Hi Paul,
Adult brine shrimp, which approach a half inch in length, are far too large
to be ingested by filter feeders.  Actually, brine shrimp themselves are
filter feeders, and the particle size they are capable of ingesting is
probably about the same size most bivalves would be capable of handling. If
you wanted to go to the trouble of buying dried brine shrimp eggs and
hatching them, the newly hatched larvae, which are about 0.4 mm in length,
might be suitable as food for bivalves. But they grow rapidly, and within a
couple of days would certainly be too large.  Pectens don't require animal
food, as far as I know, and I think it might be easier to sustain them with
a diet of algae.
Paul M.