Life process of bony fish (Osteichthyes “Tilapia”), SHS elective biology.

Tilapia is a common freshwater bony fish found in lakes and rivers and brackish (slightly salty) water lagoons. Tilapia is poikilothermic (cold-blooded), that is the body temperature varies with the temperature of its surrounding.

Scientific classification.

Phylum Chordata

Reason;

• They possess pharyngeal silts.
• A hollow nerve cord is present at the dorsal side.
• They also possess notochord.
• They are bilaterally symmetrical.
• Presence of well-developed sense organs.
• They also have a post-anal tail.

Class Osteichthyes.

Reasons;

• They have cycloid scales.
• Possess homocercal tail.
• They also have an operculum.
• Presence of bony fins on the body.
• Presence of terminal mouth for feeding.
• Their skeleton is made of bones.

Other examples in the same class Osteichthyes are;

• Mudfish
• Herring
• Catfish
• Tuna
• Salmon

The external structure of a tilapia.

• The body is streamlined, with a rounded head and trunk, which tapers towards the tail.
• The skin is covered with thin, overlapping, circular, bony scales (cycloid scale).
• The ventral surface of the body is often lighter in color than the dorsal surface.
• The mouth is terminal with a pair of nostrils above.
• The nostril is used for smelling only, to detect the presence of food and other chemicals.
• Behind the nostrils are two lidless eyes for vision.
• The head joins the trunk of the body directly; there is no neck.
• Has opercula at the sides of the head.
• On each side of the body is a lateral line.
• The lateral line marks the position of tiny pores in the scales leading to receptors sensitive to vibration and pressure in the water.
• It enables the fish to escape danger.

• There is a dorsal fin along the dorsal midline.
• An anus is located at the ventral side, behind which is the anal fin.
• The pectoral and pelvic fins are just behind the operculum.
• The tail bears a large tail fin (caudal fin).

General adaptive features of tilapia.

• Presence of fins for balancing, steering, and swimming.
• Streamlined body for easy movement in water.
• Light ventral and dark dorsal colors for countershading to escape danger.
• Presence of lateral line to detect pressure and vibrations in water.
• Their eyes are large and laterally positioned to give wide vision.
• They have nostrils for smelling to escape danger.
• They also have a terminal wide mouth for feeding.
• Presence of gills for gaseous exchange.
• Presence of backwardly overlapping scales for easy forward movement and protection.

Nutrition in tilapia.

Tilapia feeds on minute aquatic protoctists, plants, and animals, including plankton, mosquito larvae, and water weeds.

• Water containing food is taken into the mouth.
• The water passes into the buccal cavity and then through the gill slits, into the gill chamber, and to the outside.
• As it flows through, slender projections called the gill rakers prevent the food particles from escaping.
• The food then moves into the pharynx behind the buccal cavity and is swallowed.
• This is called filter feeding.

Adaptation of bony fish to its mode of nutrition

• Wide mouth for trapping food.
• Numerous small teeth for holding or nibbling food
• Nostrils for smelling food
• Wide eyes give a wide vision of the location of food.
• Gill rakers trap or filters food particles.
• Also, the presence of operculum.

How tilapia move.

Tilapia move 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 muscle blocks on both sides of the flexible vertebral column. They are especially well developed in the tail region.
• As the muscles on the left side contract, those on the right side relax, and the tail bends towards the left.
• By alternate contractions and relaxations of the muscles, the tail bends left and right.
• In bending, the tail pushes against the water and drives the fish forwards.
• The tail or caudal fin with its large surface area increases the push and the resulting forward movement.
• 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.
• When extended they slow down or stop the fish.
• A change of direction is brought about by stretching out only one of the paired pectoral and pelvic fins on one side.
• This action slows water movement on that side, causing the fish to turn towards that direction.
• In slow movement, all the fins except the tail fin are used in balancing.
• In very slow movements, the paired fins may sometimes be used as paddles.
• The dorsal and anal fins are concerned with the control of yawing and rolling.
• Yawing refers to the lateral or side-to-side movement of fish.
• Rolling is the rotational movement of the body around its longitudinal axis.
• Pitching (the tendency of the head of the fish to rise and fall) is controlled by the pectoral and pelvic fins

Note:

Tilapia, like other bony fish, has an air bladder or swim bladder, which enables it to move up and down in the water column without swimming.

Gases (air) diffuse from the blood and inflate the bladder.

Thus, the fish becomes less dense and more buoyant and can rise easily.

When the bladder is deflated the fish becomes denser and sinks.

Adaptive features for movement

• Caudal or tail fin for population.
• Paired fins for steering, braking, and controlling pitching.
• The dorsal and anal fins for balancing.
• Streamlined body shape to reduce the resistance of water current for easy and fast movement.
• Cycloid and backward overlapping scales for less resistance in the water.

Gaseous exchange

• The gills are the organs of gaseous exchange, allowing the fish to respire.
• Each gill chamber contains four gills.
• Each gill is made up of two rows of red, soft, slender structures called gill filaments.
• Each row of filaments is attached to a bony bar called the gill bar or gill arch.
• The gill filaments are well adapted for gaseous exchange in the following ways;
• Their outer covering is only one cell thick.
• They have a large surface area which is always moist.
• They are well supplied with blood.

The process:

There is a continuous flow of water into the mouth, through the buccal cavity, over the gills, and out of

the fish through the opercular opening.

For water to enter the mouth;

• The fish opens its mouth
• The floor of the buccal cavity and the pharynx are lowered and
• The opercula are closed.

After the water enter the mouth;

• The mouth is then closed,
• The floor of the buccal cavity and pharynx are raised and
• The opercula opens as water is forced over the gills.

As the water flows past the gill filaments;

• Oxygen diffuses into the blood and carbon(lV)oxide diffuses out.

Function of the gills.

• For gaseous exchange
• For feeding
• For osmoregulation

Reproduction;

In most bony fish, the females lay their eggs and the males shed their sperm into the water.

The sperm swim to the eggs and fertilize them (external fertilization).

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

• Certain species of Tilapia exhibit some degree of parental care (mouth-brooders).
• During the breeding season, the male and female dig a shallow pit in the bed of the pond or lake.
• The female lays eggs and the male sheds sperm over them.
• The male then carries the fertilized eggs in his mouth for about two weeks until they hatch.
• During this period, the male cannot eat.
• After hatching, the young fish (fries) swim around near the mouth of the parent.
• They quickly retreat into the parent’s mouth when there is danger.
• When they are sufficiently grown, they leave the parent and become independent.
• As a result of this parental care, many of the young fish survive.
• Fewer eggs (about 1000) are laid than in fish that do not care for their young.

Excretion;

• Nitrogenous waste is mainly excreted as ammonia in bony fish.
• Other excretory substances are excess salts and water.
• These are removed by the kidney in urine.
• Carbon (IV)oxide is excreted by diffusion out of the body through the gills.

Adaptive features to escape danger.

• Lateral line to detect vibration in water.
• Large eyes and laterally positioned for wide vision.
• Fins to swim away from predators.
• Also, the presence of nostrils to smell danger.

Economic importance of Tilapia

1. They serve as a ready source of protein for humans.
2. Tilapia are a key species in many foods chain and food webs.
3. Fish farm is a source of income.
4. Tilapia introduce into ponds to control mosquito larvae population.

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