Hi Wes,
Well I don't have an in-depth understanding of this pnenomenon, but I
can mention a few observations.  First, our "splash zone" is
relatively devoid of molluscan life.  Our mollusks stay in the ocean
where they belong.  No Siphonaria clinging to the rocks above high
tide level, no Neritina scooting up tree roots, or Littorina doing
arboreal gymnastics.  Just about everything lives below mean high
tide level.  If you go to the beach at high tide, you generally won't
find a living mollusk exposed.  Therefore, every mollusk gets
submerged for at least a few hours every day.  Although I never
really thought it out before, I suspect your question is a key to why
this is so.  In winter, the splash zone above the high tide level can
be covered by a thick layer of dense ice, which would entomb anything
living there.  However, the intertidal area never develops any
appreciable ice buildup because it is submerged several hours a day
in sea water which is, obviously, higher in temperature than the
freezing point of sea water.  The salt in sea water has some
protective value, reducing the freezing point from 0 degrees C (the
freezing point of fresh water) to about -2 degrees C (from +32
degrees F to about +28 degrees F), and some intertidal mollusks can
have higher salt concentrations in their tissues than in the
surrounding water.  Also, some intertidal gastropods tend to stay low
on the sides of rocks, close to the substrate in winter, rather than
crawling on top of them; or they congregate in deep fissures or tidal
pools; and some may migrate to a lower area of the intertidal zone.
Presumably all these bahaviors are aimed at having a longer total
time of submersion.  Also, some of them remain under dense growths of
rockweed.  Whether that has any insulative value, I have no idea.  So
there are some mechanisms that help such animals survive harsh winter
conditions.  Still, when there is a sudden drop to unusually cold
conditions, especially sub-zero conditions (that's sub-zero
Fahrenheit, about -18 Celcius), many intertidal mollusks do in fact
die.  Sometimes there are massive localized kills of various species.
 One time after a freeze, I saw a drift line about 4 feet wide,
several inches deep, and a quarter of a mile long, composed almost
entirely of Crepidula fornicata (slipper limpet) shells.  Another
time, hundreds of thousands of Mytilus edulis (blue mussel) washed
in.  And another time, a beach was completely covered with Pecten
irradians (bay scallops), all gaping open with the frozen soft parts
still inside, and every one of them with its hinged side facing into
the wind, which was substantial!
Yes, we do have a fair number of species here, if you know where to
look.  But nothing comparable to the tropical zones.  The number of
species, diversity of form, and range of colors of mollusks in the
tropics far outweighs what we have here.
Rhode Island isn't exactly Arctic conditions (thank goodness!).  We
usually have snow only three months or so.  Maybe one of our more
northern correspondents could add something to this.  Ross - are you
there?
Regards,
Paul