External Fertilization

You dive into the sea along a coral reef. You notice gorgeous corals that are vibrantly coloured and alive. However, you realise that these creatures do not move. How do non-moving animals find a mate?

They achieve this by discharging eggs or sperm into the water in the hopes that the cells will pair and the eggs will be fertilised. This mode of fertilisation is used by the majority of aquatic species that reproduce sexually. We call this “external fertilisation” since it occurs outside of the body.

What is Fertilization?

Fertilisation is described as the fusing of two haploid gametes, i.e., male and female gametes, to produce a new diploid individual organism or progeny and begin its development. It is sometimes referred to as generative fertilisation, syngamy, or impregnation.

Fertilisation is the process of reproduction that involves the union of the male and female gametes, resulting in the development of a zygote. Mitosis and meiosis are both involved in the fertilisation process. Fertilisation is a sexual reproduction process that is essential for the continuation of a species’ generation, whether it be plants, animals, or humans.

There are two types of Fertilization i.e. Internal and External but here we will talk about external fertilization.

What is External Fertilization in Animals?

External fertilisation is the process by which male and female gametes fuse outside of the female body in an external medium, usually water. External fertilisation occurs in animals.

Example: fish, frogs, and starfish.

External fertilisation occurs mostly in aquatic environments and necessitates the release or broadcast of gametes by both male and female organisms (usually water). This is known as spawning. External fertilisation is used by amphibians, fish, and coral to reproduce. External fertilisation is beneficial because it produces a high number of offspring. However, because of different environmental risks like predators and severe weather conditions, offspring created in this manner suffer several challenges, and many die.

Typically, spawning animals do not care for their young. The level of protection provided to an egg after fertilisation has a direct influence on its survival. Some species hide their eggs in the sand, while others carry them in pouches or their mouths, while still others just breed and never see their offspring again. An organism that has been nourished by a parent has a significantly higher chance of survival.

The Importance of Fertilization in Organisms

• Fertilisation preserves the newly created individual’s diploid number of chromosomes.
• It is important for the survival of the species.
• It produces genetic variants by combining the characteristics of the two parents, resulting in the emergence of a new species.
• It contributes to the preservation of organism biodiversity.
• Fertilisation also contributes to the sex of the child.

External Fertilization Examples

1. Sea Urchins

To avoid the sperm and eggs being washed away and diluted, sea urchins have evolved a mechanism to bring the gametes together, including synchronising spawning and chemotaxis of the sperm towards the eggs. During sea urchin fertilisation, processes occur that allow sperm to find and fuse with the egg, followed by responses that prevent more sperm from entering the egg.

2. Coy Corals

Corals can reproduce both inside and externally, as well as fertilise one another. When corals spawn, a significant number of gametes are released into the water. Although only a tiny proportion of the eggs produced will be fertilised by sperm, this is sufficient to keep coral populations healthy.

3. Fishes

Fertilization occurs externally in the majority of fish species. Females and males both discharge large amounts of eggs and sperm into the water at the same time. During spawning, individuals of a species form a big group and release their eggs and sperm simultaneously.

4. Starfish

The starfish is an echinoderm that self-fertilizes. The majority of starfish species release their eggs straight into the ocean. The male starfish next spits sperm onto the released eggs, fertilising and creating zygotes.

Advantages

• Producing gametes takes a little amount of energy.
• The gametes can travel long distances.
• Young ones can quickly occupy vast water regions.
• Females have a far greater success rate since they lay hundreds of eggs.

Disadvantages

• Some gametes do not meet zygote because of the aquatic environment and kind of water.
• Different animals normally consume the zygote, or it dies from natural reasons.
• Predators do not safeguard the offspring.

Conclusion 

External fertilisation is the process by which male and female gametes fuse outside of the female body in an external medium, usually water. They achieve this by discharging eggs or sperm into the water in the hopes that the cells will pair and the eggs will be fertilised. This mode of fertilisation is used by the majority of aquatic species that reproduce sexually. External fertilisation occurs mostly in aquatic environments and necessitates the release or broadcast of gametes by both male and female organisms. It is important for the survival of the species. It produces genetic variants by combining the characteristics of the two parents, resulting in the emergence of a new species. External fertilisation can occur exclusively in water because the eggs produced by the female require water to live, and the sperm requires water to swim and reach the eggs.