A customer with a shellfish allergy must avoid crab because it is classified as a shellfish, which includes both crustaceans and mollusks. These types of seafood are known to cause allergic reactions in some individuals. Crab, alongside other shellfish like shrimp, lobster, and oysters, contains specific proteins that trigger allergic responses in sensitive individuals.
On the other hand, options like tuna, mackerel, and salmon are categorized as fish, which belong to a different category altogether in terms of allergies. While some people may have allergies to specific types of fish, this is distinct from shellfish allergies. Therefore, the safest route for someone with a shellfish allergy is to avoid all types of shellfish, which includes crab.