Shellfish allergy: two groups, declared separately

Shellfish is not one category. Crustaceans and molluscs are separate allergens under European labelling law, and many people react to one and not the other.

The two groups

Crustaceans are shrimp, prawn, crab, lobster, crayfish, langoustine and krill. Molluscs are mussel, clam, oyster, scallop, squid, octopus, cuttlefish, snail and abalone. Both appear among the fourteen allergens that must be emphasised on a European or British label, and they are listed as two separate entries precisely because they are two separate allergies.

Crustacean allergy is the more common of the two, and tropomyosin is the protein usually responsible. It is among the most persistent food allergies, much less often outgrown than milk or egg, and among the more common causes of severe reactions in adults. Cross-reactivity with dust mite and cockroach tropomyosin is well documented, which is occasionally how someone discovers the allergy in the first place.

The condiments are the problem

Shellfish is used across East and Southeast Asian cooking as a source of savoury depth rather than as a visible ingredient. Oyster sauce is in a great many stir fries and marinades. Belacan and terasi are fermented shrimp pastes fundamental to Malaysian, Singaporean and Indonesian cooking, and they are inside sambal, rojak, laksa and countless curry pastes. XO sauce contains dried scallop and dried shrimp. Many fish sauces include shrimp alongside the anchovy.

A dish can be described as vegetable and be built on one of these. This is also the reason a restaurant kitchen is a harder environment than a packet: the same wok and the same sauce jars serve every dish on the menu.

The non-food sources

Glucosamine supplements, widely taken for joints, are usually manufactured from the shells of crustaceans. Chitosan, sold for weight management and used in some food processing, has the same origin. Some calcium supplements are made from oyster shell. None of these are food products and none carry food allergen labelling, so they are easy to miss.

Whether shell-derived products actually provoke reactions is debated, since the allergenic protein is in the flesh rather than the shell and purity varies. That uncertainty is itself the reason to raise them with a clinician rather than decide alone.

One myth worth dispelling

Shellfish allergy has nothing to do with iodine, and having one does not mean reacting to iodinated contrast dye used in medical imaging. This was believed for decades and is still repeated, including occasionally by clinicians, but the allergen is protein and iodine is an element present throughout the diet. Anyone who has been told to avoid contrast imaging on these grounds should ask again.

Names worth knowing

Shellfish worth recognising on sight, with the other names each appears under. The condiments at the end are the ones that arrive without announcing themselves.

Shrimp

BelacanPrawnShrimp PowderShrimp PasteDried ShrimpTerasiKapiBagoongHae BeePrawn Paste

Crab

Crab MeatCrab ExtractCrab Paste

Lobster

Lobster MeatLobster Extract

Crayfish

CrawfishYabby

Krill

Krill Oil

Mussel

Green Mussel

Clam

CockleLala

Oyster

Oyster Extract

Scallop

Dried ScallopConpoy

Squid

CalamariSotong

Abalone

Oyster Sauce

Oyster Extract Sauce

A selection, not the full list.

Check a whole label at once

Recognising these by eye is the hard way to do it. Photograph the ingredients panel and NoshWise reads every line against this profile, names what to avoid, and says why.

Scan a label

Common questions

Are crustaceans and molluscs the same allergy?

No. They are listed as two separate allergens on European and British labels because they are separate allergies, and many people react to one and not the other. Crustacean allergy is the more common.

Are crab sticks safe with a shellfish allergy?

Crab sticks are surimi, made from white fish rather than crab, so they are often not a crustacean risk. Shellfish-derived flavourings are sometimes added, so the ingredients list still needs reading, and they are a genuine risk for a fish allergy.

Is glucosamine made from shellfish?

Usually yes. Most glucosamine supplements are manufactured from crustacean shells, and supplements do not carry food allergen labelling. Shellfish-free versions made by fermentation are available.

Does shellfish allergy mean I react to iodine contrast dye?

No. This is a long-standing myth. Shellfish allergy is a reaction to protein, not to iodine, and it does not predict a reaction to iodinated contrast used in medical imaging.

Does oyster sauce contain shellfish?

Yes, it is made from oyster extract, and it appears widely in stir fries, marinades and sauces. Shrimp paste, sold as belacan or terasi, is likewise inside many sambals, curry pastes and laksa.