i have three...50% of the time i dont seen them...its a good thing...thats the way u tell there not dead is if they stay down in the sand...(gravel might be to tough for them to move through...yes they move...a lot)...they are great filterers and u will occasionally see them moving about and maybe even show up on the surface...no need to worry about feeding them they get everything from filtering water through there bodies...because of their movement u will often see depressions in the sand from where they were digging...its a great means to keep the sand turned...because of this be sure all plants, *rocks*,and ornaments are anchored down
the thing that sucks with clams is if u have to medicate your tank...you either need to move the infected fish out of the main tank into a hosp. tank or dig for clams as most medicines will kill the clams because of it passing through their bodies...if u get some that pop up out of the sand and have their clam open and if something sudden goes by them they shut instantly...
if ya have any more questions about them let me know