As someone with an allergy to shellfish it's actually really tricky to figure out what I can and cannot eat. Fried calamari does not seem to bother me, for example. Small portions of other things will make me very uncomfortable.
Our younger son developed an allergy to shellfish and these days can't eat anything made with fish sauce, either. I occasionally have problems with shrimp (which started with a bad reaction I had to some shrimp Creole many years ago) but I'm ok with lobster, crab and oysters. Never cared for the taste of clams.