Clams whose shells have opened before being cooked are already dead, meaning that they are bad and need to be eliminated from the batch. Cooking bad clams with good clams can spoil the whole lot.
After purchasing a batch of clams, chefs suggest looking for any open shells. These can be tapped on the counter, and if they close, they are still good. Clams that do not close need to be eliminated as bad. Clams whose shells are still firmly shut are the freshest. According to Fine Cooking, clams need to be refrigerated, preferably in a bowl covered with a wet towel. They last up to two days stored this way.