mystery plant from new jersey

[<envisible at earthlink_net>]

alright, i've finally decided to appeal to the wisdom of the list. i found
this plant growing in a pond in allaire state park in new jersey and i can't
figure out what it is. here's the pics, sorry for the quality. 

http://home.earthlink.net/~envisible

the only thing i can find that remotely resembles it is maybe a callitriche
species?
except for the thin elongated leaves i would say it is very much like
hemianthus micranthemoides in manner of growth. it is a rather small and
delicate stem  plant, two leaves per node opposite to one another, leaves on
consecutive nodes perpendicular. same color green as hygrophila difformis,
underside of leaves also silvery. no red color whatsoever. tolerates low light
well. can be trimmed down easily with branching growth. i am cultivating it as
a low foreground plant. any ideas or is it obvious?

elie