I'll have to disagree with Helmut on this.  We have plenty of ponds in this
area that dry up every year, sometimes for well over a year, and yet when it
rains and they fill up, the common pond snail is back.  There are some of
these ponds where I have watched this take place that are small (1 meter by
2 meters by 75-100mm deep) depressions in sandstone.  They are typically dry
and our normal rains merely get them a bit damp and then the sun drys them
quickly.  Yet when we get a good strong thunderstorm that hits that area
(maybe once a year or two) and fills the dpression there will be adult pond
snails in these little temporary pools within a day.  There are no plants
and the closest permanent water is miles away.  So I think at least some
species of mollusks have great powers of aestivation.  By the way, these
small depressions are also un-shaded and the temperature gets unbelievably
hot in the summer as the sun beats down on the rock.  Of course we have
other small ponds of water -- some permanent that never show any snail life
due to the high levels of arsenic and/or alkaloids and salts.

Tom Eichhorst in New Mexico, USA (where we have had two days of very strong
winds which means tomorrow I'll be clearing tumbleweeds out of the backyard)