External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation.

In sexual reproduction, there must be some way of getting the sperm to the egg. Since sperm are designed to be mobile in a watery environment (they have tails and are streamlined), aquatic animals can make use of the water in which they live. Many plants make use of external fertilization, especially ones without bright flowers or other means of attracting animals. In many aquatic animals such as coral or hydra, eggs and sperm are simultaneously shed into the water, and the sperm swim through the water to fertilize the egg in a process known as broadcast fertilization. In many fish species, including salmon, the female will deposit unfertilized eggs in the substrate and the male will swim by and fertilize them.

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