EXTERNAL FERTILIZATION: CHARACTERISTICS, ADVANTAGES AND DISADVANTAGES - BIOLOGY - 2025

The external fertilization is a type of fertilization that unlike internal takes place outside the body of the mother. Under the name of fertilization, the process by which two gametes, one female and the other male, merge during sexual reproduction with the aim of creating a new being with a genome derived from both parents is known.

Reproduction occurs when sperm and ovum combine in fertilization, allowing the reproductive cycle to begin. In the case of internal fertilization, the gametes unite within the mother's body as in the case of humans, mammals, birds, reptiles or insects.

However, external fertilization involves a whole different process, being a form of fertilization that generally occurs in humid environments where the sperm and egg are released into the water.

External fertilization is common in fish and amphibians, as well as in some marine vertebrates. The objective of carrying out this type of reproduction in water is to ensure that the eggs do not dry out due to the lack of a humid maternal environment.

The process begins when the female throws her eggs into the water and the male sheds his sperm on them in a phenomenon known as spawning.

How does external fertilization occur?

Fertilization takes place outside the body of the female, who begins the reproductive process by depositing her eggs in an underwater nest or simply abandoning them in ocean currents.

Subsequently, the male pours his sperm on them and fertilization takes place. It may well be that the male knows the eggs he wants to fertilize, so he pours his sperm into them, or he simply throws them into the water, waiting for the sperm to travel to reach an egg to fertilize.

In the case of fish, fertilization often occurs through a prior approach, however, fertilization usually occurs spontaneously in the water.

In the case of amphibians, they usually make the dorsal hug, that is, they mate on land and then they must deposit the eggs in the water so that later the sperm is released on them to achieve fertilization and reproduction.

It should be noted that the fertilization of amphibians occurs in fresh water since this type of water protects the eggs during development.

Both in the case of fish and amphibians, the female's eggs have a rather thin thin covering that allows the sperm to penetrate easily.

Reproduction usually occurs at certain intervals of time, being influenced by factors such as temperature, light, climate and food.

External fertilization is usually also typical in species such as crustaceans, echinoderms and mollusks. An example of this type of reproduction is given in salmon:

Characteristics of external fertilization

Gamete production

First, the male and female are required to produce a large number of gametes. In the case of the male, she must produce large amounts of sperm to ensure that as many sperm as possible travel in the water until she finds an egg to fertilize.

In the case of the female, it is vital that she lays dozens of hundreds of eggs to ensure reproductive success.

Although generating a large number of gametes requires more energy, doing so is crucial for this type of fertilization.

Suitable environment

For fertilization to take place requires an aquatic environment. Although fish and some aquatic invertebrates live in the water as their natural environment, in the case of amphibians it is necessary for them to return to the water to deposit the fertilized eggs.

Water is the main requirement that is needed to carry out this fertilization, since its currents allow the sperm to travel more easily, and also protects the eggs during their development by providing a warm, humid and suitable environment similar to the belly maternal.

Fertilization process

The main thing is that the female pours her eggs into the water and the sperm are released on them. A certain degree of proximity between the egg and the sperm is essential for fertilization to be successful.

In this type of fertilization, the animals disperse their gametes in the water, and precisely this dispersion makes it difficult for the sperm and the egg to meet, causing many gametes to die before being fertilized.

Therefore, for external fertilization to take place it is necessary for the nest to be near the area where the sperm was released.

Advantages and disadvantages

External fertilization has some advantages, among them is the fact that if the reproductive process is successful, many eggs are fertilized.

In turn, since it occurs outside the mother's body, fertilization is usually easier and with no complications that put the female's life at risk.

Likewise, the fertilization process is usually simple after no hormonal changes, mating cycles or rituals intervene in the process, with an egg and a sperm being able to spontaneously fertilize under water.

As disadvantages, this type of fertilization has two main disadvantages:

The first of these is that as sperm and eggs are released freely into the water, they run the risk of never meeting.

The second disadvantage has to do with the marine environment and its predators, always ready to eat the eggs shed by the female.

Hence, it is necessary for it to hide them well in a nest, since otherwise they usually die before being fertilized.

It is a more insecure type of fertilization since the mother's womb is not a lair for future offspring, which reduces the chance that they will reach adulthood.

References

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