Morphometric analyses of Staurosira inflata comb. nov. (Bacillariophyceae) and the morphologically related Staurosira tabellaria from north-western Russia

Abstract

ABSTRACTThe morphology and ontogenetic allometric trends of a rare diatom Fragilaria heidenii Østrup and the morphologically related Staurosira tabellaria (W. Smith) Leuduger-Fortmorel were compared using conventional and semilandmark-based geometric morphometric analyses. Fragilaria heidenii was studied in detail by light and electron microscopy using type material and recent samples from Lake Ladoga and Lake Ilmen (north-western Russia). The taxon is transferred into the genus Staurosira Ehrenberg as Staurosira inflata comb. nov. on the basis of its valve morphology. This taxon is characterized by the absence of rimoportulae, lack of perforated copulae, spines located on the interstriae, internal vola occlusion in the areolae and the features of the areolae and apical pore fields. Conventional morphometric analysis showed considerable overlapping of S. inflata and S. tabellaria in their frustule characteristics such as length, width, length-to-width ratio and striae density. Moreover, at later stages of the vegetative life cycle, S. inflata has a tendency to resemble S. tabellaria by its valve outline that makes it difficult to separate these two taxa. The geometric morphometric analysis revealed two shape groups corresponding to S. inflata and S. tabellaria that were separated by a clear gap. Semilandmarks representing shape of the middle part of the valve were primarily responsible for discrimination between species. Apart from differences in valve shapes, S. inflata and S. tabellaria also differed significantly in their ontogenetic allometric trajectories. Overall, our results demonstrate that the semilandmark-based geometric morphometrics is sensitive enough to distinguish species by their outlines, when traditional morphometric parameters are not able to discriminate them with confidence.