How Does External Fertilisation Work in Nature?
Fertilisation is the union of eggs and sperms. Therefore, it is the process responsible for reproduction among animals and humans. This process is further classified into two subtypes, internal and external fertilisation.
What Is External Fertilisation?
External fertilisation is a method of reproduction in which a male organism’s sperms fertilise a female organism’s eggs, but outside the female body. This process is precisely opposite to internal fertilisation. In internal fertilisation, sperms combine with eggs via insemination within the female body.
Moreover, external fertilisation takes place in aquatic environments. Here, both sperms and eggs are released into the water, and once sperms reach the eggs fertilisation takes place.
Additionally, most of the external fertilisation occurs during the spawning process. Here, one or more female organism releases eggs and similarly male organisms releases their sperms in the same region. This process takes place at the same time.
However, this release of the reproductive substance may be affected by the length of daylight or water temperature.
Animals Capable of External Fertilisation
Some of the prominent external fertilisation examples are –
Starfish
Starfishes use a more synchronized technique for their spawning. They gather at a place on a specific day and at a particular time. After that, they use chemical signal to indicate other members of their species to know that they are ready to spawn. Additionally, they use the daylight as an indicator. This synchronized process improves their chance of successful fertilisation.
Sea urchins
Since sea urchins live in places with a relatively fast flow of water, they run the risk of losing their sperms and eggs. Thus, they use the chemotaxis process to attract sperms towards eggs.
Corals
Even though corals can reproduce internally, they practice external fertilisation. They release a substantial number of gametes into the water while spawning. However, a tiny amount of those eggs fertilises via sperms.
Frogs
The external fertilisation in frogs occurs when female frogs release eggs into the water. At the same time, male frogs also release their sperms to fertilise those eggs.
Fishes
Almost all kinds of fishes use external fertilisation for reproduction.
Advantages of External Fertilisation
The external fertilisation advantages are –
It creates more genetic variations.
Produces a substantial number of offspring.
Additionally, the gametes can easily drift, thus, the chances of finding a suitable mate is high.
Disadvantages of External Fertilisation
The problems of external fertilisation are –
A large number of gametes stays un-fertilised, and are subsequently wasted.
There is no guarantee that the sperms and eggs will come into contact.
Moreover, the chance of desiccation of zygotes or gametes are superior.
Various environmental hazards and predators reduce the life expectancy of new-born organisms.
External Fertilisation in Human Beings
The rate of external fertilisation in human beings is low, as it is not organic. In case any female has blocked oviducts, then her chance of conceiving is lower. The reason being, male sperms cannot reach the eggs.
In such a scenario she can opt for an inorganic process named IVF (In Vitro Fertilisation). In this process, freshly released eggs and sperms are collected and kept together for fertilisation to start. When it occurs, the zygote is then transferred into the uterus of the female. After that, further development of that zygote occurs within the female’s body.
External fertilisation is not a common phenomenon among mammals; it occurs primarily among aquatic creatures. Apart from this topic, students can visit the website and the app of Vedantu to access other topics of biology.
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FAQs on External Fertilisation Explained: Definition, Examples & Benefits
1. What is external fertilisation?
External fertilisation is a mode of sexual reproduction where the fusion of a male gamete (sperm) and a female gamete (egg) occurs outside the female's body. This process is most common in aquatic animals, which release their gametes into the surrounding water, where fertilisation takes place by chance.
2. What is the main difference between internal and external fertilisation?
The primary difference lies in the location of gamete fusion. In external fertilisation, the fusion happens in the external environment, usually water. In internal fertilisation, the fusion occurs inside the female reproductive tract. This fundamental difference affects the number of gametes produced, the chances of survival for the offspring, and the level of parental care required.
3. What are some examples of animals that undergo external fertilisation?
External fertilisation is common in many aquatic and amphibious species. Key examples include:
- Fish: Most bony fish, such as salmon, cod, and trout, release eggs and sperm into the water in a process called spawning.
- Amphibians: Frogs and toads exhibit external fertilisation, where the male releases sperm over the eggs as the female lays them in a pond or stream.
- Invertebrates: Many marine invertebrates like corals, sea urchins, and starfish also reproduce this way.
4. What are the main advantages and disadvantages of external fertilisation?
The main advantages include producing a large number of offspring with relatively little energy investment in parental care. However, the disadvantages are significant:
- Low Survival Rate: A vast number of gametes are wasted as they may never meet. The resulting embryos are also highly vulnerable to predators and harsh environmental conditions.
- Uncertain Fertilisation: The process is highly dependent on chance and requires the synchronised release of gametes.
- Environmental Dependence: It is restricted to moist or aquatic environments to prevent the gametes from drying out.
5. What environmental conditions are necessary for successful external fertilisation?
Successful external fertilisation critically depends on the environment. The most crucial factor is a watery medium, which prevents gametes from desiccating (drying out) and allows sperm to swim to the egg. Other important conditions include suitable water temperature, pH, and the absence of strong currents that could disperse gametes. Synchrony in gamete release between males and females is also vital, often triggered by environmental cues like lunar cycles or seasonal changes.
6. Is external fertilisation a form of sexual or asexual reproduction?
External fertilisation is a form of sexual reproduction. A common student misconception is to confuse the external location with asexual processes. However, because it involves the fusion of two distinct gametes (sperm and egg) from two parents, it results in genetically unique offspring, which is the defining characteristic of sexual reproduction.
7. Why do animals that use external fertilisation produce a very large number of gametes?
Animals that rely on external fertilisation produce a massive number of gametes as a survival strategy to overcome the low probability of successful fertilisation. The external environment is filled with risks, including predation, dispersal by water currents, and sudden changes in temperature. By releasing millions of eggs and sperm, these animals increase the statistical likelihood that at least some gametes will meet, fuse, and develop into viable offspring, thereby compensating for the high mortality rate.
8. Can external fertilisation occur in humans?
Naturally, humans only undergo internal fertilisation. However, a form of external fertilisation is applied in modern medicine through Assisted Reproductive Technology (ART), specifically In-Vitro Fertilisation (IVF). In this procedure, eggs are retrieved from the female and fertilised with sperm in a laboratory setting (in a petri dish). The resulting embryo is then transferred into the uterus. This process mimics external fertilisation but in a highly controlled, artificial environment.
