"Ordinary" snails won't usually bother them. But the ones that Radha posted are parasitic and may come in on (in) the snail. The sort of eat it from the inside out - often boring up through the clam, entering from the bottom around the foot.
Do you have any angels in the tank? They will often pick at clams.
Do you monitor your calcium? Clams are calcium hogs.
Also, how large (or small) was the clam? Usually the smaller ones need to be fed with phyto, more so than the larger ones.
Jenn