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There is archaeological evidence that humans have utilised mussels as a source of food for thousands of years. Nowadays marine mussels remain a popular seafood, especially in Belgium and the Netherlands, and France (called moule marinere), where they are consumed with french fries (”mosselen met friet” or “moules frites”). During the Second World War in the United States, mussels were commonly served in diners. This was due to the unavailability of red meat related to wartime rationing.
As with all shellfish, mussels should be alive just before they are cooked because they quickly become toxic after they die. A simple criterion is that live mussels, when in the air, will tightly shut when disturbed. Open, unresponsive mussels are dead and should be discarded. Closed mussel shells that are unusually heavy should be discarded as well, because they usually contain only mud and can be tested by slightly moving the two shells away from each other. A thorough rinse in water and removal of “the beard” is suggested. Mussel shells will open by themselves when cooked, revealing the cooked soft parts. (Wikipedia)