Main Difference – Internal vs External Fertilization
Internal fertilization and external fertilization are the two mechanisms involved in the fusion of male and female gametes. Fertilization is one of the rearmost events of the sexual reproduction, which forms the zygote. The zygote develops into a new organism. The main difference between internal and external fertilization is that internal fertilization occurs inside the female organism whereas external fertilization occurs outside the female organism. Oviparity, viviparity, and ovoviparity are the three methods of internal fertilization. Internal fertilization occurs in mammals, reptiles, some birds, and some fish. External fertilization occurs in frog, fish, mollusks, and crustaceans. Internal fertilization shows high survival rates of the embryo than the external fertilization.
Key Areas Covered
1. What is Internal Fertilization
– Definition, Mechanism of Fertilization, Role
2. What is External Fertilization
– Definition, Mechanism of Fertilization, Role
3. What are the Similarities Between Internal and External Fertilization
– Outline of Common Features
4. What is the Difference Between Internal and External Fertilization
– Comparison of Key Differences
Key Terms: Egg cell, External Fertilization, Internal Fertilization, Oviparity, Spawning, Sperms, Viviparity, Zygote
What is Internal Fertilization
Internal fertilization refers to the fusion of gametes inside the female organism during sexual reproduction. This is seen in most terrestrial animals as well as some aquatic animals. The three methods of internal fertilization are oviparity, ovoviviparity, and viviparity. In oviparity, eggs are laid outside, following fertilization. The yolk is responsible for the nourishment of the embryo. Some mammals, birds, many reptiles, most amphibians, and some cartilaginous fish use oviparity. The eggs of the reptiles and insects are leathery. Birds produce eggs with a hard shell, which is made up of calcium carbonate. The egg hatches to release the new organism. A reptile egg is shown in figure 1.
Figure 1: Reptile Egg
In ovoviviparity, the embryo remains inside the female but, the egg’s yolk is responsible for the nourishment of the embryo. Oviviparous animals include some snakes, lizards, bony fish, sharks, and some invertebrate animals. Once the egg hatches, the new organism is born. In viviparity, the embryo develops into the fetus inside the female organism. Mother’s blood supplies the nourishment to the developing young through the placenta. Most mammals, few reptiles, and cartilaginous fish are viviparous animals. Internal fertilization of humans is shown in figure 2.
Figure 2: Internal Fertilization
In plants, internal fertilization occurs inside the ovary in higher plants such as angiosperms, gymnosperms, pteridophytes, and bryophytes. Internal fertilization of angiosperms is shown in figure 3.
Figure 3: Internal Fertilization of Angiosperms
The major advantage of the internal fertilization is the protection of the embryo from dehydration on land. The isolation of the embryo inside the female also protects the young from predators. The sperms come from a specific male in the internal fertilization. Although fewer offspring is produced from the internal fertilization, internal fertilization shows a higher survival rate.
What is External Fertilization
External fertilization refers to the fusion of gametes outside the female organism during sexual reproduction. External fertilization ideally occurs in aquatic environments and both male and female gametes are released to the external environment at the same time. In this process, gametes, as well as embryos, are protected from dehydration by aquatic environment. All fish, echinoderms, mollusks, and crustaceans use external fertilization. The eggs and sperms deposited in water of these animals are called the spawn. An anemonefish is protecting its spawn is shown in figure 4.
Figure 4: Anemonefish is Protecting its Spawn
Spawning results in a great genetic diversity within a particular species. In some sessile organisms such as sponges, spawning is the only method of colonizing in a new environment. Predation causes the loss of offspring in external fertilization. However, a large number of offspring is produced by external fertilization. Some algae also use external fertilization during their sexual reproduction.
Similarities Between Internal and External Fertilization
- Both internal and external fertilization are mechanisms of fusing female gametes with male gametes.
- Both internal and external fertilization occurs in animals as well as plants.
- Mobile sperms and immobile egg cells are produced in both internal and external fertilization.
- The final result of both internal and external fertilization is the zygote.
Difference Between Internal and External Fertilization
Definition
Internal Fertilization: Internal fertilization refers to the fusion of gametes inside the female organism during sexual reproduction.
External Fertilization: External fertilization refers to the fusion of gametes outside the female organism during sexual reproduction.
Fertilization
Internal Fertilization: Internal fertilization occurs inside the female organism.
External Fertilization: External fertilization occurs outside the female organism.
Methods
Internal Fertilization: Oviparity, viviparity, and ovovivipary are the three methods of internal fertilization.
External Fertilization: External fertilization occurs in the external environment.
Examples
Internal Fertilization: Internal fertilization occurs in mammals, reptiles, some birds, and some fish.
External Fertilization: External fertilization occurs in frog, fish, mollusks, and crustaceans.
In Plants
Internal Fertilization: Internal fertilization occurs in bryophytes, pteridophytes, gymnosperms, and angiosperms.
External Fertilization: External fertilization occurs in most algae.
Rates of Survival
Internal Fertilization: Internal fertilization shows high survival rates of the embryo.
External Fertilization: External fertilization shows lower survival rates of the embryo and egg.
Importance
Internal Fertilization: Internal fertilization is successful in harsh environmental conditions as well.
External Fertilization: External fertilization is successful in moistened environments.
Number of Gametes
Internal Fertilization: Less number of gametes are produced during the internal fertilization.
External Fertilization: A large number of gametes are produced during the external fertilization.
Genetic Diversity
Internal Fertilization: The male gametes are received by a selected individual in the internal fertilization.
External Fertilization: External fertilization results in a great genetic diversity.
Conclusion
Internal and external fertilization are the two mechanisms of fertilization of gametes during sexual reproduction of animals and plants. Internal fertilization occurs in higher animals and higher plants. External fertilization occurs in lower animals and plants. During internal fertilization, the fusion of gametes occurs inside the female organism. In the external fertilization, gametes are fused in the external environment. Thus, the main difference between internal and external fertilization is their mechanism of fusing gametes.
Reference:
1. “Fertilization.” Fertilization | Boundless Biology, Available here.
Image Courtesy:
1. “Tortoise-Hatchling” (CC BY-SA 3.0) via Commons Wikimedia
2. “Blausen 0404 Fertilization” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia
3. “Double Fertilization” By The original uploader was Triploid at English Wikipedia – Transferred from en.wikipedia to Commons (CC BY 3.0) via Commons Wikimedia
4. “Anemone Fish protecting it’s spawn” by prilfish (CC BY 3.0) via Flickr
Leave a Reply