Morphological and taxonomic implications of the formation of colonies in Cyclostephanos and Stephanodiscus (Bacillariophyta)

The importance of diatoms in bioindication and palaeolimnology is significant. For these applications, and species identification in general, a sound understanding of morphological variability is essential. Some species of the centric diatom genera Cyclostephanos and Stephanodiscus can form long, chain-like colonies. In these genera colony formation is associated with the morphology of the spines, which hold the cells together. One objective of this study was to investigate other possible differences in the valve morphology of solitary and colonial cultures. In addition, all strains were sequenced to account for the possibility that solitary and colonial forms may be different species, using three different loci (LSU, rbcL, cox1). The results show that colony formation not only affects the spine morphology but also other morphological features of the valve. The current interpretation that C. invisitatus and C. delicatus are facultative colony formers is supported by the results of this study. The evidence furthermore suggests that C. incognitus is a morphotype of C. invisitatus. A similar morphological pattern leads to the hypothesis that S. hantzschii and S. binderanus could possibly be solitary and colonial forms of the same species, which is also in agreement with the molecular analysis. These findings have implications for the taxonomy of the group and its application and open up a way for future investigations of morphological changes due to colony formation in other groups and taxa.