i agree that clams of that size are best to purchase, because they can handle shipping and other stresses better but not about there need for phytoplankton. shortly after clams metamorphasize (days) there mantles are fully developed and full of zoox and they can support them selves on light alone. check out this study of juvi clams that were kept in micro filtered water and never received any particulate matter, just light.
http://links.jstor.org/sici?sici=000...2-N&size=LARGE