Define external fertilization. Mention its disadvantages.

Hint: The female releases a large number of eggs in the outside environment and the male releases the sperms in the same environment that is usually aquatic so that the desiccation of the released gametes is protected.

Complete answer:
External fertilization is a type of fertilization that occurs in the external environment i.e. outside the body of the organism. The sperms and eggs are released in the same environment which is usually aquatic so that the desiccation of the eggs can be protected and the two gametes meet in the same environment to form the zygote. It occurs in organisms like sea urchins, frogs, salmons, starfishes, etc. A figure showing egg and sperm release in fishes has been given below –

The most common disadvantages of the external fertilization are –
1) In this fertilization, since a large number of eggs are released, most of them fail to get fertilized by the sperms and thus a major quantity of the eggs are wasted.
2) Since fertilization is taking place externally, most of the time it is affected and diminished by the environmental hazards and also by the predators present in that environment.
3) There are chances that the released eggs may not come in contact with the right sperm i.e. they may fuse or mix with the eggs of other animals living there.
4) Because the progenies are developing in an external environment, in the absence of proper care their chances of survival are very low and most of the time they become prey for the predators.

Note:
Though external fertilization has many disadvantages, it is still beneficial for some animals because it results in increased genetic variations and also a large number of offspring can be produced at the same time.
Also, this is an easy mechanism to reproduce as compared to internal fertilization because it does not involve strategies like relying on hormones, mating rituals, and other similar factors that are considered in the internal fertilization.