Shellfish

What a shellfish allergy is

“Shellfish” is an everyday word covering two groups of animals that are not closely related, and the distinction matters more here than for any other allergen because the labels are drawn along it.

Crustaceans are the hard-shelled, jointed ones: prawns and shrimp, crab, lobster, langoustine, crayfish and krill. The protein most often involved is tropomyosin, which is heat-stable, so cooking does not defuse it, and it is also what makes reactions across different crustacean species common.

Molluscs are the soft-bodied ones: bivalves such as clams, mussels, oysters and scallops; cephalopods such as squid and octopus; and gastropods such as snails and abalone. Mollusc tropomyosin is a different enough protein that many people react to one group and not the other.

The EU and UK declare these as two separate allergens. The United States declares only the crustaceans. That asymmetry is the single most important thing on this page, because it means a US label can be completely compliant while giving no allergen warning for squid or oyster sauce.

The names that do not say “shellfish”

Most crustacean aliases are simply the animal under a regional or menu name. Scampi in Britain usually means langoustine tails in breadcrumbs, though in the US “scampi” often describes a garlic-butter prawn dish. Langoustine, Dublin Bay prawn and Norway lobster are the same creature. Crawfish, crayfish and écrevisse are one animal with three names. Krill is a small crustacean, and krill oil is sold as an omega-3 supplement.

Shrimp paste is the alias most likely to be invisible. It is fermented, intensely savoury, and fundamental to South East Asian cooking, appearing as belacan in Malaysia, terasi in Indonesia, kapi in Thailand and bagoong in the Philippines. It is inside curry pastes, sambals and dipping sauces where no English word suggests it.

Oyster sauce is the mollusc equivalent — a staple of Chinese stir-fry sauces and marinades, and commonly present in ready meals and takeaway dishes. Vegetarian mushroom versions exist and are labelled as such.

Calamari is squid. Escargot is snail. Abalone appears as a dried ingredient in Chinese broths and sauces.

Surimi is made from white fish, not shellfish, but crab sticks are routinely flavoured with crab extract or crustacean powder, so the product that looks like crab may in fact contain some.

Glucosamine is usually derived from crustacean shells, as is the thickener and coating agent chitosan.

How the label is worded where you are

In the EU and UK both categories must be emphasised inside the ingredient list, normally in bold, so belacan has to declare crustaceans and oyster sauce has to declare molluscs. There are no named exemptions for either category — unlike fish, milk or soy, every crustacean- and mollusc-derived ingredient is declarable.

In the United States, crustacean shellfish must be named in plain English in the ingredient list or a “Contains” line, and the species is identified — shrimp, crab, lobster. Molluscs fall outside the rule entirely. Elsewhere there may be no emphasis convention at all, and a paste or sauce name in the local language may be the only trace.

Cross-contamination and “may contain”

Shellfish cross-contact is overwhelmingly a kitchen problem rather than a factory one. Shared fryers, shared grills, shared steamers and shared tongs in seafood restaurants, sushi counters and takeaways do most of it, and shellfish stock is often the base of a soup that contains no visible shellfish. Fishmongers cut and display crustaceans and molluscs on the same ice. In packaged food, precautionary wording such as “may contain crustaceans” is voluntary and unstandardised in most jurisdictions, no phrasing is legally safer than another, and the absence of a warning is not evidence that no shellfish came near. Loose and counter-sold items carry no ingredient list at all. PNut reports any such wording as May contain, and what “may contain traces” actually means explains why that verdict cannot be sharpened any further.

Reading a shellfish label abroad

Abroad you need two words rather than one, because the two categories are printed separately. Crustacés and mollusques, Krebstiere and Weichtiere, crostacei and molluschi, 甲壳类 and 软体动物. Beware the catch-all terms: fruits de mer, marisco and frutti di mare mean seafood generally and may cover either group or both, so they are a prompt to read further rather than an answer. The table below gives the two category words plus the commonest individual animal for each language, and reading a label in a language you don’t speak covers how to read the list they appear in.

How PNut flags shellfish

Scan the barcode and PNut looks up the product’s published ingredient list; photograph the ingredient panel and it reads the text off the label instead, in any of 40+ languages. It checks crustaceans and molluscs separately, along with every menu name, paste and derivative on this page, then gives one of four verdicts — Safe, Contains allergen, May contain, or Unknown when the label cannot be read. An unreadable or incomplete label returns Unknown rather than a guess.