Fish, molluscs and crustaceans: allergen advice for caterers

Our allergen series looks at all things from the sea – the watch-outs and easy substitutions

A seafood allergy is one of the most common in adults but can also affect children. It can start at any time in your life and you can react to a seafood you’ve had before when you might previously have had no reaction.

An allergy to seafood is one of the more serious as it can potentially cause life-threatening anaphylaxis. So it’s definitely worth swotting up on the more unusual ingredients seafood is hiding in.

Seafood splits into fish and shellfish, and those with shells can be molluscs or crustaceans. If someone is allergic to fish, it doesn’t necessarily mean they will also have an adverse reaction to shellfish, and vice versa.


A backbone is what defines fish as fish – a vertebrate. They also often have scales and fins. Most types of fish are easily identifiable on menus and in food, however, there are a few watch-outs, especially anchovies which can be hiding in sauces.

Look out for:

  • Caesar salad dressing which often contains anchovies
  • Worcestershire sauce also contains anchovies, even though it doesn’t have a fishy taste
  • Anchovies can also be used in stew or sauce bases to add a salty, umami element
  • Isinglass is a gelatine type substance that comes from fish bladders and is used to clear and brighten alcoholic drinks, such as beer. Not everyone who has a fish allergy would react to isinglass as it is often the flesh of a fish that people are allergic to. However, in some severe cases, a reaction could happen. More and more brewers are moving away from using isinglass but it is always worth checking if drinks that you are selling contain it. Some jelly-based desserts might use isinglass as a setting agent
  • Surimi is a processed product that is sometimes used as a cheaper alternative to crabmeat. It often contains several types of fish. Look out for it in processed products such as ready-made meals or pizzas
  • Avoid frying food in the same oil as fish or shellfish as it can present a risk to allergy sufferers.
Download and print the second part of our allergen guide


Molluscs are invertebrates which means they have no backbone. They have soft bodies and are often covered by a shell. Some have a shell that opens and closes – bivalve molluscs. Examples of molluscs include mussels, octopus and squid.

This type of seafood is often the star of a dish so it’s easy to identify but there can still be hidden forms in sauces so be vigilant.

Look out for:

  • Oyster sauce – as the name suggests, this does contain real oysters. It adds flavour to many Asian dishes.
“If you buy products imported from Asia, allergens should be labelled correctly for the UK but always check. Look out for nam pla or nuoc mam which are other names for fish sauce.”
- Les Mohammed, Senior Trading Controller, Unitas Wholesale


Similarly to molluscs, crustaceans are invertebrates but they have a segmented body and jointed legs. Crabs, shrimp, lobster and prawns fall into this category, among others.

Like molluscs, crustaceans generally form a central part to a meal but they are regularly used in other ways, especially in Asian dishes.

Look out for:

  • Shrimp in Asian foods. Dried shrimp is commonly used in many Thai curry pastes and sauces such as sambal.


Seafood substitutions

Simulating that signature fishy taste and aroma can be hard to replicate but there are several ways you can offer customers that same salty, umami-esque flavour.

Fish sauce is a pungent flavour in Southeast Asian dishes. Although not a direct swap, you can use soy sauce or tamari instead. The latter has the extra bonus of being suitable for customers with wheat and gluten allergies or intolerances.

A mushroom and soy broth can give a similar flavour to fish-based sauces and stocks. Coconut aminos is a salty, savoury seasoning to look out for as well.

Learn more about the upcoming allergen law change in October 2021.

Stay up-to-date with the latest allergen legislation at the Food Standards Agency website.

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