CUC Question/ Problem?

I have been getting snails and small hermits from my LFS. I have added about 15 or so hermits and about 15-20 snails. Over the last few weeks I have notices a few of the hermits are now sporting new "rides." A few are now in snail shells and I see the small hermit shells they discarded on the sand bed. Are the hermits killing the snails for the shells or are the snails dying and the hermits are moving in to the vacant shells? I still have active snails I see on the glass and rocks.