External Fertilization

What is External Fertilization?

Fertilization that occurs outside the body of an organism is known as external fertilization. It usually requires a water body for successful fertilization.

The sperms and eggs are released in an external environment.

External fertilization usually occurs during spawning when the female releases the egg and the male releases the sperm in the same environment.

External fertilization in an aquatic environment protects the desiccation of eggs.

Broadcast spawning leads to higher genetic diversity due to larger mixing of genes within a group. The chances of survival of the species also increase.

For organisms like sponges, broadcast spawning is the only process for fertilization. Millions of eggs are produced by individuals because many are lost by predation.

Features of External Fertilization

  • The success rate of fertilization is very low.

  • Unlike internal fertilization, a large number of gametes need to be produced by the male and female to ensure reproductive success.

  • A water body is required to initiate external fertilization. The sperms would die on land.

  • It is a reproductive disadvantage for most of the animals because most of the gametes die without being fertilized.

  • External fertilization is, however, a simple reproductive strategy which does not require the involvement of any hormones or mating rituals.

Advantages of External Fertilization

  • It results in increased genetic variations.

  • It produces a larger number of offsprings.

  • The gametes released can drift and therefore it is easy to find mates.

Disadvantages of External Fertilization

  • A large number of gametes are left unfertilized and wasted.

  • Predators and other environmental hazards reduce the chances of fertilization.

  • The sperm might not necessarily come in contact with the eggs.

  • The gametes or the zygote dessicate.

Examples of External Fertilization

The examples of external fertilization are given below:

Sea Urchins

They use chemotaxis to attract the sperms towards the eggs. Their spawning is synchronized to prevent the eggs and sperms from diluting or drifting away.

Frogs

The female releases the eggs into the water. The male also releases the sperms in the water to fertilize them. The larval life of the frogs is in water whereas the adult life is on land.

Salmon

They have an opening right in front of their anal fin through which the gametes are released and then fertilized.

Coy Coral

A large number of gametes are released in water during spawning of corals. After fertilization, the coral larvae move up to the surface for maturation.

Starfish

They gather in groups and use chemical signals to indicate the other members of the group that they are ready to spawn. This synchronized spawning increases the chances of fertilization success.

External Fertilization in Humans

External fertilization also takes place in humans but it is not natural. A few females have blocked oviducts. The sperms cannot reach the eggs and the female cannot conceive. In this situation, the doctors collect freshly released eggs and sperms and keep them together for several hours for in-vitro fertilization. When the fertilization occurs, the zygote is transferred to the uterus of the female after about a week, and the development process occurs inside the female’s body.

All the fish and amphibians fertilize externally. However, it is not as successful as the internal fertilization and the probability of survival is reduced.

For more information on External Fertilization and related topics keep visiting BYJU’S Biology website or download the BYJU’S app for further reference.

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