Bivalves are marine animals that have one shell on the top and one on the bottom. Oysters, which are the most valuable of all bivalves, are common in shallow, warmer waters. Females lay five million to fifty million eggs in a year, but few mature. The shell starts to harden one day after being fertilized. They swim a few days after hatching. Then they attach to a rock and spend the rest of their lives there. Once fully grown, they are two to ten inches long.