Molecular and morphological data reveal hidden diversity in common North American <i>Frustulia</i> species (Amphipleuraceae)

<i>Frustulia</i> Rabenhorst is an established diatom genus that is common and widespread across North America. As for many diatom genera, taxa within <i>Frustulia</i> have been the subject of taxonomic confusion. Although recent studies have examined species from Europe and New Zealand, there exist no detailed genetic data for North American individuals. Using both molecular (<i>rbcL</i> and 18S rRNA sequences) and morphological (i.e., frustule characters and shape analysis) data, we investigated common taxa of <i>Frustulia</i> in North America. European and New Zealand taxa were also included to investigate how North American species were related. We recognized nine taxa from North America in this study including two unknowns. A new species, <i>Frustulia gibsonea</i> sp. nov. is described. <i>Frustulia gibsonea</i> was observed in previous studies and described as <i>F.</i> cf. <i>krammeri</i> based on morphology. The use of molecular characters in this study demonstrates that this taxon is a distinct species, and not a morph of <i>Frustulia krammeri</i>. Despite differences in <i>rbcL</i> and 18S, <i>F. gibsonea</i> and <i>F. krammeri</i> are quite similar morphologically, showing overlap with both traditional measurements and shape analysis. This suggests that the combination of molecular and morphological data can help in deciphering these morphologically cryptic taxa. <i>Frustulia saxonica</i> and <i>F. krammeri</i> could be weakly separated based on sequence divergences, although in this case morphological characters were more useful. As evidenced by the low sequence divergence values obtained between the two taxa, they are closely related. Future molecular research, focusing on less conserved markers, will be necessary to resolve the relationships of these taxonomic complexes.