One of several animal reproductive methods, external fertilization generally only occurs in aquatic creatures as water acts as a vehicle through which sperm can travel. The process involves the releasing of sperm and eggs -- or gametes -- near each other so fertilization can occur. It's sometimes also known as spawning.
Although corals can reproduce internally, they're able to practice external fertilization, too. When corals spawn, a huge amount of gametes is released into the water. Only a very small proportion of the eggs released will be fertilized by sperm, but it's still enough to keep coral numbers up. Once the fertilized eggs have hatched, the coral larvae head up to the surface to mature, after which they sink back down to the rocky seabed to find a place to anchor themselves.
Almost all species of marine fish reproduce using external fertilization, although a few exceptions exist. Fish such as tuna, cod, wrasses, groupers and flounders all fertilize their eggs using this method. Some types of fish, such as parrotfish, are able to spontaneously change gender from female to male if there aren't enough males in the area to fertilize the females' eggs.
Starfish have several methods to ensure the success of their external fertilization. Rather than simply releasing their gametes and hoping for the best, they synchronize their spawning to increase the chances of fertilization. They gather themselves in groups on certain days and at certain times -- using the length of the day and the amount of light as indicators -- and use chemical signals to let other members of their species know they're ready to spawn.
Sensual Sea Urchins
Since sea urchins live in tide pools or reefs, where there's a relatively fast flow of water, there's a risk of all their gametes getting washed away and no fertilization occurring. To avoid this, they've adapted several methods to increase the chance of eggs being fertilized. They synchronize their spawning, so there's not enough time for the eggs and sperm to drift away or become diluted. They also use a process called chemotaxis to help attract the sperm toward the eggs.
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