Animals with external fertilization

Animals with external fertilization

Reproduction is a fundamental process in all the species that inhabit the planet since this process is what guarantees its permanence. The reproductive event is not general in the animal world, on the contrary, within each group, some variants have developed depending on the characteristics of the species and the environment where it lives because the latter plays a fundamental role.

Within reproduction, we find fertilization, which is divided into two types, internal and external, each with particular aspects and raised in different groups of animals. In this Bestpetslover article, we want to present you with information about animals with external fertilization, so we invite you to continue reading about this interesting topic.

What is external fertilization?

Fertilization is the union between the female and male gamete, also known as ovules and sperm, from which the zygote and later the embryo will originate. This fusion process between both cells can occur inside the female’s body or outside of it and, depending on it, is called internal or external fertilization. 

Thus, external fertilization is the process of the union of the gametes outside the body of the female, so that it occurs in the environment where both animals are. The foregoing indicates that the conditions of the place where external fertilization takes place must be appropriate because otherwise the process would be limited or prevented.

About this type of fertilization we can allude to oviparous animals, that is, those that produce eggs, which develop outside the mother’s body. Within the species that have this type of reproduction, there are some with internal fertilization, as is the case of birds, but there are also oviparous with external fertilization, such as certain fish, among others. Now, the

characteristics of the egg vary depending on the type of fertilization of the animal :

  • Oviparous species with internal fertilization have hard eggs, with covers or shells resistant to desiccation, so they can remain out of the water.
  • Oviparous animals with external fertilization produce eggs without this protection, having thinner membranes, so they generally require an aquatic environment or a humid environment for their development.

This difference in the cover between one and the other also suggests that, in the case of the former, since the egg comes out already fertilized, it is ready for its development, while in the latter the fusion between the gametes must still occur, requiring a smaller cover that allows the fertilization process.

Vertebrates with external fertilization

Although internal fertilization is common in many vertebrates, there are also various species of vertebrates with external fertilization, which can be found among certain species of fish and amphibians. Next, we know examples of vertebrates with external fertilization:

Fish with external fertilization

Fish are amphibian creatures that occupy both freshwater and saltwater conditions. They are a diverse group, not only from a taxonomic point of view but also in terms of the characteristics they present. Regarding fertilization, several species carry out the process externally, however, it is quite a feat since several factors play against it.

On the one hand, the aquatic environment itself can act as a dispersant for both the egg and the sperm. On the other hand, the gametes are very short-lived, so fertilization must occur as soon as possible, and the tiny sperm has a limited range to reach the female gamete.

Additionally, we must mention that various species feed on the eggs released by the females, but, despite everything mentioned, the life of this group of animals continues on its way and they manage to reproduce effectively.

To solve the above, aquatic species with external fertilization are provided with something known as a chemotactic factor, which consists of a chemical attraction released by the ovules to attract male gametes. These compounds are specific to each species. It is also common for spawning to be massive to try to increase the chances that some will manage to develop.

Among the fish that have external fertilization we can mention:

  • Perca fluviatilis )
  • Salmo salar )
  • Gadus morhua )
  • Salvelinus fontinalis )
  • Pink salmon ( Oncorhynchus gorbuscha )

Amphibians with external fertilization

Within amphibians, there are several examples of animals with external fertilization, although it is not an absolute rule because there are also species of this type that carry out the other type of fertilization.

Among the amphibian species with external fertilization we find:

  1. Bufo bufo )
  2. Siren intermedia )
  3. Rana temporaria )
  4. Fischer’s mauled lizard ( Onychodactylus fischeri )

Invertebrates with external fertilization

In invertebrates, we also find various groups of animals with external fertilization it is quite common in this type of animal that inhabits aquatic environments. Although there may be certain particular aspects in each group, since some have sessile life and others do not, the process, in general, is similar: the animals must release their gametes into the water so that they merge and fertilization occurs, then a series will occur. of transformations to originating the embryonic form.

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Invertebrate species with external fertilization are also subject to environmental influences, which can limit or prevent the process from occurring, but in many cases, they also manage to overcome them and successfully reproduce. Instances of spineless creatures with outer treatment are:


It is common for species of aquatic mollusks to have external fertilization and some of the most representative examples are:

Clam ( Mercenaria mercenaria )

Pacific oyster ( Magallana gigas )

Common tusk shell ( Antalis Vulgaris )


On account of echinoderms that are prepared external the female’s body, we can specify the accompanying models:

Common starfish ( Asterias Rubens )

Fire urchin ( Astropyga radiata )

Donkey manure ( Holothuria Mexicana )


Inside the gathering of marine arthropods, we track down specific instances of this kind of preparation, among which we feature:

  1. Pycnogonum littorale )
  2. American horseshoe crab ( Limulus polyphemus )

Sea anemones and corals

In this gathering of creatures with only amphibian propensities, outer treatment is normal, so we can specify:

  • Sublime ocean anemone ( Heteractis Magnifica )
  • Knotty brain coral ( Pseudodiploria close )
  • Elkhorn coral ( Acropora palmate )


Polychaetes are segmented worms of the Annelid phylum. They compare to the most assorted class of polychaetes, the greater part being in the marine climate. Fertilization of the entire group is external.

Why do most aquatic animals have external fertilization?

Animals, without a doubt, through their evolutionary and adaptation processes, develop different mechanisms or strategies that are beneficial for their lives. The aquatic environment offers a possibility of mobilizing the gametes for fertilization to occur, something that does not happen in the terrestrial environment, so fertilization is possible in the water without the creatures contacting one another.

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On the other hand, after the fusion between ovules and spermatozoa, the aquatic environment also offers in some cases another advantage, the possibility of dispersal of the newly formed zygote. As we have as of now referenced, these creatures discharge enormous amounts of eggs into the water, so that the endurance of the species is ensured both by the quantity and by the dispersion offered by the environment.

Many species, when developing in this type of habitat with these possibilities to fertilize, take advantage of it for their benefit, which is why evolution has allowed them this type of development.