Disadvantages of external fertilization include a reliance on water and the large amount of wasted sperm and eggs that never reach a corresponding gamete, even when the organisms releasing the eggs and sperm are in close proximity. Additionally, the eggs released can have no shells, as these would stop sperm.
External fertilization is used only by animals that breed in water, since the sperm require water to swim through to reach the eggs. Animals that use this reproductive strategy include most fish, most amphibians, and many other types of aquatic invertebrates. Because of the uncertain fertilization of any particular egg, animals that use external fertilization typically produce very large numbers of sperm and eggs.
External fertilization also has advantages over internal fertilization. In internal fertilization, a male of a species directly inserts sperm into a female, which then swim in the fluids surrounding the reproductive tract to reach the eggs. This is the strategy commonly used by animals that breed on land. While this is a more reliable method in terms of the likelihood that each egg gets fertilized, it is more complicated than external fertilization. It requires close coordination between males and females in terms of behavior and physiology, which requires extensive hormonal controls.