It's a crustacean.
There are 2 kinds of shellfish - crustaceans & mollusks.
Lobster, shrimp, prawn, crab & crayfish are crustaceans relatives of insects and characterized by having claws. The other group of shellfish, the mollusks, include clam, cockles, oyster, mussels, scallops, snails, squid, octopus & calamari. If you are allergic to crustaceans you may not be allergic to mussels and vice versa.