Structure and life Processes of Fish (Tilapia)

 Objectives       

This  blog post provides readers with the following objectives. The reader will be able to:

o   Describe the external features of Tilapia 

o   Discuss the adaptation of Tilapia to its habitat.

FISH

There are two main groups of fish, bony fish (Osteichthyes) and cartilaginous fish (Chondrichthyes). The skeletons of Osteichthyes are made of bone e.g., Tilapia, goldfish, tuna, while the Chondrichthyes have cartilaginous skeletons e.g., sharks, rays and dogfish. Fishes are poikilothermic or ectothermic (cold-blooded) meaning; their body temperature depends on the temperature of their environment.

Tilapia sp.

Phylum: Chordata
Sub-phylum: Vertebrata

Class: Osteichthyes

Habitat of Tilapia

Habitat: Tilapia is freshwater bony fish found in lakes, ponds, rivers and lagoons.

Structure of a Tilapia

The body has a rounded head and a trunk which tapers to the tail. This gives the fish a streamlined shape which enables it to move smoothly through the water.

The skin is covered with flattened, discoid overlapping scales.

It has wide terminal mouth and small sharp teeth. The pair of nostrils above the mouth helps the fish to smell food. It plays no part in breathing.

It has operculum (or gill cover) at each side of the head which covers the gills. It plays an important part in the breathing mechanism.

The lateral line is a clear line or tube that runs along each side of the body, behind the head to the tail. It is found just beneath the skin. It is filled with a jelly like liquid and has sensory nerve endings. It detects vibrations in the water or pressure changes enabling the fish to escape from enemies.

It bears fins at various points on the body. 

It has in its body cavity a long air filled bladder (swim bladder) just beneath the backbone. The swim bladder makes a fish buoyant. It enables the fish to move up and down in the water without swimming. Gases diffuse from the blood into the bladder to inflate it, rendering the fish less dense and when deflated the fish becomes more dense.

Types of fish fins

a. Unpaired fins or median fins: are the dorsal fin, the ventral fin or anal fin and the caudal or tail fin. They control yawing and rolling movement.

b. Paired fins: are the pectoral fins and the pelvic fins. They are used for steering, balancing, braking and diving. They are also used to control pitching.

Nutrition in fish

Tilapia feeds on minute aquatic plants e.g., planktons and animals e.g., mosquito larvae and aquatic protoctist. 

Water containing food is taken into the mouth. As it flows through the slender projections called the gill rakers, it prevents the food particle from escaping.

The food then moves into the pharynx behind the buccal cavity and it is swallowed. This is filter feeding.

Excretion in fish

Ammonia is the nitrogenous waste substance excreted. Other excretory substances are excess salts and water. These are excreted by the kidney in urine. Carbon dioxide is excreted by diffusion out of the body through the gills.

Respiration in fish

Structure of the Gill

The gills are the organs of gaseous exchange. Each gill chamber contains four gills. Each gill is made up of two rows of red, soft and slender structures called gill filaments. Each row of the filament is attached to a bony bar called the gill bar or gill arch.

Adaptation of Filaments for Gaseous Exchange

o They have a large surface area.

o   They are well supplied with blood. 

o   They are thin walled to enable rapid diffuse into the blood.

Gaseous Exchange in fish 

During gaseous exchange, the mouth opens to take in water containing dissolved oxygen, whiles the operculum is pressed to the head.  As water enters the gill chamber, the mouth closes and floor of the mouth is raised, building up pressure in the mouth. The pressure forces the water to flow over the oil filaments, oxygen in the water diffuses into the blood capillaries of the filaments. At the same time, carbon dioxide in the capillaries diffuses into the water. Oxygen in the blood stream is carried to various parts of the body for tissue respiration.

Movement in Fish

Tilapia moves in water by swimming. Rapid forward movement is brought about by the side-to-side lashing of the tail. This is controlled by the contraction of the muscle blocks on both sides of the flexible vertebral column. They are well developed especially in the tail region. The caudal fin also works with the paired fins in steering the fish through the water. During rapid movement, the paired fins are held close to the body and when extended, they slow down or stop the fish.

o The unpaired fins control yawing (the tendency of the body to zigzag from side-to-side. They also control rolling (rotation of the body around its longitudinal axis).

o The paired fins control pitching (tendency of the head to rise and fall).

The air or swim bladder enables up and down movement in the water without swimming. Gases diffuse from the blood into the bladder to inflate it, rendering the fish less dense and can rise easily. When deflated the fish becomes denser and sinks.

These images are credited to D G Mackean www.biology-resources.com

Reproduction

Usually, the females lay eggs and the male shed the sperms into the water. The sperms swim to the eggs and fertilize them (external fertilization).

The fertilized eggs are abandoned by the parents and most are eaten by predators.

Certain species of Tilapia exhibit some degree of parental care and therefore called mouth brooders.

The male and female dig a shallow pit in the bed of the pond or lake. The female lays eggs and the male sheds sperms over them. 

The male then carries the fertilized eggs in his mouth for about two weeks until they hatch into tiny fish called fry. The fry retreat into the parent mouth when there is danger.

Adaptation of Fish to Its Environment

¨    The streamlined body shape enables it to move with ease.

¨     The overlapping scales protect the fish from mechanical injuries.

¨     The slimy body surface together with the backwardly pointing scales, allows easy movement in water.

¨      It possesses large eyes detecting predators, prey and mating partners.

¨     Lateral line for detecting vibration in water and changes in temperature.

¨      It possesses a nostril for smelling food.

¨      Presence of fins for locomotion. 

¨   Presence of gill rakers to trap food particles.

¨   Presence of gills which aids in respiration.

¨    The swim bladder maintains its buoyancy and enables it to move up and down without swimming.

¨  The ability to camouflage its predators; the dorsal surface is darker to match the color of water from above whiles the ventral surface is lighter to match the color of water from below. This makes detection from above and below difficult.

 

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