Bristle worms can and will attack clams. They will enter the clam from below, through the bysal opening. That is why some people say not to put clams on sand, as this is an invitation to the bristle worms in the sand. Make sure you know what clams need. Most species of Tridactna clams require intense light, and will only do well under metal halide lights at least 250 watts. Water conditions need to be perfect, and calcium levels need to be over 400 ppm. Smaller clams need supplemental feeding, so beware of them. Larger clams feed on the sugars produced by the algae that live in the mantle, and they have to have the proper spectrum and intensity of light to do well.
Good luck,
Sonny