Fish life cycle - a set of all phases of development, covering all periods of fish life - from birth to death. Life cycle is composed of periods: embryonic, larval, fry, immature organism, adult, old age. Within a period, subperiods are distinguished, which, in turn, are divided into stages.
1. embryonic period - from the moment of fertilization of the egg to the transition of the young to external feeding. The embryo feeds on the yolk - the food supply received from the mother organism. This period is divided into two sub-periods:
a) the sub-period of the egg, or embryo, when development takes place within the shell;
b) the sub-period of the free embryo (pre-larva), when development takes place outside the shell.
2. the larval period begins from the moment of transition to feeding on external food; the external appearance and internal structure have not yet taken the form of an adult organism. Larvae have specific larval organs, which later disappear.
3. Fry period - the external appearance is close to that of an adult organism. Disappear larval organs, appear characteristic of adult organs and functions. Sexual organs are almost undeveloped. Energy resources are spent mainly on growth. Secondary sex characteristics are usually absent.
4. Period of semi-adult (immature) organism: more or less rapid development of sex glands and secondary sex characteristics begins, but the organism is not yet capable of reproduction.
5. Adult period (sexually mature organism - the state in which, at a certain period of the year, an organism is able to reproduce; secondary sex characteristics, if peculiar to the species, are present. Energy is spent primarily. Energy is spent mainly on development of the sexual system and creation of reserves to maintain vital activity during migrations, wintering, and reproduction.
6. Period of old age - sexual function fades; growth in length ceases or is extremely slowed down.
Fish life cycle
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