Some species, however, are incapable of movement. These include edible mussels and oysters. As they cannot move voluntarily they usually stay on the same rock for their entire lives. Cockles, periwinkles and whelks can leap short distances and they move by the thousands with the tide. Clams and cockles are usually harvested from beaches at low tide.
Scallops can swim.
Mollusks are usually eaten raw or very briefly cooked. They must therefore be very fresh. Mussels, oysters and clams are usually sold live. Their shells must be tightly closed and any that are open should be tapped sharply. Shellfish should always be eaten fresh.