Why does external fertilization require a large number of sperm?

With external fertilization, sperm and eggs are released into the water (i.e., into the external environment), where they merge. A significant part of gametes die from adverse environmental conditions, therefore, in animals with an external type of fertilization (bone fish, amphibians, many aquatic invertebrates), a large number of germ cells are produced. For example, cod swallows about 10 million eggs, and moon fish – up to 300 million.

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