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 there are variants that 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 arising in different groups of animals. In this article on our site, we want to present you with information about the 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 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.
In relation to 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 then, the characteristics of the egg vary depending on the type of fertilization of the animal:
- The oviparous species with internal fertilization have hard-boiled eggs, with covers or shells resistant to desiccation, so they can remain outside the Water.
- The oviparous animals with external fertilization produce eggs without this protection, having thinner membranes, so they generally require the aquatic environment or a humid environment for its 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 development, while in the latter it must still occur the fusion between the gametes, needing 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 amphibiansNext, let's see examples of vertebrates with external fertilization:
Fish with external fertilization
Fish are aquatic animals that inhabit both freshwater and s altwater environments. 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 there are several factors that 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, gametes are very short-lived, so fertilization must occur as soon as possible, furthermore, the tiny sperm has a limited range to reach the gamete feminine. 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 chemotactic factor, which consists of a chemical attraction released by the ova to attract male gametesThese 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:
- European perch (Perca fluviatilis)
- Atlantic salmon (Salmo salar)
- Atlantic cod (Gadus morhua)
- Brook trout (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:
- Common Toad (Bufo bufo)
- Minor Siren (Intermediate Siren)
- Common frog (Rana temporaria)
- Chinese giant salamander (Andrias davidianus)
- Fischer's clawed salamander (Onychodactylus fischeri)
Invertebrates with external fertilization
In invertebrates we also find various groups of animals with external fertilization, in fact, 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 fuse and occur. fertilization, then, a series of transformations will take place to originate the embryonic form.
Invertebrate species with external fertilization are also subject to the influences of the environment, which can limit or prevent the process from occurring, but in many cases they also manage to overcome them and successfully reproduce. Examples of invertebrates with external fertilization are:
Mollusks
It is usual for species of aquatic molluscs to have external fertilization and some of the most representative examples are:
- Clam (Mercenary Mercenary)
- Pacific Oyster (Magallana gigas)
- Common tusk shell (Antalis vulgaris)
Echinoderms
In the case of echinoderms that are fertilized outside the female's body, we can mention the following examples:
- Common Starfish (Asterias rubens)
- Fire urchin (Astropyga radiata)
- Donkey manure (Holothuria mexicana)
Arthropods
Within the group of marine arthropods we find certain examples of this type of fertilization, among which we highlight:
- Sea spider (Pycnogonum littorale)
- American Horseshoe Crab (Limulus Polyphemus)
Sea anemones and corals
In this group of animals with exclusively aquatic habits, external fertilization is common, so we can mention:
- Magnificent sea anemone (Heteractis magnifica)
- Knotty Brain Coral (Pseudodiploria clivosa)
- Elkhorn Coral (Acropora palmate)
Polychaetes
Polychaetes are segmented worms of the Annelid phylum. They correspond to the most diverse class of polychaetes, the majority being in the marine environment. Fertilization of the entire group is external.
Why do most aquatic animals undergo 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 that inside the water fertilization is possible without the animals having to touch
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 already mentioned, these animals release large quantities of eggs into the water, in such a way that the survival of the species is guaranteed both by the quantity and by the dispersion offered by the environment. When many species develop in this type of habitat with these possibilities to fertilize, they take advantage of it for their benefit, which is why evolution has allowed them this type of development.