Define external fertilization. Mention its disadvantages.

Hint: To derive the correct answer, you must first breakdown the word external fertilization into two parts. Fertilization generally occurs somewhere internal. But here we can see the word external before fertilization signifying that fertilization occurs somewhere other than its normal site especially outside of its natural environment.

Complete answer:
By external fertilization, we mean the fusion of the male and female gamete outside the body of the female organism. It is differentiated from internal fertilization where the fusion of the male and the female gamete occurs inside the body of the female organism. External fertilization mainly occurs in wet or moist areas especially in the sea which facilitates the movement of the male gamete towards the female gamete. When the gamete is of sessile species ,it must be released in the vicinity of the female gamete to allow fertilization. External fertilization has disadvantages in addition to its advantages. They are as follows:
1. A large amount of gamete is wasted, is eaten, lost or it remains unfertilized.
2. The chances that fertilization will occur is reduced by predators thriving in the sea and also environmental conditions.
3. There is no guarantee that the male and the female will come in contact with one another.
4. There leads to the desiccation of the gametes or the zygote formed.

Note:
In humans, internal fertilization occurs where the male and the female gamete fuses in the fallopian tube of the female. However in the seahorse that resides in water, fertilization is mostly of external type and occurs in the open seas. The development of the eggs of the seahorse occurs inside the brood pouch of the male seahorse.