Chirality: an informative and underutilized morphological feature of raphid diatoms (Bacillariophyceae)

Chirality, the inability to superimpose an object on a mirror image of itself, is an underutilized feature in diatom taxonomy. Many different groups of diatoms display chirality in valve outline, raphe path, striae orientation, as well as proximal and distal raphe end orientation. These patterns appear to reflect mitotic processes and valve deposition and therefore may offer highly conservative features useful for understanding phylogenetic relationships at the genus level or higher. Unfortunately, many authors have unawarely failed to accurately depict the orientation of chiral features, and the orientation of these features as presented in the literature cannot be taken at face value. Here we use the genus Neidium to identify specific chiral valve features, as well as the significance and variability of those features. We also discuss how these features have been misrepresented in the literature. We also review all genera currently recognized on diatoms.org to identify the presence of chirality in other genera. It appears that all Neidium taxa with oppositely deflected proximal raphe ends display levorotatory chirality. Chirality is most abundant in the monoraphids, where many groups seem to display at least genus-level trends. More research is required to understand the taxonomic utility of chirality, as published images are not a viable avenue for investigating trends in chirality.