Generally speaking, bivalves should be purchased alive, since these creatures decompose exceptionally quickly once dead, even when properly stored on ice and refrigerated. Most of the qualities listed below indicate whether or not the animal inside the shell is still living.
Things to Bear in Mind When Buying Clams, Oysters, or Mussels
- In their raw state, these bivalves should feel heavy for their size.
- Like all seafood, these should smell of the ocean — briny and sweet, like seaweed — and not off-putting or "fishy" in any way.
- If shellfish are prepackaged in mesh bags, ask to open up the bag to get a better look, as it's tougher to tell the condition of the shells when bagged.
- Shells should be tightly closed, with no chips or cracks present. An open shell indicates that the creature is already dead (and will have begun to decompose). Once ready to prep or eat raw, sharply tap any that are slightly ajar; if alive, shells should close — and if any don't, make sure to discard. Likewise, once cooked, the shells should open up slightly — this indicates that the shellfish was alive when cooked — any that stay closed should be discarded.
Keep reading for guidelines for purchasing scallops.
What to Know About Scallops
Scallops differ slightly from their bivalve brethren, as they are typically sold already shelled, but if you can find scallops still in their shells, that's your best bet.
- Flesh should be translucent with a white cast and firm (never shriveled).
- Follow the same odor guidelines as outlined above, but also avoid any that smell like iodine; this indicates rot.
- Buy "dry" scallops — this means they haven't been brined, which is sometimes used as a preservation method, and will keep the scallops looking white and plump longer. "Wet" scallops will not sear or brown properly.
- Diver scallops are the best quality, as well as being the most ecologically sound: divers harvest scallops by hand, which disturbs the aquatic environment less than other harvesting methods. Day boat scallops are also a good choice, as this indicates that they were harvested on a boat that makes daily trips in and out of the harbor (rather than some fishing boats that can be out to sea as long as 10 days at a time).
For an excellent guide on the sustainability of fish and shellfish, check out the Monterey Bay Aquarium's Seafood Watch. It's even downloadable as a PDF and available as an iPhone and Android app.