* Aquatic environment: External fertilization primarily occurs in water. This is because the sperm needs a moist environment to swim to the egg.
* Proximity of gametes: The male and female animals need to be close enough for the sperm and egg to meet. This often involves synchronized spawning behavior where both males and females release their gametes into the water simultaneously.
* Suitable water conditions: The water must be of the right temperature, salinity, and pH to allow the sperm to function and the eggs to develop.
* Protection from predators: Once the eggs are fertilized, they are vulnerable to predators. Some species have strategies to protect their eggs, such as laying them in nests, attaching them to plants, or releasing them in large numbers.
Examples of animals that use external fertilization:
* Fish: Many fish species release their eggs and sperm into the water for external fertilization.
* Amphibians: Frogs and toads lay their eggs in water, and fertilization occurs externally.
* Invertebrates: Many invertebrates, such as sea urchins, starfish, and corals, use external fertilization.
Let me know if you'd like more details on any of these factors or specific examples!