Clams feed by filtering water thru their gill structures and sifting out the edible particles. Think of the gills as a sieve with a given "mesh size", so for instance, if the mesh has 80 micron holes in it, anything over this size will become "caught" in the gills and consumed, while smaller particles will pass thru the gills and out the clam's excurrent siphon. Different critters are different sizes, so there's a "size range" that the clam in question is able to consume.