Molecular-assisted alpha taxonomy, which combines molecular species delimitation with post-hoc morphological examinations, has proved to be an effective tool for the classification of morphologically similar species. We employed this approach to examine the diversity of the genus Frustulia in northern Europe. First, we used two molecular markers to delimit species and then characterized their morphology using conventional and geometric morphometrics. Next, we employed machine-learning methods to identify valves in benthic diatom communities in order to infer the distribution and pH preference of individual species. Unlike previous studies using automated identification of diatom species, we examined the performance of the semi-supervised classifier. Supervised methods, which have been used before, only employ labelled valves to train the classification algorithm. The semi-supervised approach is, in addition, able to benefit from unlabelled valves in the natural populations. It is usually superior in cases in which there are few labelled data available. Finally, we compared the classification accuracy of the algorithms and five volunteer specialists. We found five molecular lineages, the F. crassinervia-saxonica species complex, F. gaertnerae, F. septentrionalis, F. krammeri, and F. cf. maoriana. The most valuable characteristics for species identification were length, width, striation pattern, and allometric shape changes described by the first axis in the geometric morphometric analysis. We found that a semi-supervised approach that does not rely solely on the morphology of isolated cells, but also accounts for variation among valves from natural populations, has superior performance. Based on valves from natural populations, we observed marked differences in species abundances and pH tolerances that have a bearing on their geographical distributions.

During studies of the diversity of the diatom genus Frustulia in Europe and New Zealand we recognized two species that did not fit any existing description. These species are described in this paper as F. curvata sp. nov. and F. paulii sp. nov. based on examinations of the morphology of natural populations and isolated strains. A combination of morphological features enables discrimination of F. paulii and F. curvata from previously described taxa. Frustulia curvata was found at a single locality in Ireland and is probably rare. Frustulia paulii has been identified from wetland pools in both the South and North Islands of New Zealand. The two new species are compared with similar taxa and our current knowledge of their ecology and distribution is discussed. These formal descriptions complement the results of a separate study of the molecular diversity of the genus Frustulia, which confirmed distinct lineages for both species. GenBank entries for published sequences are included in the descriptions. In the future, both frustule morphology and DNA sequences can be used to test presence of these species elsewhere in the world and examine their ecology and distribution in detail.

Irregularities in cell division can produce asymmetry in symmetric structures, such as outlines of diatom cells, which can reflect genetic, environmental, or random variability in developmental processes. This study examined 12 phylogenetic lineages of the diatom genus Frustulia using landmark-based geometric morphometrics to assess the variation between cell segments separated by apical and transapical axes. Although asymmetric variation within cells differed in some lineages, these irregularities most likely did not reflect the evolutionary history of the lineages. The intraclonal phenotypic plasticity of diatom frustules was induced rather by nongenetic factors, i.e., inherited valve abnormalities, constraints of siliceous cell walls, and random developmental instability during morphogenesis. The positive correlations between the symmetric and asymmetric components of shape variation indicated that the morphogenesis of diatom cell walls affected irregularities within cells and variability among the symmetrized cells to a similar extent.