Because mussels are severely affected by pollutants, they are now being commercially farmed in "safe" waters. When conditions are optimal, these cultivated mussels are usually superior in quality to wild mussels. Although available year round, mussels are best and most plentiful from October through May; in late spring, their spawning season, they tend to be of inferior quality. In their natural state, mussels attach themselves to surf-washed rocks and spend half their lives submerged and half exposed to the air. Mussels are sold live, fresh (shucked), and cooked as well as smoked. You can also buy canned mussels. When you buy mussels, it is imperative to keep them alive--or cold--until you are ready to cook and serve them. Mussels, because they are sold live, offer specific signals of freshness: the shells should be tightly closed (pulling mussels from the shell destroys them), or should close tightly when the shell is tapped; don't buy mussels with open or cracked shells. Freshly shucked mussels should smell perfectly fresh, with no trace of ammonia or a "fishy" smell. A simple criterion: pulling mussels or opening their shells kills them and they should be discarded.
Live mussels can be stored in the refrigerator, covered with wet kitchen towels or paper towels. Don't put them in an airtight container or submerge them in fresh water, or they will die. You can freeze shucked raw mussels in their liquid in airtight containers. They should keep for two months in the freezer. To prepare mussels for cooking (most commonly cooked without pulling mussels from their shells), scrub the shells and rinse under cold running water. Pull the stringy "beards"--the fibrous dark tufts protruding from the shells. When it comes to cooking mussels, the trick is to heat them sufficiently to destroy harmful organisms but not so long as to make the flesh too tough. This requires careful monitoring, as mussels can be toughened by just seconds of overcooking.