Ohhhh. Somebody who speaks really good Italian could probably answer this. What I did in Venice was manage to ask for 'no fish' at the restaurant, and I received a wonderful dish. I have no clue what it was, right? But I didn't die, and I can't do shellfish.
Part of that will depend on your allergy level, of course. I can eat something that's been made in a shellfish dish. I always carry my industrial strength antihistamine with me just in case, but I can live. I don't have an anaphylactic reaction, thank heaven. I think the level of your allergy may dictate what you're comfortable eating.
I have found that I travel that while I like (and can eat) many types of fish, it's often safer in other countries where my language skills are poor simply to say - no fish and put my hands to my throat. It's worked every time so far.