There was an article in Coral magazine a few months ago about the species of snails that prey on clams. I don't think I still have that issue, perhaps someone can point you to the online version so you can read up on these vermin.
Those are pyramidal snails and NOT a good thing. If you have that many, you also have eggs, and the cycle is going to continue.
The article Slice is referring to can be found in Coral Magazine's March/April 2011 Issue. It's called "Vampires of the Sea".
Certain wrasses do help with the issue, but I'd do some google searches. There's differing opinions on what works best.
Go to first thread link here.
"Natural predators of Pyramid snails include several species of wrasse within the genus of Halichoeres and Pseudocheilinus. A few of the more popular species that are reportedly very efficient at controlling these snails are; Six and Eight Lined Wrasse (P. hexataenia and P. octotaenia), and the Green Wrasse (H. chloropterus)."
Pyramidellae are the bane of a clam keeper's existence. There are wrasses that will eat them, but unfortunately, the snails come out at night when the fish are sleeping, so they aren't 100% effective. At this point, manual removal is all you can do. Remove the clam to a bowl or bucket of tankwater and go over it with a magnifying glass to remove any snails. Check beneath the mantle and the shell margins, and especially the byssal area ("hinges"). You'll also want to scrub any egg masses from the clam with a brush.
One problem is that these snails often live in the substrate, so you'll need to make this a nightly activity until you eradicate them.
One more thing...if you have snails in the tank, check them as well...some Pyramidellae are reported to be species specific, but some aren't so picky.
A friend of ours wondered why he couldn't keep clams...