its a myth that clams below size X need to be feed. this was started when Dr. Shimek wrote an article for DT's phyto, to sell phyto. if you read the article and then check the references he used, the references contradict the article.
clams do filter feed but are primary photosynthetic and are capable of sustaining them selves on light alone when they are just millimeters in length.
heres a study of larval clams that were kept in micro filtered water and not allowed to get any particulate, and they did just fine.
http://links.jstor.org/sici?sici=0006-3185(198110)161:2<213:SDAAOZ>2.0.CO;2-N&size=LARGE