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I have seen some comments to the effect that salt may have anti-caking
additives which may include gluten (this was in reference to salt in
restaurants).   Food processing industries are notorious for adding
numerous additives to improve processing, so it seems reasonable to suspect
that the salt added to the canned oysters and sardines may have anti-caking
ingredients added.
 
One should never, of course eat seafood from a can that either bulges or
fails to give a good "hiss" of air sucking IN when it is punctured!