External Fertilization – An Overview

External fertilization occurs in fish, frogs, and certain invertebrates like sponges. External fertilization is the process of reproduction where the fusion of the male gamete and female gamete takes place outside the body in an external environment. This type of reproduction has various benefits as it has great biodiversity and the chances of survival are also higher. In this article, we will study the process of external fertilisation, examples, features and more.

What is External Fertilization?

External fertilization is a widespread reproductive method that takes place in the aquatic environment. The fertilization method takes place during the process of spawning where the females release their eggs and the male releases sperm in the same area and same time. External fertilization takes place in crabs, shrimps, oysters, squid, sea urchins, and sea cucumbers.

The release of the reproductive material egg and sperm is triggered by the temperature of the water or the duration of the daylight. The fertilization process in the aquatic environment protects the eggs from desiccation and the chances of survival of the species also increase.

External Fertilization

Characteristics of External Fertilization

The features of external fertilization are as follows:

• Fertilization has a meager success rate.
• Unlike internal fertilization, the male and the female have to produce a large quantity of gametes to increase the reproductive rate.
• An aquatic environment is required for this fertilization as the sperm and egg would perish on land.
• A major number of gametes perish without being fertilized.
• External fertilization is a straightforward reproductive method and does not require any mating rituals or hormones.

External Fertilization Examples

Many aquatic species follow the external fertilization process. Some of them are:

External Fertilization in Sea Urchins

The external fertilization in sea urchins use chemotaxis to attract the sperm towards the eggs, their spawning is synchronized to prevent the eggs and sperms from drifting away.

External Fertilization in Frogs

The female frogs release the eggs into the water and the male releases the sperm in the water to fertilize them, the tadpole-larval stage of the frog lives in water and the adult frog lives on land.

External Fertilization in Coy Coral

These animals release a large number of gametes in water during spawning and after fertilization, the coral larvae move up to the surface of the water for maturation.

External Fertilization in Salmon

Salmon reproduces through external fertilization, where eggs and sperm are released into water simultaneously during spawning. The salmon fishes have an opening right in front of the anal fin from which the gametes are released and then they are fertilized.

External Fertilization in Starfish

These species use chemical signals to other members to inform that they are ready to spawn, this synchronizing method of spawning increases the chances of fertilization.

External Fertilization Process

The reproductive process where the male gamete sperm fuses with the female gamete ovum outside the female body. The steps involved in this process are:

• Synchronization: The most important aspect of external fertilization is the perfect timing and synchronization with the environment. Most of the species rely on this method and they are tuned to certain specific triggers like temperature change, daylight duration, and lunar cycles. The coral reefs exhibit mass spawning during lunar cycles and it ensures the release of gametes by many individuals at the same time. This increases the chance of the gametes meeting and results in successful fertilization.
• Release of Gametes: This is the primary step in the external fertilization process, the male and the female species release the sperm and egg into the surrounding environment. The release is done by complex courtship behavior which signals the spawning. In certain species of fish, the males and females come together in groups and release their gametes at the same time which maximizes the chances of fertilization.
• Gamete Fusion: After the release of the gametes into the water, the sperm finds the egg and fertilizes it. This process is influenced by water currents, the concentration of gametes in the water, and the viability of the sperm and eggs. In certain species, eggs emit chemical signals that attract sperm and they fuse to form a zygote.
• Protection and Development: After fertilization the resulting zygote faces challenges in surviving in the open environment so many species develop certain strategies like laying eggs in protective cases or attaching to rocks or plants. In some cases both the parents guard the egg until they are hatched. The development of embryos into larvae continues in the aquatic environment.

