It seems unlikely that fluctuation in water chemistry or temperature, or any
kind of toxic runoff problem could be so species-specific in what it kills.
Could viruses be responsible for some of these kills?  I don't know much about
invertebrate virology, but I know there are viruses that attack various
species of just about anything you can name, both plant and animal.  There are
even viruses that attack species of bacteria.  So I'm sure there must be a
number of them associated with molluscan species.  And most viruses are
extremely species-specific.
Paul M.