You are correct, it most certainly can (and does).  However, this happens when
the decaying animal rests upon the exterior of the shell.  This can happen
"out-of-the-water," or when you leave a shell soaking in still water in a
container at home, but is extremely unlikely in a natural, watery habitat.  First
of all the predators and scavengers will have the shell cleaned out long before
decay begins.  Next, the natural action of the water moving over the shell, or
the shell rolling around the bottom, flushes and washes it clean or the acids of
decay away if the scavengers haven't done their job.
 
Marlo
 
ParkAver wrote:
 
> Someone set me straight if I am wrong, but I thought the decay of the dead
> mollusk also could somewhat dull the nacre or the translucency of the shell.
> I've noticed this on some species that have had to soak for a while in fresh
> water to remove the rest of the tissue. Or is this a reaction to the soaking
> in fresh water? Or maybe a reaction to the fact that I was brain dead for two
> weeks and forgot about them in the garage til the smell put back the electro-
> chemical impulses into my cranium?    Scott