Advantages of External Fertilization

External fertilization has certain advantages:

• High Productive Output: This is the primary advantage of external fertilization the species that use this method release a large number of gametes into the environment that the likelihood of fertilization. This compensates for the high mortality rate that is often due to predation, environmental hazards, and other factors.
• Genetic Diversity: This fertilization method promotes genetic diversity as the gametes of many different individuals mix in the environment and it will result in a wide range of genetic recombination. This is crucial for adaptation and also enables them to withstand environmental changes, disease outbreaks, and other threats
• Low Energy Expenditure: Compared to internal fertilization the organisms require less energy to produce the gametes as it does not need any complex mating rituals or internal egg development.
• Adaptation to Aquatic Life: This method of reproduction is well suited in aquatic environments as the water facilitates the dispersal of gametes, this is advantageous because the direct contact of the individuals might be limited in the vast environment. The aquatic environment also supports the development of eggs, larvae, and dispersal.

Disadvantages of External Fertilization

Although external fertilization has many advantages it also has certain disadvantages, like:

• A large amount of gametes might remain unfertilized.
• The chase of fertilization is reduced by environmental problems and predators.
• Desiccation of the gametes and zygote is possible.
• The eggs and sperms produced by the organisms may not come in contact.

External Fertilization in Humans

External fertilization is also seen among humans but it is not natural. When the female’s oviducts are blocked the sperm cannot reach the eggs to fertilize it. In this situation the doctors collect the eggs and sperm and keep it together for several hours for the fertilization to take place this is known as in-vitro fertilization. After fertilization, the zygote is transferred into the uterus of the female after a week and the development takes place inside the body of the female.

External Fertilization in Plants

In plants external fertilization is very evident in primitive forms like algae and mosses where the male gametes are released into the environment and they finally reach the egg cell through chemotaxis. When fertilization occurs, the zygote develope individually independent of the mother plant. Pollination is also sometimes considered as an advnce form of external fertilization where the pollen is carried by an agent to the female plant.

External Fertilization in Fish

Fishes usually exhibit external fertilization. In this process the female fish lays egg in water and the male fertilizes it by releasing sperms on the cluster of egg. Once fertilization is successful, the male parent cares for the egg until they hatch.

Difference Between Internal and External Fertilization

The difference between internal and external fertilization in points is listed below:

Criteria	External Fertilization	Internal Fertilization
Occurence	External fertilization occurs outside the female body	Internal fertilization occurs inside the female body
Process of sperm transfer	Sperms are released into the environment and they fertilize the egg.	Sperms are released inside the female reproductive tract.
Vulnerability of offspring	Offsprings are highly vulnerable to exteral threats.	Offsprings are less vulnerable as they develop inside the mothers womb.
Examples of organisms	Fishes, amphibans, etc.	Humans, dog, cat, etc.

Conclusion – External Fertilization

External fertilization is the method of fertilization that takes place outside the female body. Here, the male and the female gametes are released into the aquatic environment and the fusion takes place. This type of fertilization is seen among aquatic organisms like fish and amphibians. External fertilization has high productive output and genetic diversity but it also has significant risks and challenges for the survival of offspring.

FAQs on External Fertilization

What are Internal and External Fertilization?

Internal fertilization is the fusion of gametes that takes place inside the female body example mammals. External fertilization is the fusion of gametes that takes place in the external environment example fish and frogs.

Is a Bird an Example of External Fertilization?

Birds reproduce by internal fertilization where the egg is fertilized inside the female but the development takes place outside the body.

What is Internal Fertilization?

The process of fertilization takes place inside the body of the female where the sperm is ejaculated in the female reproductive tract of the female and it travels towards the oviduct where fertilization takes place.

What are the Examples of External Fertilization?

This type of fertilization takes place in fish and amphibians, some of them are salmon, codfish, starfish, sea urchin, and trout fish.

What is External Fertilization?

The fusion of the male sperm and female egg that takes place outside the female body usually in an aquatic environment is called external fertilization.

What are the Advantages and Disadvantages of External Fertilization?

External fertilization allows for a large number of offspring to be produced, enhancing species survival, but it also exposes eggs and sperm to predation and environmental hazards, reducing individual survival chances.

What are the 5 Examples of External Fertilization Animals?

Examples of animals that reproduce via external fertilization include fish, amphibians, mollusks, echinoderms, and many marine invertebrates like corals and sea urchins.