The best way to choose a live fish or crab or lobster is to look for, well, life. Is it scampering around in its tank? Swimming happily? Or is it sulking in a corner or hanging motionless and panting? If so, don’t buy it. Lobsters and crabs starve themselves in tanks and often can be almost empty inside when you crack open one that’s been imprisoned in a tank for weeks.

Your best bet is to make friends with the fishmonger and find out when the new shipments arrive. Plan on being there to meet it and buy then. You will be rewarded for your extra effort

Buy only at the finest fish markets. These are the places where turnover is so rapid or their dedication to quality is so high that you can be assured of fresh mussels, clams or oysters. You may still get a dead one, but the ratio will be far lower. If you have any questions about the shellfish origin they should be able to tell you where it came from, and in mussels, clams and oysters they should be able to produce a tag of origin.

What is a dead shellfish? Shellfish are sold alive, so they should react to you. Put them on the countertop and back away for a moment. Then tap the shell: If it opens at all it should close tighter than it was when you tap it. Oysters are a little tough to do this with, but clams and mussels will definitely react. If they are open before cooking and do not respond to tapping, toss them. You can also tell a dead shellfish after you’ve cooked them all. Dead ones do not open after being cooked. Throw them away.

Scallops, a Special Case

Scallops are almost always sold shucked, so what you are looking for are “dry packed” scallops, meaning they are not shipped and stored in brine. Better to buy frozen, vacuum-sealed scallops, which are perfectly good by the way, than an inferior wet-packed scallop.Ask your fish monger about their scallops. If they don’t know the difference between a dry and wet scallop, you should go elsewhere.

Shrimp

This one is easy. Buy them whole and frozen, and if not frozen no more than a day thawed. Whole because the shell protects them from the rigors of being frozen without losing too much moisture, and frozen or recently thawed because shrimp cook – and rot – very rapidly.

Should you be near a shrimping region, or have access to truly magnificent fresh shrimp, by all means buy them. Head on if possible. Why? Because head-on shrimp stay moister. Remember: Nothing says boring like a dry, overcooked shrimp.

Everything I said about shrimp applies to crayfish, too. Unless you can get them live, in which case follow the instructions for lobsters or crabs

Squid or Octopus

These are almost always sold to the wholesaler pre-frozen, so you should buy them frozen. Both squid, commonly known as calamari, and its more richly flavored cousin the octopus freeze exceptionally well.

Again, if you can buy squid and octopus on the coast– not to mention cuttlefish – fresh, do it! They are rare treats even at fine fish markets and should be appreciated as such. Like finfish, you should look first at their eyes, which should be clean and bright.