Not that I am aware of, if they do they will be of such minute quantities as to be of no use, if I see any empty shells I crush them up so that they can help a little with drainage, but I,d rather not see them, particularly when occupied, Derek.

The shells of snails, clams, oysters etc are mostly composed of calcium carbonate with only a small quantity of protein less than 2 percent.

calcium in itself probably helps to a degree .