony fishes, including Tilapia, have a long. narrow air sac called air bladder or swim bladder, situated just below the vertebral column in the body cavity. The swim bladder provides buoyancy to the iish and enables it to alter the depth at which it swims. The swim bladder _is inflated when gases diffuse from the blood into it. This makes the fish less dense and more buoyant, enabling the lish to rise to the surface. When the gases diffuse out of the swim bladder, it is deflated and the fish becomes denser and less buoyant. enabling the fish to sink. Therefore, by altering the volume of the swim bladder, the depth at which the fish swims in the water is also altered.


Nutrition: Tilapia feeds on planktons that live near the

water surface, such as mosquito larvae and pupae, cyclops and waterweeds. It also feeds on shrimps. TiIapia is a filter-feeder. It has a terminal wide-gaped mouth, which enables the fish to swallow its food whole. The small, sharp homodont teeth prevent the food or prey from escaping. Food particles contained in Water are filtered by the gill rakers as the water passes over the gill and out of the body through the opercular openings. The gill rakers are used in rasping the food and directing it into the oesophagus or gulleth The oesophageal sphincters, at the entrance of the stomach, prevent water from entering the stomach.

The cardiac sphincter also allows food to enter the stomach, where protein food is converted into peptones by the enzyme pepsin, secreted by the stomach. Pyloric sphincters or pylorus allows the partially digested food from the stomach to enter the duodenum. The surface area of the duodenum is increased by the presence of pyloric'caecae. ‘ Digestion is completed by enzymes secreted by the pancreas in pancreatic iuice. Lipase converts fats and oils into fatty acids and glycerol. Trypsin convert proteins into amino acids and amylases convert carbohydrates into glucose and fructose.

The end products of food digestion are absorbed in the ileum. Undigested food materials or faeces, stored in the rectum are egested by the anus.

Respiration:
The organs for gaseous exchange in the Tilapia are the gills. Each gill chamber on either side . of the head contains four overlapping gills.

Each gill consists of a gill arch or gill bar, which is slightly curved and bony. Attached to the gill arch are two rows of numerous fine gill filaments. On the inner edge of the gill arch are rows of flattened comb-like gill rakers. The gill filaments are thin-walled and highly vascularised, i.e. contains a large number of blood capillaries.


The gills are covered by the opercula or gill covers. There is a continuous flow of water through the buccal cavity and over the gills. The water passes out of the body through the opercular openings or gill slits. The mouth and opercula openings open and close alternately, to allow the ~water to flow continuously.

Oxygen dissolved in water diffuses into the blood in the gill filaments. The oxygen is used to oxidize absorbed food to release energy. Carbon (IV) oxide evolved, diffuses out of the blood through gill filaments into the water.


Excretion:
Tilapia possesses two brown, elongated kidneys, located above the swim bladder. The kidneys open separately behind the anus. The main nitrogenous‘waste substance is ammonia. Other waste substances, such as water and salts, together with ammonia are removed by the kidneys as urine. The nitrogenous waste substance in the lagoon species is trimethylamine oxide.

Carbon (IV) oxide, evolved during respiration, is removed by diffusion into the water through the walls of the gill filaments.



Reproduction:
During the breeding season, the matured male and female Tilapia separate themselves to the pawning grounds, which are usually part of the habitat where the water current is very slow. Laying of eggs by the female is preceded by a successful courtship display, which involves the male and female butting each other. When the female lays its eggs, male follows it and immediately sheds sperms over the eggs in the water. Fertilization is therefore external.

In some species of Tilapia, both parents guard and protect the developing eggs in the nest. In other species, either the male or female keeps the fertilized egg in its mouth, where they develop and hatch. Which parent incubates the eggs depends upon the species.

The eggs hatch into tiny young fish called fry. The fry swim near the mouth of the parent and quickly return into the mouth' of the parent when an enemy approaches. Tilapia therefore shows some sort of parental care.



ADAPTATION OF TILAPIA TO AQUATIC LIFE.

ADAPTATION OF Tilapia FOR MOVEMENT.
Tilapia possesses a streamlined body shape and backward pointing and overlapping scales which enable the fish to move easily or smoothly through the water with reduced resistance.

The paired fins, pectoral and pelvic fins, are used for steering, braking and slow movements in water. The paired fins are also used to control pitching. The dorsal and anal fins are used for balancing or stabilizing and thus prevent wobbling or rolling over side ways. The tail or caudal‘fin beats to propel the fish forcefully forward. The tail fin is also used for steering in water.

The swim bladder enables the fish to alter the depth at which it swims, while the lateral lines enable the fish to detect pressure variations, vibrations and movements in water.




ADAPTATION OF Tilapia FOR PROTECTION.

The dark dorsal surface of Tilapia blends with the dark bottom of its habitat, while the white or silvery ventral surface blends with the light entering into the habitat from above. The dark dorsal surface and white, pale or silvery ventral surface make detection of Tilapia in its habitat difficult, when viewed from above or below respectively, by predators or enemies. The colour is therefore used for camouflaging.

The two eyes, lateral lines and the pair of nostrils enable Tilapia to detect the approach or presence of its enemies.

The bony plates of backward pointing and overlapping scales prevent mechanical injuries as the fish moves in water.



ADAPTATION OF Tilapia FOR GASEOUS EXCHANGE
Tilapia possesses gills with numerous gill filaments, which provide a large surface area, for efficient gaseous exchange. The gill filaments are thin-walled and highly vascularised or supplied with numerous blood capillaries for efficient absorption and transportation of respiratory gases.

The presence of mouth and opercular openings or gill slits which open and close alternately, ensure a continuous flow of water, containing dissolved oxygen, over the gills for efficient gaseous exchange in water.



ADAPTATION OF Tilapia FOR FEEDING The presence of a terminal, wide-gapped mouth ensures that food or prey is swallowed whole. The small, sharp and similar or homodont teeth in the mouth, hold onto the food or prey and prevent it from escaping. The gill rakers filter food particles, contained in water flowing through the buccal cavity and direct them into the oesophagus.



ECOLOGICAL SIGNIFICANCE OF Tilapia
Tilapia plays an important role in the aquatic eco-system. It is an integral part of food chains in freshwater bodies, controlling the growth and population of zoo-planktons and phyto_ planktons in its habitat. By feeding on .. mosquito larvae and pupae, it helps to control diseases like Malarla, Yellow Fever, Dengue, Fever, Encephalitis and Filariasis or Elephantiasis, The pathogens that cause all the above diseases are transmitted by the adult mosquito.

TiIapia serves as a major source of protein to humans. Tilapia also serves as food for other larger fishes, such as cat, fish in its habitat.

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