Some people are allergic to one type of shellfish, crustaceans or mollusks, while others are allergic to both. Most people seem to be allergic to crustaceans only, and the reactions to these foods tend to be more severe. They can include vomiting, diarrhea, stomach pain, shortness of breath, coughing, chest pain, and swelling of the face, lips, tongue, or throat. An allergy can worsen over time, so everybody should check in with their nutritionist for further evaluation and testing.