Allergy to shellfish: crustaceans, molluscs, and the hidden sources in restaurant food
By the Menulta editorial team · Facts checked 15 October 2026 · Updated 15 October 2026
The short answer
An allergy to shellfish is an immune reaction to proteins in crustaceans, such as prawns, shrimp, crab, lobster and crayfish, or in molluscs, such as mussels, clams, oysters, squid and octopus. EU law treats crustaceans and molluscs as two separate allergens. Shellfish allergy is often reported as the most common food allergy in adults, frequently starts in adulthood, and usually lasts for life. It hides in stocks, sauces, paella, risotto, fish soups and Asian condiments.
Crustaceans and molluscs are different allergens
Everyday English uses 'shellfish' for both groups, but Annex II of Regulation (EU) 1169/2011 lists them separately: crustaceans (entry 2) and molluscs (entry 14). Some people are allergic to one group and not the other, and some to both.
| Group | Examples | Annex II entry |
|---|---|---|
| Crustaceans | Prawns, shrimp, crab, lobster, langoustine, crayfish, krill | No. 2 |
| Molluscs: bivalves | Mussels, clams, oysters, scallops, cockles | No. 14 |
| Molluscs: cephalopods | Squid, calamari, octopus, cuttlefish | No. 14 |
| Molluscs: gastropods | Snails, whelks, limpets, abalone | No. 14 |
Why shellfish allergy matters in adults
The main crustacean allergen, tropomyosin, is very stable: it survives boiling, frying and grilling. That is why cooked prawns are as allergenic as raw ones. Tropomyosin is also found in house dust mites and cockroaches, which may explain why some people develop shellfish allergy as adults without having eaten much shellfish.
Reactions can be severe. Some highly sensitive people react to cooking steam or vapour from boiling shellfish, so seating a shellfish-allergic guest near an open kitchen boiling prawns can be a real issue.
The iodine myth
A shellfish allergy is not an iodine allergy. The reaction is to proteins such as tropomyosin, not to iodine. This matters mostly in medicine, but it also comes up in kitchens: iodised salt and seaweed are not shellfish, although some seaweed snacks and seasonings contain shrimp or fish, so check their labels.
Where shellfish hides in restaurant food
- Seafood stock: the base of many risottos, paellas, soups, sauces and some rice dishes.
- Fish soups: Maltese aljotta is fish-based, and some versions add prawns or mussels.
- Bisque and seafood sauces: usually made from crustacean shells.
- Asian condiments: shrimp paste (belacan), some fish sauces, XO sauce, dried shrimp.
- Prawn crackers and some seasoning mixes.
- Surimi ('crab sticks'): mainly fish, but often contains crab extract, egg and wheat.
- Mixed fried seafood: shared fryer oil transfers proteins between calamari, prawns and everything else.
- Garnishes: a prawn or crayfish on top of a fish dish, or squid ink in pasta (a mollusc product).
Serving a guest with a shellfish allergy
Ask which group the guest reacts to: crustaceans, molluscs or both. Then check the allergen record for every component, including stocks and sauces made in advance. Fried food is a common failure: if calamari or prawns go through the same fryer as chips, the chips are not safe for a shellfish-allergic guest.
Use clean pans and utensils, keep the guest's dish away from shellfish on the pass, and be honest if the kitchen's seafood volume makes cross-contact impossible to rule out. Seafood restaurants should consider whether they can safely serve guests with severe shellfish allergy at all, and say so clearly rather than improvise.
Questions to ask before service
Shellfish allergy questions are often about the base of a dish rather than its visible ingredients. Before service, the kitchen should know which stocks, sauces and soups contain crustaceans or molluscs, which fried items share oil with seafood, and whether any condiments, such as fish sauce or XO sauce, contain shrimp. Record these in the allergen record per dish so front-of-house staff can answer without guessing.
Keep the two groups separate in the record. Writing 'shellfish' hides whether a dish contains prawns, squid or both, and a guest allergic to only one group loses the ability to choose.
Seafood platters and shared dishes
Platters, mixed grills and sharing plates mix crustaceans and molluscs with fish and other foods on the same board, under the same juices and dressings. A guest with a shellfish allergy should not eat from a shared platter, even the items that seem unrelated, because juices and garnishes travel across the plate. Offer an individually plated dish prepared separately. Use clean utensils and keep it away from the seafood station on the pass.
Sources
This guide is general information, not medical or legal advice. Guests with an allergy should follow their doctor's advice. Operators should confirm their own setup with their food safety adviser or local authority.