Abstract
An unknown heteropolar Staurosirella taxon was observed in a survey of Lake Pavin (Massif Central, France) with peculiar marginal spines that could not be identified using the currently available literature. Additional populations of the same taxon were observed in samples from Iceland and Flanders, Belgium. The unknown taxon is described as a new species, named Staurosirella pavinensis, based on detailed light and scanning electron microscopy observations of all populations. This new species is characterised by heteropolar, slightly clavate valves, with a narrow foot pole and a more broadly rounded head pole. The marginal spines are individually located in a small depression. Only at the foot pole, a small apical pore field is present, whereas the headpole is typically depressed. A detailed comparison with morphologically similar species is provided together with notes on its ecology, based on the analysis of the associated diatom flora and physico-chemical data.