For sexual reproduction to take place, male and female cells must come together. Many marine species spawn, or discharge one or both of their sex cells into the water. For this strategy to be successful, eggs and sperm must be released at the same time, which is why spawning usually occurs once a year at a specific time.
|Clown Fish Egg|
Animals are cued to release gametes by specific environmental factors, such as the Moon’s phase, length of daylight, or temperature.
The alternative strategy to spawning is brooding. Animals that brood release sperm into the water, while eggs remain within the mother. Sperm swim around until they find a female, enter her body, and fertilize the eggs. Eggs are brooded within the mother’s body until time for them to hatch. Fertilized eggs that are brooded have the advantage of protection from predators during development.
In comparison, eggs fertilized in the water column or on the seafloor are at high risk from predators. For this reason, animals that brood their developing eggs only produce small numbers of gametes, while those that spawn discharge hundreds of thousands of gametes, a strategy that ensures that a few of the resulting offspring will survive until adulthood.