Conch is a gastropod, a very large marine snail, so it is shellfish in the mollusc sense rather than the crustacean sense. That matters on a label. EU rules require molluscs to be declared as their own allergen, but US labelling law only compels crustacean shellfish to be named, so conch can appear in a chowder or a fritter without a shellfish warning on the front of the pack. If your diagnosis is crustacean allergy, conch is a separate question for testing, not an automatic no and not an automatic yes.

It depends which shellfish. Snails are molluscs, so they sit with mussels, clams, oysters, squid and abalone rather than with prawns, crab and lobster, which are crustaceans. The two groups are often lumped together on menus but allergy to one does not automatically mean allergy to the other. Molluscs do share tropomyosin as a major allergen among themselves, so if you react to mussels or squid, snail is a genuine risk. If your allergy is specifically to crustaceans, snail is less predictable and worth discussing with an allergist rather than testing over dinner.

Moderate, not high, and the number moves with handling rather than with the oyster. An oyster is alive until it is shucked, which is unusual among foods and keeps histamine low right up to service, so a shuck-to-order oyster is about as low as shellfish gets. Once open, the meat and its liquor start accumulating amines like any other seafood. Tinned, smoked and pre-shucked oysters, and anything held in a tub for a day, are the versions that cause trouble for histamine intolerant people.

For a healthy adult the odds of a serious Vibrio vulnificus infection are low, and most raw oyster illness is unpleasant norovirus rather than anything dangerous. The picture changes sharply with certain conditions. Chronic liver disease raises the rate of vibrio illness after raw oysters by orders of magnitude, and among people who do develop a V. vulnificus infection about one in five die. If you have liver disease, haemochromatosis, cancer treatment, diabetes or any immune suppression, the sensible line is cooked oysters only. The old rule about eating oysters in months with an R in them is a rough seasonal proxy for water temperature and not a safety system.

It often can. Cephalopods discolour quickly once dead, so processors treat them to keep the flesh white, and EU food additive rules explicitly permit sulphur dioxide and sulphites on fresh, frozen and deep-frozen cephalopods up to 150 mg/kg. Our data grades squid low-moderate for sulfites for exactly this reason. Packaged squid has to declare sulfites above 10 mg/kg, so read the label. At an unpackaged counter, ask whether the squid has been treated, and if nobody knows, treat it as if it has been.