About 95 percent of oysters come from oyster farms, which tend to be very well managed. Unlike farmed salmon, which pollute the surrounding waters, oysters actually benefit their aquatic environment, and so the risk of pollution is very small. Eating them raw, however, does always carry some risk. Oysters can contain the bacteria vibrio vulnificus, which occurs naturally in warm coastal waters and can bring on those classic food-poisoning symptoms most of us would rather not have to envision right now.
Verdict: If cooked, no worries. As for raw oysters, they're generally safe, especially if you check each oyster before slurping it down to make sure it doesn't look or smell "off" (brackish odor, meat or liquor that's cloudy and brown, black, or red). But avoid raw oysters if you're pregnant or your immune system is compromised due to a medical condition.
Clams and Mussels
Like oysters, commercial clams and mussels are harvested from clean waters, whether farmed or wild. When purchasing clams and mussels, the shells should not gape open and should close if you gently tap them (unless you're buying soft-shell clams that don't totally close due to the neck that protrudes from the shell). Avoid any that have broken or damaged shells. And after they're cooked, the shells should open by themselves--don't shove a knife in there to pry them open, as you'll just be met with something that's been dead for who knows how long and therefore shouldn't go in your mouth.
Verdict: Fine to eat. And as with oysters, raw clams and mussels are okay for healthy individuals as long as the meat looks and smells fresh.
Shrimp is the most popular seafood in America. About 10 percent of the shrimp we eat comes from fisheries and farms in the U.S., which are held to strict environmental standards. The other 90 percent, however, is imported from Latin America and Southeast Asia, where environmental regulations are either lax or not enforced.
Verdict: Buy American. If the only shrimp you can find is imported from Thailand or Vietnam, just know that while they're likely totally fine, there's no guarantee these tasty crustaceans are being raised in an optimal manner.
Lobster and Crab
There's no real issue with lobster or crab meat. But you know that green stuff in the lobster, and that yellow "mustard" inside the crab? Also known as tomalley, this organ, which functions as both liver and pancreas, can potentially be contaminated with dangerous levels of the toxins that cause paralytic shellfish poisoning (PSP). And these toxins are not eliminated when cooked, and cause way more trouble (like possible death within two hours) than the aforementioned vibrio virus.
Verdict: Enjoy your lobster and crab, but steer clear of the green and yellow. Ick.