Although it's often grouped with finned fish, shellfish comes from a different family. Just because you have an allergy to one family doesn't mean you will to the other. However, doctors recommend being tested for both at the same time anyway. Testing involves a skin prick test or a blood test.
The shellfish family encompasses not only crustaceans like crabs, shrimp and lobster, but also mollusks like oysters, squid and mussels. As with any seafood allergy, reactions might include hives, asthma, atopic dermatitis or anaphylactic shock.
Shellfish allergies usually don't show up until adulthood. Nonetheless, they stand as the most common food allergies among adults [source: Wood].