Note: Information is dated. Do not rely on it.Oyster. A well-known edible shellfish. The principal breeding time of the common oyster is in the spring, when their spawn is usually cast. The young oyster is at first a free-swimming organism which after a short time develops a shell and attaches itself to stones or other hard objects upon the sea bottom. Very commonly they adhere to adult shells, and thus are formed the large masses termed oyster banks. In about a year and a half they attain a size fit for the table.