Taxonomy of Elodea spp

As an FYI to folks involved in identification and/or taxonomy of aquatics:

In their latest (Second) edition of Aquatic and Wetland Plants of Northeastern North America (2023, University of Wisconsin Press), authors Crow and Hellquist note the difficulty distinguishing between the two main species of the aquatic plant Elodea (Waterweed) found in our area:

“Differentiating our two most common species, E. canadensis from E. nuttalli, by vegetative specimens is very unreliable; accurate identifications require flowering/fruiting material. D. Les (pers. comm., 2020) found that fertile populations were needed to corroborate DNA information with reproductive morphology to confirm species; that DNA studies do confirm that there are two genetic lineages, but that hybrids detected by DNA analysis are not uncommon.”

In Britain, I’d call Elodea with short round-tipped leaves canadensis, but as leaves get longer and more pointy, it is harder to draw a distinction between canadensis and nuttalli. Does that match your experience?

Different identification guides use different characters. One claims the number of rows of transparent cells along the leaf margin is characteristic. You do sometimes wonder what are these distinctions based on? Has fully verified material been examined or is it just an author’s arbitrary decision on where to draw the species boundary?

We have had lots of threads arguing about lumping vs splitting.

I think that the authors previously used the same vegetative characteristics that you describe in differentiating between E. canadensis and E. nuttallii, which is pretty consistent in North American keys that I have seen. However, I think analysis of phylogenetic data has indicated that those vegetative distinctions are unreliable.