Environment

External fertilization: what is it and animals

Just as there is a great variety of ecosystems, there is also a great variety of organisms. These have adapted to different types of habitat to achieve optimal development. Within these adaptations is the type of fertilization for the species to reproduce and continue to survive. You can find species with internal fertilization and species with external fertilization.

On this occasion we will explain what external fertilization is , which animals have this type of fertilization and what advantages it provides to these species. Continue reading this article to find out more.

Interesting: Are mammalian animals oviparous or viparous

What is external fertilization and its characteristics

External fertilization is the process where the male sperm fertilizes or fertilizes a female egg on the outside . Thus, this occurs outside the female’s body, unlike internal fertilization. The process of external fertilization can also be called spawning or spawning . These eggs have a layer that protects from the outside, but that can be easily penetrated by the sperm.

This type of fertilization depends largely on the environment. To facilitate the process, it must be carried out in water so that there is a means of transport for the sperm. The presence of water also prevents the gametes from drying out. There are animals that do not move and that depend on water currents so that gametes can meet.

The eggs can be laid in a nest, among algae, on the substrate or on rocks . These sites are called spawning sites and are chosen because they provide the ideal environment for external fertilization in terms of oxygenation, shelter, temperature, depth and the possibility of thriving. On the other hand, the gametes can simply be poured onto the water. The choice of site and strategy depends on the species and the organism.

How external fertilization occurs

External fertilization works with a male and a female organism , both of the same species . The process is the following:

  1. The female lays her eggs in water , in her outside environment.
  2. The male releases the sperm into the water so that the eggs can be fertilized. Normally there is synchronization so that the release of gametes occurs at the same time, but it can happen that the sperm are released earlier or later, but that is a clear disadvantage.
  3. Once the sperm enters the ovules, fertilization occurs . The result of external fertilization is a new organism that is the son of both parents.
Examples of animals with external fertilization

External fertilization examples: To better understand external fertilization, it is convenient to indicate some examples of animal species with external fertilization, although we can already imagine that they are animals that live always or seasonally in the water.

Corals

Corals are aquatic animals without movement . With the information we have given you so far, you can already imagine what its fertilization is like. These organisms are made up of polyps that can be male or female, in the same hermaphroditic coral . Each one releases its corresponding gametes and with the help of the marine currents they manage to meet. When fertilized, they produce a so-called planula larva that will spend a few days free and then settle in the substrate to grow as sessile coral. Here you can meet the hermaphrodite animals .

In addition to having external fertilization, corals are important spawning sites for tropical fish , as they are in shallow, warm waters with nooks and crannies so that the fry can develop quietly. For this reason, coral reefs have an incomparable wealth of biodiversity.

Frogs and toads

External fertilization examples: Frogs have a courtship process, unlike most animals with external fertilization where gametes are released more easily. Once the female accepts the male , she lays her eggs on top of the water while the male hugs her. This act is known as an amplex embrace . As the eggs hatch from the female, they are fertilized by the male . This is a case where there must be acceptance of the female for fertilization to occur.

Salmon

In salmon, the external fertilization process is particular because they swim upstream to spawn . They return from the ocean to the rivers because here there are rocky surfaces on which it is more certain that external fertilization can occur successfully. In this case, the acceptance of the female is not necessary, only the sperm and the ovules are released and these are fertilized by the closest sperm.

Advantages and disadvantages of external fertilization

To finish talking about external fertilization, we will comment on its advantages and disadvantages for these species, although, obviously, for these the advantages outweigh them and, therefore, evolution has led them to this type of fertilization.

Advantages

  • In order to increase the probability that both gametes will unite, the males release a number of sperm and the females a similar number of eggs. This strategy considers that some gametes may be lost by being released into open water.
  • This type of fertilization avoids the wear and physical load that internal fertilization implies.
  • It allows the safest possible development of the eggs and offspring, taking into account the environment in which they live.
  • It also avoids the need to go through courtship processes, except in cases such as some amphibians.

Disadvantages of external fertilization

  • There is a risk that the gametes, both female and male, are ingested by other animals. Many aquatic animals are filter feeders and may eat them, thus gametes may be lost.
  • The gametes are exposed to strong sea currents, which can be counterproductive and affect fertilization.
  • The production of multiple gametes is a high energy expenditure.
  • Many ovules can remain unfertilized because they are thrown out, which is a waste of energy and gametes.
  • There is no parental care, leaving children vulnerable to external dangers.
  • Not all hatchlings survive.

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