Which animal groups commonly use external fertilization?

External fertilization is a reproductive strategy in which a male releases sperm over eggs that have been shed by a female into the environment, typically in aquatic habitats. This method is common among several animal groups:12

• Fish: Most bony fishes utilize external fertilization, releasing their eggs into open water or onto a substrate where they are fertilized by the male's milt.3
• Amphibians: The majority of amphibians rely on external fertilization to reproduce.1
• Invertebrates: Many invertebrates employ this strategy. For example, among coelenterates (such as certain hydrozoans), eggs may be shed directly into the water for external fertilization.14

While these groups frequently use external fertilization, it is important to note that reproductive strategies can vary significantly even within these classes. For instance, some fish and amphibians have evolved internal fertilization or specialized parental care behaviors to increase offspring survival.356