Dilution Effect: In aquatic environments, the chances of sperm encountering and fertilizing an egg are relatively low due to the vastness of the water body and the unpredictable movement of gametes. Producing a large number of sperms increases the probability that at least some will reach the vicinity of an egg and successfully fertilize it. The sheer volume of sperms released helps to overcome the dilution effect of the water.
Sperm Competition: In many aquatic species, multiple males may release sperms simultaneously, resulting in intense sperm competition. Producing a large number of sperms increases the male's chances of fertilizing the eggs even in the presence of competitors. The greater the number of sperms released, the higher the likelihood that some will be able to reach and penetrate the eggs before sperms from other males.
Egg Mortality and Predation: External fertilization occurs in open water, exposing both eggs and sperms to environmental hazards and predators. Many eggs and sperms are lost due to predation, wave action, and other environmental factors before they can successfully meet and fertilize. Producing a large number of gametes compensates for this high mortality rate, ensuring that at least some eggs will survive to be fertilized.
Synchrony and Timing: In many aquatic species, females release eggs at specific times or in synchronized cycles to increase the chances of encountering sperms. However, due to environmental variations and the unpredictable nature of reproduction, the release of eggs and sperms may not always overlap. Producing a large number of gametes increases the chances that at least some eggs will be fertilized during the optimal window of time.
Adaptations to Different Environments: Different aquatic habitats present unique challenges for successful external fertilization. For instance, in high-energy environments like wave-swept shores or fast-flowing rivers, the chances of eggs and sperms encountering each other are lower compared to calmer waters. Producing a large number of gametes helps overcome these challenges and increases the likelihood of successful fertilization in diverse habitats.
In summary, animals that undergo external fertilization produce a large number of eggs and sperms as a reproductive strategy to increase the chances of successful fertilization in the challenging aquatic environment. This strategy compensates for various factors, including dilution, sperm competition, egg mortality, and environmental unpredictability, ultimately enhancing reproductive success and ensuring the survival of the species.