Morphological variation in Eunotia serra, with a focus on the rimoportula

Several populations of Eunotia serra Ehrenberg were examined from the eastern and central USA, revealing significant morphological differences from the different sites investigated. Populations from some sites have two rimoportulae per valve, as opposed to other sites where populations have one rimoportula per valve. This last observation raises the question about whether the number of rimoportulae is an important characteristic to use for distinguishing species of Eunotia Ehrenberg. It also raises interesting questions about the phylogenetic relationships between species of Eunotia and other taxa in the Eunotiophycidae D.G. Mann, especially with respect to the significance of rimoportulae in the evolution of the raphe.