Mussel farming is becoming a popular business. They are raised on ropes, which keep them from the silty bottom, thus making them cleaner and more salable. When grown in this manner, they are also twice the size of ordinary mussels.
If a shell isn't tightly closed, test to see if the mussel is alive. Thump the shell with your fingers or insert the tip of a knife. The shell should close immediately. Discard any mussels that stay open or have cracked shells.
To clean mussels, scrub with a stiff brush, then pull off the beards or remove them with a sharp knife or kitchen shears. Place the mussels in very cold water and rub the shells together to remove any remaining debris. Or scrub with a vegetable brush.