For simplicity, all occurrences of mosquito fern in the Boise Front are dumped into what is currently known to be the most common species regionally, Azolla filicoides.  It is, however, quite possible that other species are also present, such as A. mexicana and A. caroliniana, but the distinguishing morphological characters can only be properly observed at high magnification.  Furthermore, the most reliable characters are found only on the reproductive megaspores, which only present during a brief part of the life span.  A recent widespread genomic study of the genus in California (Song et al. 2025) shows the situation may be even more complicated; instead of the two species currently recognized in the state, only A. filicoides was among the three clades that showed up in the results.  The other two clades could not be confidently identified, and could potentially indicate either undescribed or non-native species.

https://www.inverse.com/article/47027-azolla-small-fern-big-impact-climate-change

Song, M.J., C. M. Tribble, I S. Gonzalez-Ramirez, F. Freund, F.-W. Li, and C. J. Rothfels.  2025.  Landscape genomics and demography of California Azolla.  Madrono 72: 162-168.