About 7 million Americans are allergic to seafood, both finned fish and shellfish.  Shellfish allergies can cause severe reactions, including anaphylaxis.  The allergy is typically life-long and is first experienced as an adult.   There are two kinds of shellfish:  crustacea (such as shrimp, crab, and lobster) and mollusks (such as clams, mussels, oysters, and scallops).  Shrimp, crab, and lobster cause most shellfish allergies, and the reactions to crustacean shellfish tend to be particularly severe.   Some people who are allergic to one group of shellfish might be able to tolerate shellfish from the other group.  However, to prevent a reaction, strict avoidance of shellfish and shellfish products is essential.  Finned fish and shellfish do not come from related families of foods, so being allergic to one does not necessarily mean that both should be avoided.  Salmon, tuna, and halibut are the most common kinds of finned fish to which people are allergic. 

A common question from people who are allergic to fish is if omega-3 fatty acids from fish oil are safe to consume.   Since the allergy is caused by proteins, it would follow that fish oil should be safe.  However, if fish oil is not handled and manufactured correctly, there is a risk of cross-contamination and exposure.   So most experts recommend that people allergic to fish avoid fish oil omega-3 fatty acids and instead consume spirulina, or vegetable omega-3s from flaxseed, pumpkins seeds, walnuts, and other sources.