The deep blue sea might be teeming with delicacies, but seafood and shellfish are the most common causes of food allergies. Reactions include hives, eczema, asthma, digestive problems, and anaphylactic shock. Shellfish allergic reactions can be triggered either by mollusks (clams, mussels, and oysters) or by crustaceans (shrimp, lobster, and crab). Common seafood allergens are scaly and bony fish like cod, salmon, trout, and halibut. It's not uncommon for a person to be allergic to only one seafood species, which makes avoidance a bit easier. Still, anyone with this type of allergy should beware — fish by-products can lurk in foods, medicines, cosmetics, and lotions, so read labels carefully.