Dont rightly know mate...Would imagine they are just filter feeders, catching anything that goes by..But suppose if they are massive, they must need more that just filtering...
They rely on filter feeding / small particle capture for food. Phytoplankton feedings (DTs Phytoplankton or home grown) are commonly used to keep them alive and well.
in my personal opinion, all those dry filterfeeder products are crap. even a lot of the liquids. many filter feeders (especialy mollusks) are very very very picky about the size food they injest and will often die of starvation if you are feeding them the wrong type of food. most of the powerdered products are too small because they are mearly pieces of the actual plankton. feather dusters are much less picky, so what you will may work, but the live stuff would be better. it would also pollute your tank less than already dead stuff.