Oysters mate through releasing millions of sperm in attempt to fertilise the 100 million eggs each female oyster produces per year
All oysters are born males, later becoming female, they have organs containing two sexes that can release both sperm and eggs in order to reproduce. Technically, this means that oysters can fertilise their own eggs. The oysters actually produce several thousand different varieties of egg and sperm. Once fertilised, millions of eggs are released into the water.
Oysters have a huge number of different proteins that coat the surface of their sperm and eggs. These proteins help determine whether or not an individual egg or sperm can fertilise another egg or sperm. A sperm with a specific type of protein can only fertilise an egg that has a corresponding protein that accepts the same type of protein sperm. If an egg is fertilised by more than one sperm, the embryo is most likely going to die. Having a variety of proteins protects the egg from being fertilised by more than one sperm and makes the baby more likely to survive.
Image from www.flickr.com/photos/stuutje
Photograph: Jeremy Keith