While many allergies first appear in childhood, an aversion to shellfish often crops up in adulthood. Along with peanuts and tree nuts, shellfish can produce some of the strong anaphylactic reactions that are considered most dangerous to people with food allergies. There are two kinds of shellfish: crustaceans, like shrimp, crab and lobster, and mollusks, like clams, mussels, oysters and scallops. People may be allergic to crustaceans or mollusks or both. Doctors generally recommend consulting a medical professional to help determine tolerance. Further, if you are allergic to shellfish, that doesn’t mean you will be allergic to bony fish.