Mussels should be kept cool at all times prior to cooking – on ice or refrigerated. If you anticipate your mussels will spend time in a warm car or room after purchasing, consider bringing an ice pack or cooler along to ensure they remain cold and fresh.
Mussels that are kept cool can stay fresh for a number of days; however, it is ideal to prepare them as soon as possible after purchasing.
Fresh mussels have a tendency to open upon packaging (this is when their respiratory system switches over to extract moisture from the air). To ensure your mussels are still alive and fresh, tap them with a utensil such as a knife, or on the side of your sink – if they are still fresh, they will close.