Plants of the genus Aponogeton are strictly emersed and can not live out of water. They evolved in Australia then spread all over the world, fossils have been found in Peru, Labrador and Greenland; then migrated back to Australia, leaving species all the way there and back.
Aponogeton distachyos, described in 1792 is the foundation species and exists in the oldest fossils found in Peru, Greenland and Canada. More recent species have smaller and more complex flowers.
A. distachyos flowers are sold canned in South Africa where they are one of three ingredients in a regional lamb stew.
The only sure method of distinguishing this species from Azolla filiculoides is to examine the trichomes on the upper surfaces of the leaves. Trichomes are small protuberances that create water resistance. They are unicellular in A. filiculoides but septate (two-celled) in A. cristata