Shellfish is an umbrella term for any aquatic invertebrate with a shell, spanning two main groups: crustaceans and mollusks. A few less common animals, like sea urchins, also qualify. The term is culinary rather than strictly scientific, which is why it covers creatures as different as shrimp, oysters, and squid.
Crustaceans
Crustaceans are the joint-legged, hard-shelled animals most people picture when they hear “shellfish.” They belong to the same broad animal phylum as insects and spiders. The ones you’ll find on menus include:
- Shrimp and prawns: The most widely consumed shellfish worldwide, spanning dozens of species across several families.
- Crab: Blue crab, king crab, Dungeness crab, snow crab, and stone crab are among the most popular varieties.
- Lobster: American (Maine) lobster, spiny lobster, and slipper lobster are distinct species with different flavor profiles and price points.
- Crayfish: Freshwater relatives of lobster, central to Cajun and Scandinavian cuisines.
- Barnacles: Less common in American kitchens, but percebes (gooseneck barnacles) are a prized delicacy in Spain and Portugal.
Mollusks
Mollusks are soft-bodied animals, many of which grow a protective shell. This group is broader and more varied than crustaceans, splitting into three culinary subcategories.
Bivalves
Bivalves have two hinged shells and filter water for food. They rank among the most commercially important shellfish in the world. This group includes oysters, mussels, clams, scallops, and cockles. Because they’re filter feeders, bivalves are the shellfish most associated with absorbing marine toxins from algal blooms.
Gastropods
Gastropods are single-shelled animals. Snails (escargot), abalone, conch, periwinkles, and whelks all fall here. They’re less commonly eaten in the U.S. but are staples in French, Caribbean, Korean, and West African cooking.
Cephalopods
This is where the definition surprises people. Squid, octopus, cuttlefish, and nautilus are all mollusks, which places them in the shellfish family even though they have no visible external shell. Squid have a thin internal structure called a pen, and cuttlefish have an internal “bone,” but octopus have lost their shell entirely. Despite looking nothing like a clam, they share the same phylum and, critically, some of the same allergenic proteins.
Echinoderms and Other Outliers
Sea urchins and sea cucumbers belong to a third animal group called echinoderms. They’re technically classified as shellfish by some definitions, and sea urchin roe (uni) is widely served in sushi restaurants. Sea cucumbers are a valued ingredient in Chinese cuisine. These animals are spiny-skinned rather than shelled in the traditional sense, but they’re invertebrates harvested from the ocean floor, which puts them in the shellfish conversation for both culinary and allergy purposes.
Why the Groupings Matter for Allergies
Shellfish allergy is one of the most common food allergies in adults, and the distinction between crustaceans and mollusks is more than academic. The primary trigger is a muscle protein called tropomyosin, which is found across nearly all shellfish species. Because tropomyosin is so structurally similar from one species to the next, someone allergic to shrimp may also react to crab, lobster, clams, or even squid.
That said, crustacean allergy is far more common than mollusk allergy, and the two groups don’t always cross-react. Research published in Frontiers in Allergy found that antibody levels against shrimp tropomyosin explained roughly 35% of the immune response to clams and about 50% of the response to crab. In practical terms, this means cross-reactivity is real but not guaranteed. If you react to shrimp, you have a meaningful chance of reacting to crab and a lower but still notable chance of reacting to clams or oysters.
Interestingly, tropomyosin also exists in dust mites and cockroaches, which is why some people with shellfish allergies also test positive for dust mite sensitivity. The proteins are evolutionarily related.
Nutritional Differences Across Shellfish
Shellfish are generally high in protein and low in fat, but their micronutrient profiles vary dramatically. Clams are an exceptional source of vitamin B12, delivering about 99 micrograms per 100-gram serving. For context, the daily recommended intake for adults is 2.4 micrograms, so a single serving of clams provides more than 40 times that amount. Shrimp, by comparison, offer about 1.7 micrograms per serving.
Zinc content also varies. Oysters are famously rich in zinc (often cited as the single best dietary source), while shrimp contain a more modest 1.6 mg per 100 grams. Both clams and shrimp provide similar amounts of omega-3 fatty acids, around 275 to 280 mg of combined EPA and DHA per 100-gram cooked serving.
Mercury and Contaminant Levels
One of the practical advantages of eating shellfish over large predatory fish is their low mercury content. FDA monitoring data from 1990 to 2012 shows that scallops averaged just 0.003 parts per million (ppm) of mercury, clams and shrimp averaged 0.009 ppm, and oysters averaged 0.012 ppm. For comparison, swordfish averages around 0.995 ppm.
Crustaceans sit slightly higher. Crab averaged 0.065 ppm, and American lobster averaged 0.107 ppm. These levels are still well below the threshold that warrants limiting consumption, making shellfish as a group one of the safer seafood choices for mercury exposure.
The more relevant safety concern for shellfish, particularly filter-feeding bivalves, is marine biotoxins produced by algal blooms. These toxins cause four distinct types of shellfish poisoning: paralytic (from saxitoxin), diarrheic (from okadaic acid), neurotoxic (from brevetoxins), and amnesic (from domoic acid). Commercial shellfish are monitored for these toxins before sale, but wild-harvested shellfish carry more risk, especially during warm-water algal bloom seasons.
Quick Reference: What Counts as Shellfish
- Crustaceans: shrimp, prawns, crab, lobster, crayfish, barnacles, krill
- Bivalve mollusks: clams, mussels, oysters, scallops, cockles, geoduck
- Gastropod mollusks: snails (escargot), abalone, conch, periwinkles, whelks
- Cephalopod mollusks: squid (calamari), octopus, cuttlefish, nautilus
- Echinoderms: sea urchins, sea cucumbers
Fish with fins and bones, no matter how they’re prepared, are never classified as shellfish. The dividing line is simple: shellfish are invertebrates, while fish have a backbone.