For picking mussels, you best tools are your eyes and nose. The mussels should be tightly closed or just barely open. If they are wide open, they are dead or close to it; pass those by. Ask to handle a couple and take a sniff. If they smell like the sea they are nice and fresh. If they smell fishy, they're past their prime. Pass those by as well.
To store those nice fresh mussels, take them home and get them out of the bag they are packed in. Any live shellfish will suffocate in plastic. Just simply place them in a bowl and cover them with a damp towel then pop the bowl in the fridge. If they are fresh when you pick them, they will last for 2 days stored this way. You'll want to cook them as soon as possible to make sure you eat them at their peek.
To get mussels ready for cooking, place them in a colander and rinse them under cold water. You'll want to give them a good scrub with a brush to get all the sand and grit off. You might notice a weird wiry bit hanging out of or off the shell. This is the "beard". Remove that bit with a pair of scissors. If any of the mussels have opened wide, discard those.
When they are cooked, the mussels will open wide. If any are still closed when the dish is done, throw those out.
Never cooked with mussels? Here is a quick, easy dish that can be used as an appetizer for up to 8 or a main dish for 4 (it's pretty easily doubled or halved).
Mussels with Cilantro Cream Sauce
4 lbs. mussels, cleaned and debearded
2 Tbsp olive oil
1 small poblano pepper, diced (can substitute jalepeno for more spice)
3 cloves garlic, crushed or minced
Zest and juice from 1 lime (can substitute with lemon)
1/2 cup white wine (can substitute with water, veggie or chicken stock)
1/2 cup heavy cream (can use whole milk in a pinch)
1/3 cup fresh cilantro, chopped (no substitutes here :O)
Salt to taste
Heat oil in a large pan over medium heat. Add poblano pepper and saute, stirring occasionally until it's soft (about 2 minutes or so). Add the garlic and zest and cook until you can smell the garlic (about 30 seconds or so). Pour the wine in and turn the heat up to high. As soon as it comes to a boil, add the mussels, cover and let steam for 5 minutes or so. You might want to give the pan a shake now and again.
Turn the heat off and use a slotted spoon to remove the mussels to a large bowl. Cover it with foil or a lid to keep them warm (discard any still closed mussels now).
Turn the heat back on to medium/medium high and add the cream to the pan juices. Bring to a boil, then reduce the heat to medium and let the sauce reduce for about 3 minutes.
Turn off the heat and quickly add about 1 Tbsp of lime juice, a pinch of salt and the cilantro. Try the sauce and adjust to taste, adding more salt or lime juice if needed. To add a bit more spice, you can throw in a dab of your favorite hot sauce here as well.
Place mussels in bowls (number depending on if you are serving as appetizer or main course) and ladle sauce over the top. Serve immediately. If serving as a main dish, serve plain or over a small serving of fettucine noodles with a slice of crusty Italian bread.
This dish also works well with littleneck clams, or a mixture of seafoods! Add the clams first, let steam for 3 minutes, add the mussels and let steam 3 more minutes (or until they are all opened up). Add large raw shrimp (deveined and peeled) in when the sauce is reducing. Add calamari rings after the heat is turned off, when you add the cilantro. Any combination is delicious!