Maxx,
Couldn't have said it better
Great info.
"Pinched Mantle" IMO as well as others, do not think this is a pathogen but parasitic problem. I have seen this with some clams and did not spread to others in the same tank. Some good success has been done with FWD but I would not recommend this but as a last resort. IME, it take several days for the clam to recover and if the speciment is weak to begin with, some will not survive the dip.