I had a different type of predator. Originally, I thought I had some sort of bacterial infection, but actually I found they were these really tiny snail-like critters. They were about 3mm long by 1mm wide. The white shell had brown stripes.
The little b a s t a r d s would only come out at night. So I didn't know what was attacking my clams until one night I decided to take a quick peak with a flashlight. One of my T. derasas was under attack by at least 30 of these things. It was completely overwhelmed. I lifted the clam out of the water, and it basically dissolved as I lifted it out.
I've noticed that these things are more likely to attack my smaller clams. Indeed, they've been attacking all my clams. The mantles have tell-tale signs of their attacks. They look a little withdrawn and maybe as if a chunk had been bitten off. They killed off two of my clams already.
They actually like to coordinate group attacks. Don't ask me how. I pulled a stomatella off it's shell by accident, and it fell to the substrate. I walked away for about one minute. When I came back, that poor stomatella was engulfed by these things. These guys are like vultures... they didn't care about daylight in light of this easy meal.
These aren't like those little white snails. Those guys are nothing compared to these brown striped critters. I've been unsuccessful in removal them manually. But what I've noticed is that they cannot traverse over any SPS. So I moved/mounted all clams on top of or into a montipora capricornis. They've been fine ever since.
I've changed bulbs, performed water changes, never fed, and things are fine. They've been extremely hardy ever since I moved them to their "protected" sites. The mantles have grown back and the shells are growing fine.
So in my honest opinion, if a snail is going to go down, there has to be some predator involved. Keep your eyes open.
