Hello Geoff,

The good news is - fish stomachs are not highly acidic like mammalian
stomachs.  Shells taken from the intestine, way back near the exit, are not
usually damaged.  Most fish in the market are already gutted though, aren't
they? (at least here they are).  Gastropods with tight-fitting opercs have
actually been known to pass through the digestive tract of fishes, and
emerge alive and apparently unharmed.

Paul M.