I recently came across two articles that dealt with geographic range
expansion in marine snails.  It seems that those with tight-fitting
opercula can survive predation by birds and fish by sealing themselves off
from the digestive juices in the stomach of the predator.  Apparently,
many snails are regurgitated alive or come out the other way unscathed.
The authors of both articles suggested that this may be a mechanism for
dispersal.  Bird eats snail.  Bird flies to a faraway place.
Bird regurgitates snail in different place. You get the idea.
 
Greg Herbert