Eh. I like snails and all but i definitely don't ever want another pond snail outbreak. And that's what you usually end up getting off plants.
Usually you can get some Trumpet snails or even ram's horn snails for free from a lfs or petsmart if you want them. I just got both types that way.
Some insects can survive freezing, not saying that daphnia can, but i can't figure out where else they could have come from. If i could nail that down i would get a colony going for my upcoming spawn. Unfortunately all the ones i had got eaten once i cleared the cover in that tank.
My one Petsmart i got the trumpet snails from had a tank taken over by some really freaky looking white worms.