What Are Signs of a Bad Oyster?

Oysters are enjoyed by people all over the world. When fresh, the rough-shelled mollusks have a clean, briney taste full of the flavors of the ocean. However, when oysters have gone bad, not only are they foul-tasting; they can be deadly. There are a few signs you should look for to tell if an oyster has gone bad.

A truly bad or rotten oyster will have a noticeable odor. The odor will be similar to rotten eggs, sulfurous and offensive. This is the strongest indicator that the oyster was already dead -- or died -- shortly after it was harvested. The smell may not be very strong, but if it is noticeable, you may want to use caution and consider not eating the oyster. Smell the oyster before topping it with any other accompaniments, which may mask an odor.

When live oysters are harvested, their shells are tightly closed because the muscles in the oyster are working and keeping the shell closed. When an oyster's shell is already open before someone uses a tool to manually open it, that is a sign that the oyster is dead. If the shell is partially or completely open, be cautious. Also, an oyster's cracked or noticeably broken shell is a bad sign because the oyster is likely dead or contaminants could have entered through the crack. The open shell indicator is only useful if you are personally inspecting unopened oysters; if the oysters are pre-shucked and out of the shell, then you must rely on the other two signs.

The oyster meat itself and the surrounding liquor (the juice within the oyster) may also show signs of contamination. A good oyster is firmly attached to the shell and must be loosened with an oyster knife. If the oyster has detached from the inner walls of the shell by itself, meaning an oyster knife was not used to loosen the oyster, then it either died before or after it was harvested. The meat of the oyster should appear plump, translucent and somewhat shiny. If the meat looks dry, shriveled and discolored, it is likely rotten and should be discarded. The liquor should be clear or somewhat cloudy and should smell somewhat like seawater. If the oyster liquor is milky or mottled and has a strange smell, the oyster is no good.

Oysters are filter feeders, which means that they feed off of the waste and byproducts of the ocean waters. If the waters are contaminated with toxins, then oysters will absorb large concentrations of the toxins. Spoiled and contaminated oysters will pass these toxins on to humans who consume the oysters. Toxins such as saxitoxin and brevetoxin and bacteria such as Vibrio vulnificus can cause serious, permanent injury to a person and can even lead to death.