The terms “crustaceans” and “shellfish” are often confused and used interchangeably. However, they represent distinct classifications. Understanding their precise definitions clarifies their relationship and provides important insights, particularly for dietary considerations. This article aims to distinguish these groups and explain their connection.
Understanding Crustaceans
Crustaceans are invertebrates belonging to the subphylum Crustacea, part of the phylum Arthropoda. They possess a hard external skeleton, or exoskeleton, which they shed periodically as they grow (molting). Their bodies are segmented and often feature multiple pairs of jointed legs and two pairs of antennae.
Most crustaceans are aquatic, inhabiting both freshwater and marine environments, though some species have adapted to terrestrial life. Common examples include crabs, lobsters, shrimp, crayfish, barnacles, krill, and woodlice.
Understanding Shellfish
“Shellfish” is a culinary term for aquatic invertebrates with a shell or hard outer covering, consumed as food. It is not a strict biological classification, encompassing animals from different biological groups. This broad category includes two main types: crustaceans and mollusks.
Mollusks are soft-bodied invertebrates, often protected by a hard shell. Common edible mollusks include bivalves like clams, oysters, mussels, and scallops, which have two-part hinged shells. Cephalopods like squid (calamari) and octopus, despite lacking external shells, are also categorized as shellfish due to their aquatic habitat and culinary use.
The Relationship Between Crustaceans and Shellfish
The relationship between crustaceans and shellfish is straightforward: all crustaceans are considered a type of shellfish. “Shellfish” is an umbrella term including crustaceans, mollusks, and sometimes echinoderms like sea urchins. Therefore, while every crab or lobster is a shellfish, not every shellfish is a crustacean.
Crustaceans are a specific biological group falling under the broader culinary term of shellfish. This distinction highlights that “shellfish” is a practical descriptor for food, whereas “crustacean” denotes a biological lineage with shared anatomical characteristics.
Why This Classification Is Important
Understanding this classification has practical implications, particularly for food allergies and dietary restrictions. Shellfish allergies are among the most common food allergies in adults and can trigger severe reactions, including anaphylaxis. Approximately 2.9% of U.S. adults report a shellfish allergy, with about half experiencing their first reaction in adulthood.
Food labeling laws, like the Food Allergen Labeling and Consumer Protection Act (FALCPA) in the United States, mandate declaring major food allergens. Crustacean shellfish (e.g., crab, lobster, shrimp) are identified as major allergens and must be clearly listed on food labels. However, molluscan shellfish (e.g., clams, mussels, oysters, scallops) are not consistently required to be labeled as major allergens under the same regulations, a crucial distinction for allergy sufferers.
Beyond allergies, this classification is relevant for dietary laws, such as kosher regulations in Judaism. According to kosher law, aquatic animals are permissible only if they possess both fins and scales. Since crustaceans and mollusks lack these features, they are not considered kosher. This religious dietary guideline further highlights the importance of distinguishing between aquatic life types.