Andy,

> What kinds of changes must be made in a mollusk to let it
> live in fresh water?

Since there are no terrestrial bivalves, freshwater bivalves must have come
directly from the sea. The two routes I can think of are estuaries & inland
lakes that were once part of a sea. Animals that invade freshwaters via
estuaries must be euryhaline, that is they must be able to tolerate wide
variation in salinity. On the other hand, in a lake cut off from the sea,
salinity would fall slowly & some of the marine species surviving in the
lake may be able slowly become adapted to the lower salinities.

You may want to read Colin Little, "The Terrestrail Invasion. An
ecophysiological approach to the origins of land animals" Cambride U. press,
1990. As it is obvious from the title, the book is about terrestrial
animals, but some of the arguments could apply to freshwater systems also.

Aydin