from the minute they are born clams are fully self sufficient on light and proper water conditions. feeding is not absolutely necessary.
picking out a good clam:
check the mantle, see if there are bad damage from fish nips
check the mantle, see if there is extra holes in the mantle itself
check mantle extension, see if the clam is extending itself fully
check photo reaction, place hand over clam and see if it retracts forcefully
check incurrent siphon (the slit), check to see if is gaping badly
check the clam shell, check for pyramid snails right under mantle area in the shell and the bysuss area