All of those clams you've pictured are rock boring clams, and they shouldn't be placed directly ontop of the sandbed. You would be best off placing them high up on the rockwork, where they will be able to receive ample amounts of lighting. T. crocea the strongest byssal gland of the tridacna clams, thus it's important to place them in an area which they can attach their byssal gland to -- the rockwork.
Graham