A type of bivalve with a more elongated, oval shell than a clam. Mussels are more common in European (especially Belgian) cuisines than in the U.S. They're often served simply, steamed in wine and aromatics, but they are also an essential component of bouillabaisse and paella.
Spotting the good ones: At the fish counter, use your eyes and your nose to guide you. Fresh mussels should look tightly closed or just slightly gaping open. Make sure their shells are closed or that they close immediately with a gentle tap. That's an indication that they're still alive. If they're yawning wide, they're dead or close to it. Once you have them in hand, take a sniff. They should smell like the sea. If they're really fishy smelling, don't buy them.
Discard any whose shells open prior to cooking.
Buy more than the quantity required, since you’ll likely have to discard a few that don’t open during cooking.
Just before cooking, look for any shellfish that have opened and tap them on the counter. If they don't close, discard them. Check closed mussels by pressing on the two shells in opposing directions. Dead ones will fall apart. Once you've weeded out the bad ones, scrub the remaining mussels under cold running water with a stiff brush to get rid of any grit. If the mussels have "beards"—black hairy fibers sticking out of their shells—pinch them and yank them off.
Keeping them fresh: Store in an open plastic bag (shellfish will suffocate in a sealed bag) in the refrigerator on a bed of ice in a large bowl or dish with sides. Refresh ice as it melts, and use within a day.