What Animals Use External Fertilization?

Thomas Quine/CC-BY-2.0

Animals that live in water, such as amphibians and fish, use external fertilization. The term refers to the process by which the egg is fertilized by the sperm in an open environment and not within the bodies of the organisms.

In external fertilization, the eggs and sperm are usually released in huge numbers to increase the chance of fertilization because the open environment leaves them vulnerable to predators. Animals that practice external fertilization usually interact during the breeding period only, and the eggs and sperm are released under specific environmental conditions, such as temperature. Through chemotaxis, where certain chemicals stimulate movement, the sperm find the eggs for fertilization.