In addition to the parasitic snails (Pyramidellidae), be sure to check for aiptasia anemones. They too like to hide within the scutes and won't become evident until the clam has been in place and undisturbed for a time. Another reason to place the clam in the sandbed. In my case I simply located the anemone, and covered it with superglue gel. The gel sets up quickly under water, effectively smothers the anemone, and is harmless to the clam (as long as you avoid the mantle, hinge, and byssal), and it's tank mates.
Should you choose a more light loving clam (T. maxima, T. crocea), I would also suggest placing the clam under your 250w halide.
HTH