There are many snails that looks just the same as Pyramid Snails that prey on Clams. Some are harmless herbervoirs free living snails. Others preys on snails. Some even prey on Vermetid snails.
From my observation, these snails are very specific regarding their preys. Snail parasites do not prey on Clams.
You should have your clams on the sand for at least the first month or two. During this time periord, you should check the clams for parasitic snails and remove them. I also break all the scales off my clams initially so I can removes all the snails. After a few weeks of doing this, all the parasitic snails should be gone. and you can leave the clams along and move them to where ever you want them.
Pyramid Snails of a differece species also infested snails. They are one of the major reason for snails to died in our tank. Every body look at the shell of you Turbo. Chances are you will find some there. In the past, before I know to look for these, my snail would all died within 2 years. When I wise up, I removed 20+ parasitic snails from one of my larger Turbo. I now have snails that live as long as three years and still going strong.
I also observed Parasitic snails that attack my Vermetid snails on the live rock. I got lots of Vermetid snails and wanted to keep the number down so they won't spin their web everywhere. Because of this reason, I do not remove the parasitic snails that infest these Vermetid snails. As far as I can tell, these there types of parasitic snails and the hunreds of free living tiny snails in my tank looks identical to the naked eyes. They don't cross infected. I have being Clams parasitic snails for several years. I also have been almost parasitic snails that infected Turpo snails free for a long time, but still have lots of other snails all over my tank.
I would not consider a snail a parasitic snail and sentence him to death unless i see him cloes to the mantel of my clams. I have not kill one from my tank for years, although I allways clean and remove all the snail that often come from newly bought Clams, even from first rate vendors.
For small tank, the only method of control is manual removal. In large tank, the only wrasse that I experence taht works well is the Harlequne Tuskfish.
I hope I have not being too long winded in my reply, but I think I mention all the relevence information on these parasitic snails as I experienced it.
Minh Nguyen