Most mussels found in markets today are 'cultivated' meaning that they are raised in a controlled environment and are much less likely to be muddy or to sport "beards," the wiry growths that protrude from the shell. If the mussels you buy have beards-you'll find them sticking out from between the shells about halfway down the flat side of the mussel-simply give the beards a good, firm tug to remove them. In any case, it's a good idea to rinse the mussels in a colander under cold water before cooking them.