Spotting the good ones: At the fish counter, use your eyes and your nose to guide you. Fresh hardshell clams and mussels should look tightly closed or just slightly gaping open. 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.
Keeping them fresh: Shellfish will suffocate in plastic, so take them out of the bag as soon as you get home, put them in a bowl, cover with a wet towel, and refrigerate. It’s best to cook them as soon as possible, but if they were fresh to begin with, they should keep stored this way for up to two days.
Cleaning them up: 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 shellfish 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.