Guys, as a person that sells phytoplankton it would be in my finincial interest to tell you clams under X size have to be fed phyto to live. Not only would I be wrong in saying so, I'd be flat out lying since I have seen the latest studies on baby giant clams.
ChrisANdBarb is more up on the studies then I. You really should listen to what he is saying. He's very connnected in the giant clam world, just head on over to Clams Direct and you'll see him offerring advice on bahalf of CD as a moderator.
While old studies and books may have printed one thing current science has shown the opposite.
While feeding baby clams isn't a bad idea IMO, it's not a requirement. I've kept baby maximas with and with out phyto. I got better growth on the phyto feed, but the non grew and thrived
Most clam farms do not feed phyto to their baby giant clams. One farm I've seen in detail filterred all water prior to the livestock section. While some phyto may ahve gotten thru, the clams basicly grow on sunlight alone.