We forgot one other agency, besides algae & fungi, that undoubtedly
contributes to the dissolution of shells: the organisms that run the world,
namely, the bacteria. Shells must have colonies of bacteria on them -after
all, just about everything else in water does. And they may release acids
into their surroundings as waste products or to dissolve the shells they are
colonizing (this is how the oral bacteria damage teeth), perhaps to create
crevices for themselves or to get to the protein within the shell matrix.
 
We still haven't answered Roland's original question though: "how
long have these shells been in the bottles?"
 
A.