This is a quote from Marine Invertebrates by Ronald L Shimek, Ph.D.
These are suspension-feeding animals, and they need a lot of food to survive, particularly as juveniles. In nature, until their shell length exceeds about 4inches (10cm), the majority of their nutrition comes from feeding. When the clam is smaller, the volume of the mantle tissue is simply not large enough to provide space for enough zooxanthellae to keep the clam going. Even after that point, food in the form of phytoplankton or dissolved organic mater such as ammonium nitrate is necessary to supplement the production of algal photosynthate from the zooxanthellae. Fortunately, with the advent of phytoplankton food, feeding of the clams is relatively easy, but must be done on a regular schedule for the animal to thrive.
The clams we are talking about here are juveniles.