These are called Pyramid snails...They will attack the clam at the foot & suck juice from the clam....They will eventually kill the clam if they go untreated...(manual scrubbing of shells to remove eggs & also removing the suckers on the bottom near the foot...)If your diligent you can erradicate them...if you don't remove them they will overpopulate & eat the clam alive!!!
The big clams are not affected as much cause they are so large...the PS can't make a dent on them!!!