That maybe true for and 11 inch clam, but not for a 1 to 2 inch clam. Completely different here. Baby clams do not get much of their food source from light at this stage.
Adult clams will however get food from the light, but IMHO they should still be fed Phytoplankton whether it is home cultured or purchase to maintain their overall health and survival.
With proper care clams can live for a very long time in the aquarium.
Just my thoughts,