Reproduction and Life-Cycle of the Perch

The egg-laying, or spawning, takes place in spring when the weather is warm (the end of April and early May). The Perch gather in great shoals in shallow quiet backwaters overgrown with weeds where the water is sufficiently warm.

The eggs laid by the female remain glued together in long strings which hang on the weeds. The males excrete jellied fluid containing sperms.

Once realized into the water, the sperms swim actively towards the eggs; the different cells fuse and fertilization takes place. The nuclei and protoplasm of the sperms and ova unite. Two cells merge into one giving rise to- a new living organism.

The fertilized egg of the Perch divides into two cells. The cells redivide - so that four are formed. This process continues until a multicelled embryo is formed. The embryo develops tissues and organs. In about 5 or 6 days it becomes a small (0,5 cm. long) larval fish, with a yolk sac on its belly - the remainder of the food reserved in the egg for its development. When the supply runs out, the larva feed on microscopically-small weeds, infusorians and tiny crustaceans (Daphnia and Cyclops), and grows into a young fish which is very much like the adult Perch.

Shallow backwaters are most suitable for spawning: the temperature of the water is higher; there are plenty of weeds on which the egg-strings hang; the plants provide a rich supply of oxygen. As a rule, there is no shortage of tiny living organisms for food for larval fish and small fry.

And yet these relatively favourable conditions hold hazards for the development of the young. Some of the eggs may be left underfertilized; many of the eggs are devoured by other animals. Finally, there is always the possibility that the water level will fall due to hot weather, or oxygen supply will prove insufficient.

The only way to escape extinction is the laying of tremendous number of eggs. One ovary which weighs about 200 gr., normally contains between 200,000 and 300,000 eggs. The number of sperms discharged by the male Perch is much greater.

On the other hand, some fishes lay very few eggs, for instance, the Stickleback (only 60 or 70). But these eggs are deposited in nests composed of grass or weeds and are protected by the male.

The behavior of the Perch at spawning time is very complex and may lead to the conclusion that it is even conscious. In fact, it is promoted by a chain of hereditary unconditional reflexes. Everything is performed instinctively.

VOCABULARY:

Spawning- нерест

Shoal- косяк

Backwaters- заводь

Fertilization- запліднення

Nucleus- ядро

Ovum (pi. ova) - яйце

Yolk sac- жовчний мішок

Crustacean- ракоподібний

Small fry- мальки, молодь

Ovary- яєчник

Stickleback- колюшка

Unconditional reflex- безумовний рефлекс

I. ANSWER THE QUESTIONS:

1.What is understood by fertilization? Describe the process using the Perch as an example.

2.What changes does a fertilized ovum undergo in its development?

3.What conditions are essential for the development of the ova and the young fish?

4.Why does the Perch lay so many eggs?

II. GIVE THE ENGLISH EQUIVALENTS TO THE FOLLOWING:

Самка,самець, виділяти рідину, зливатися, запліднення, клітина, зародок, бути приданим до нересту, запас кисню, вимирання.

III. COMPLETE THE SENTENCES:

1. The fish gather in great...

2. The female lays …

3. The males excrete …

4. Fertilization takes place like this …

IV. FILL IN THE GAPS WITH THE WORDS GIVEN BELOW:

1. The process of dividing the cells continues until … is formed.

2. It develops … and … .

3. … feeds on the remainders of the food in the egg for its development.

4. Shallow backwaters are most suitable for …

5. The only way to escape … is the laying of tremendous number of eggs.

*organs, extinction, a multicelled embryo, organs, tissues, spawning, a larval fish.

V. PUT THE SENTENCES INTO UKRAINIAN WITHOUT A DICTIONARY AND USE THEM TO RETELL THE TEXT:

1. The egg-laying or spawning takes place in spring.

2. The Perch gather in great shoals in quiet backwaters.

3. The fertilized egg of the Perch divides into two cells.

4. This process continues until a multicelled embryo is formed.

5. This embryo develops tissues and organs.

6. In 5-6 days it becomes a small larval fish and then grows into a young fish.

7. Shallow backwaters are most suitable for spawning.

Text 6