Biologically speaking, shellfish are aquatic invertebrates rather than fish. They can be divided into four main groups:
e.g. crab, lobster, crayfish, shrimp, prawn
e.g. mussels, oysters, scallops, clams, paua, pipi, tuatua, toheroa
e.g. limpets, periwinkles, snails
e.g. squid, cuttlefish, octopus, kina
People who have reacted to one type of shellfish (e.g. crab) are likely to react to other members of the same group (in this case, crustaceans). If you react to crab, avoid that and the rest of the crustacean group. If you react to squid, avoid that and the rest of the cephalopod group. Shellfish from the other groups may not necessarily present a problem, although they may do for some people. It is advisable to avoid them all anyway. A special reason for being cautious is that there is a relatively high risk of cross-contamination among different types of seafood.