[Oilers]

I have recently purchased a couple crocea clams and lost both of them today. I was placing them on the rock and noticed that they were knocked down to the sand a couple times. I picked them up and put them back on the rock. This morning, they were not at where I put them last night and I found them on the sand again but only the shells were left.
What would likely to knock the clams off the rock and what would have eaten them? If I get more clams in the future, what should I do to ensure their survival? Do I need to glue them on the rock? Thanks all for your help.