Final answer:
External fertilization usually occurs in aquatic environments, where many species release eggs and sperm into the water. The process, often called spawning, is synchronized and can lead to high genetic diversity despite a high predation risk.
Step-by-step explanation:
External fertilization typically occurs in an aquatic environment. In these settings, both eggs and sperm are released into the water, and fertilization occurs when the sperm reaches the egg. This process is common in many aquatic species, including fish, crustaceans, mollusks, and echinoderms, which engage in a reproductive behavior known as spawning. Spawning involves multiple individuals releasing their gametes into the water simultaneously, often triggered by environmental cues like water temperature or daylight length. High genetic diversity is one advantage of this method, although the high risk of egg predation necessitates the production of large numbers of eggs.