What’s the difference? Shellfish, crustaceans and mollusks

Crustaceans and shellfish and mollusks — oh my!

Shellfish allergies are among the most common in the world, with countries around the globe implementing regulatory guidelines for their labeling. Interestingly, it’s an allergen that often manifests itself in the food-allergic person’s adulthood, with 60% of those allergic experiencing their first reaction after the age of 18.

There are a lot of creatures that fall under the umbrella of “shellfish,” however, and a person might find themselves allergic to some but not others. Today, we’ll take a minute to look at how this category is divided.

Crustacea

The crustaceans group includes commonly eaten seafood like shrimp, crab and lobster. Also included: prawns, crayfish, krill and barnacles (Yes, some people do eat barnacles!).

Though their name, crustaceans, refers to their hard crusts or shells, not all creatures with shells and exoskeletons fit into this category. True crustaceans usually have segmented, split limbs or appendages and develop from larvae.

Mollusks

Mollusks are soft-bodied invertebrates that exist within hard calcium carbonate shells. There are around 100,000 species in this category, including snails and slugs. The ones we eat, like in the crustacea category, come from the sea: clams, mussels, scallops, oysters and cockles, among others. Technically, squids and octopi are cephalopod mollusks, though they don’t have shells.

You can be allergic to crustaceans but not mollusks, and you can be allergic to mollusks but not crustaceans. You might only react to certain kinds of crustaceans and mollusks, or you can be allergic to species from both types of shellfish — about 15% of shellfish-allergic people are. The uniting, allergy-causing factor here is the protein tropomyosin, which is used in muscle movement.

Interestingly, there might be a connection to dust allergies and shellfish allergies. Dust mites are like crabs in that they are arthropods and have tropomyosin proteins. Some people with noted crustacea allergies also develop dust allergies and vice versa.

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