What is External Fertilisation Process Steps and Examples
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.
Moreover, they can attend the live online classes available on the app to learn from our experts.
Thus, download the app today for more interactive learning.
FAQs on External Fertilisation in Animals and Aquatic Organisms
1. What is external fertilisation?
External fertilisation is the fusion of male and female gametes outside the body of the female organism. In this process, sperm and egg cells (ova) are released into the external environment, usually water, where they unite to form a zygote.
- Common in aquatic animals such as fish, frogs, and many marine invertebrates.
- Requires a moist or aquatic environment to prevent gametes from drying out.
- Often involves the release of a large number of gametes to increase chances of fertilisation.
2. How does external fertilisation occur step by step?
External fertilisation occurs when gametes are released into the environment and fuse outside the parents’ bodies. The process typically follows these steps:
- Release of eggs by the female into water.
- Release of sperm by the male over the eggs (spawning).
- Movement of sperm toward the eggs using flagella.
- Fusion of sperm and egg nuclei to form a diploid zygote.
- Development of the zygote into an embryo outside the body.
3. In which organisms does external fertilisation occur?
External fertilisation occurs mainly in aquatic animals, especially fish, amphibians, and many marine invertebrates. Examples include:
- Bony fish such as salmon and cod.
- Amphibians like frogs and toads.
- Marine invertebrates such as sea urchins and corals.
4. What is the difference between external and internal fertilisation?
The main difference between external and internal fertilisation is the location where gametes fuse. In external fertilisation, fusion occurs outside the body, while in internal fertilisation, it occurs inside the female reproductive tract.
- External: Usually in water; produces many gametes; lower survival rate.
- Internal: Occurs inside body; fewer gametes; higher protection and survival.
- Examples: Frogs (external) vs. mammals (internal).
5. Why is water necessary for external fertilisation?
Water is necessary for external fertilisation because sperm need a fluid medium to swim toward the egg. In aquatic environments:
- Sperm motility depends on a liquid medium.
- Water prevents desiccation (drying out) of gametes.
- It helps disperse gametes during spawning.
6. What are the advantages of external fertilisation?
External fertilisation allows the production of a large number of offspring at once. Its advantages include:
- Release of thousands to millions of eggs, increasing chances of survival.
- Wide dispersal of offspring in aquatic habitats.
- Less energy spent on internal gestation.
7. What are the disadvantages of external fertilisation?
External fertilisation has a lower survival rate because gametes and embryos are exposed to environmental risks. Major disadvantages include:
- High loss of gametes due to predation or dispersal.
- Dependence on water for successful fertilisation.
- No parental protection in many species.
8. What is spawning in external fertilisation?
Spawning is the release of eggs and sperm into water for external fertilisation. During spawning:
- Females release clusters of eggs.
- Males simultaneously release sperm over them.
- Fertilisation occurs in the surrounding water.
9. How many eggs are usually produced in external fertilisation?
Organisms with external fertilisation usually produce a very large number of eggs, often thousands to millions. For example:
- A single female fish may release thousands of eggs at once.
- Some marine species like sea urchins release millions of gametes.
10. Is external fertilisation common in humans?
External fertilisation does not occur naturally in humans because fertilisation takes place inside the female reproductive system. In humans:
- Fusion of sperm and egg occurs in the fallopian tube.
- This is an example of internal fertilisation.
