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Why must aquatic animals that use external fertilization release eggs and sperm at nearly the same time?

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Diagram comparing reproductive pathways: arrows show sexual reproduction forming a diploid zygote from oocyte and sperm, haploid parthenogenesis forming a haploid individual, and two types of diploid parthenogenesis-automixis and apomixis-both resulting in diploid individuals. Symbols indicate haploid and diploid stages. — Animal reproductive system The process of sexual reproduction and several forms of parthenogenesis.

In aquatic environments, animals that utilize external fertilization must release their eggs and sperm at nearly the same time because the gametes are highly susceptible to being quickly dispersed by water currents.1

Because fertilization in most animals depends on random collisions between eggs and sperm, the rapid movement of water can easily separate these reproductive cells, significantly reducing the likelihood of successful union. To overcome this challenge, many species have evolved complex courtship behaviors that serve to synchronize the release of gametes by both mating individuals. This synchronization ensures that the sperm and eggs are present in the same location simultaneously, thereby maximizing the chances of fertilization and the subsequent survival of the species.21