How Does External Fertilization Occur?
External Fertilization occurs when an organism fertilizes outside the body. The process of external fertilization usually requires a water body. Here both the eggs and sperms are released into the water. The majority of external fertilization occurs during spawning. In the priceless spawning, several eggs are released by the females at a time when the males also release several sperms. This fertilization or reproduction process is influenced by sunlight and temperature. The benefits of performing external fertilization in an aquatic body are that the eggs do not dry up. The spawning of eggs and sperms in external fertilization leads to greater biodiversity along with higher chances of genetic survival.
What is External Fertilisation?
External fertilization is a type of reproduction in which the sperm of a male creature fertilizes the egg of a female organism outside of the female's body. Internal fertilization, on the other hand, is when sperm is delivered via insemination and subsequently mix with an egg inside the female organism's body. External fertilization is most common in aquatic habitats when both eggs and sperm are discharged. Fertilization occurs when the sperm reaches the egg. The majority of external fertilization occurs during the spawning phase when one or more females release their eggs and one or more males discharge sperm in the same location at the same time. The temperature of the water or the length of daylight may cause the reproductive material to be released. Almost all fish, as well as crustaceans (like crabs and shrimp), mollusks (like oysters), squid, and echinoderms, spawn (such as sea urchins and sea cucumbers).
Mention Some External Fertilization Examples
Aquatic animals generally release their gametes directly into the environment. It mostly takes place in fish and amphibians. Descriptive external fertilization examples are that of salmon, trout fish, and codfish. Both the female and male release their eggs and sperms respectively into the water where they diffuse and fertilize. In the list of external fertilization examples, aquatic animals such as mammals and sharks are exceptions.
Features of External Fertilisation:-
Fertilization has an extremely low success rate.
Unlike internal fertilization, the male and female must produce a significant quantity of gametes to assure reproductive success.
To start external fertilization, you'll need a body of water. On land, the sperm would perish.
The majority of gametes perish without being fertilized, putting most animals at a reproductive disadvantage.
External fertilization, on the other hand, is a straightforward reproductive method that does not necessitate the use of hormones or mating rituals.
Solved Examples
Explain The Process of External Fertilization In Frogs in Brief.
Ans: It is known that frogs lay eggs that hatch into tadpoles. The reproduction occurs by the process of external fertilization. The females release their eggs into the water which are then fertilized by the male sperm. Although the majority of the species of frogs undergo external fertilization, there are some frog species that reproduce through internal fertilization too. Scientists have discovered recently that certain frog species instead of laying eggs, carry tadpoles inside them. This finding was a very unusual one as external fertilization is a common reproduction method for amphibians.
Mention a Few Disadvantages of External Fertilization.
Ans: A few common disadvantages of external fertilization are mentioned as follows:
A very major disadvantage of external fertilization is that a large number of gametes remain unused.
The aquatic environment is often very harsh. It is. It is not always suitable for reproduction or fertilization.
The offspring are subjected to danger from predators.
There is no guarantee that the sperm will come in direct contact with the egg.
The zygote often desiccates.
Mention the External Fertilization Advantages Along with Examples.
Ans: A simple major external fertilization advantage and benefit is that the chances of union between the gametes are high as a vast number of sperms and eggs are released into the water. External fertilization has greater synchronization and is a much simpler process as compared to internal reproduction. The majority of algae reproduction are external fertilization examples.
Fun Facts About External Fertilization In Animals
Coral animals send their eggs and sleep separately. The water currents eventually carry the gamete so that they can fertilize.
The male counterparts have more gametes compared to that females.
Birds have their eggs fertilized even before they are laid.
In external fertilization, an unfertilized egg can never be laid.
A large number of offspring are produced in external fertilization.
External fertilization results in genetic variation.
Most of the species of fish fertilize externally.
Algae also show external fertilization. The reproduction in algae is basically of three types: isogamy, anisogamy and oogamy.
Algae have no embryo formation and asexual reproduction takes place through spores such as zoospores.
External Fertilisation in Humans
Humans use external fertilization as well, but it is not natural. Ovarian ducts have been obstructed in a few females. The sperms are unable to reach the eggs, and the woman is unable to conceive. Doctors gather newly released eggs and sperms and keep them together for several hours for in-vitro fertilization in this case. After fertilization, the zygote is delivered to the female's uterus and the development process begins within her body, which takes about a week.
External fertilization is used by all fish and amphibians. However, it is not as effective as internal fertilization, and the chances of survival are lower.
FAQs on External Fertilization: Concepts & Examples
1. What is external fertilization?
External fertilization is a mode of sexual reproduction in which the fusion of male and female gametes (sperm and egg) occurs outside the female's body. This process almost always takes place in an aquatic or moist environment, which is necessary for the survival of the gametes and the movement of sperm to the egg.
2. What are some common examples of animals that reproduce through external fertilization?
External fertilization is common in many aquatic animals. Key examples include:
Fish: Many species like salmon, cod, and trout release their gametes into the water in a process called spawning.
Amphibians: Animals like frogs and salamanders release their eggs and sperm in ponds or streams.
Aquatic Invertebrates: Organisms such as starfish, sea urchins, and corals engage in external fertilization, often in massive, synchronised spawning events.
3. What is the main difference between external and internal fertilization?
The primary difference lies in the location of fertilization. In external fertilization, the fusion of gametes happens in the external environment (like water). In contrast, internal fertilization involves the fusion of gametes inside the female's reproductive tract. This leads to other key distinctions: external fertilization requires a large number of gametes to be produced to ensure success against environmental odds, while internal fertilization requires fewer gametes due to the protected environment and higher chance of fusion.
4. What is 'spawning' in the context of external fertilization?
Spawning is the process of releasing eggs and sperm into water by aquatic animals. It is the method through which external fertilization is achieved in most fish and amphibians. Often, spawning is a synchronised event where males and females of a species release their gametes at the same time. This synchronisation is usually triggered by environmental cues like water temperature, day length, or tidal cycles, and it significantly increases the probability of fertilization.
5. Is external fertilization a form of sexual or asexual reproduction?
External fertilization is a form of sexual reproduction. This is because it involves the combination of genetic material from two different parents through the fusion of two distinct gametes: a male gamete (sperm) and a female gamete (egg). Asexual reproduction, by contrast, involves only one parent and produces offspring that are genetically identical to that parent, without the fusion of gametes.
6. Why is a water-based environment crucial for external fertilization?
A water-based environment is essential for two main reasons. Firstly, it prevents the delicate gametes from desiccation (drying out), which would quickly kill them on land. Secondly, the water acts as a vital medium that allows the motile sperm to swim and travel towards the eggs, a journey that would be impossible in a dry, terrestrial setting.
7. What are the major disadvantages or risks associated with external fertilization?
While effective for many species, external fertilization has significant disadvantages:
Low Fertilization Rate: There is no guarantee that sperm will find the eggs in a vast body of water, leading to a low chance of successful fertilization.
High Predation: Both the released gametes and the resulting zygotes/embryos are highly vulnerable to being consumed by predators.
Environmental Dependency: Success depends heavily on favourable environmental conditions. Changes in water currents, temperature, or pH can prevent fertilization or harm the developing offspring.
8. Why doesn't external fertilization occur naturally in mammals like humans?
External fertilization is not feasible for mammals, including humans, because they are terrestrial (land-dwelling) organisms. The terrestrial environment lacks the water medium required to prevent gametes from drying out and to facilitate the sperm's movement to the egg. Consequently, mammals have evolved internal fertilization, which protects the gametes within the female's body, provides a controlled environment for fertilization, and ensures a much higher survival rate for the resulting embryo.
