Explain why aquatic animals typically use external fertilization and terrestrial animals use internal fertilization.
Aquatic animals typically use external fertilization because it allows them to efficiently release their gametes (sperm and eggs) directly into the water. Since water is a fluid medium, the gametes can easily mix and combine to form zygotes. This method of fertilization is advantageous in aquatic environments because it increases the chances of successful fertilization, especially in species that release a massive number of gametes.
External fertilization in aquatic animals also increases the probability of encountering a mate and reproducing, as they can release their gametes simultaneously into the water, where they can mix and be carried by currents, increasing the likelihood of encountering a gamete of the opposite sex.
On the other hand, terrestrial animals typically use internal fertilization because the terrestrial environment poses challenges for the survival and successful fusion of gametes. The absence of a fluid medium like water makes it harder for the sperm to reach the eggs, increasing the chances of desiccation (drying out) and death of the gametes. Internal fertilization solves this problem by allowing direct transfer of sperm from male to female, ensuring a higher chance of successful fertilization.
Internal fertilization in terrestrial animals also reduces the number of gametes required to increase the chances of fertilization. Unlike aquatic animals, terrestrial animals cannot release a massive number of gametes into the environment, as it would be resource-intensive and energetically costly. Hence, by employing internal fertilization, terrestrial animals can reduce the number of gametes and increase the efficiency of reproduction by ensuring direct transfer of sperm to eggs.
Moreover, internal fertilization offers an advantage in providing a protective environment for the developing zygote. Terrestrial animals generally invest more in the development and care of their young compared to aquatic animals. Internal fertilization allows the female to retain the fertilized eggs internally or provide a protective structure (such as eggs, nests, or live birth) to safeguard the zygote from predation, desiccation, and other environmental challenges.
Overall, the different modes of fertilization in aquatic and terrestrial animals are adaptations to the specific challenges posed by their respective environments, where external fertilization facilitates successful gamete fusion and dispersal in aquatic environments, while internal fertilization enhances the chances of fertilization and survival in terrestrial habitats.