Interesting to observe that the mantle is separate from the soft parts of the
animal. It can be retracted or extended without the animal moving at all. You
see this most often on brooding females that are disturbed by just touching
the shell with a small stick. The mantle will retract but the rest of the
animal does not move.

In my observations the animal does not retract into the shell, even when
threatened, unless it is knocked off it's perch. The mantle was retracted,
but the animal did not retract unless moved or touched.

Most of these were shells in the inter tidal zone, so I can't speak for
shells in deeper water. If I remember correctly the several Cyp cernica that
I found in shallow water seemed braver than the other shells I observed.

Frank in Massachusetts (where it doesn't seem know what it wants to do
weather wise. But the Sox are in 1st place)