Hi Lisa,
I think the clam died, could be old clam age,
lack of food, water too warm, and then was eaten
by the critters whose job it is to clean up the deceased - fish, snails, zoo
plankton. The snails would munch on the shell for the minerals. As for the
missing snails, fish will eat them if they can fit them in their mouths, birds
if they can get at them and turtles.
The snails also could have gone on walk about. I had a japanese trapdoor that
made it all the way into the compost area. Some aquatic snails have to come to
the surface to breath and get 'lost' as it were and don't make it back to the
water.

k30a