Analysis of the type material ofAchnanthes hauckianaGrunow (Achnanthaceae, Bacillariophyceae)

The type material ofAchnanthes hauckianaGrunow has been investigated using light and scanning electron microscopy. The species, transferred to the genusPlanothidium, was frequently reported from brackish and even marine environments. The original Grunow material was collected from a karstic spring near Trieste. Analysis of the material and of samples collected from the Croatian Krka River, resulted in a better understanding of the morphology ofPlanothidium hauckianumand a better characterization of the diatom flora of which it is a dominant component. The paper compares the species with similarPlanothidiumspecies from theP. delicatulumcomplex and revises published images ofP. hauckianumin literature, making it clear that all reported and illustrated brackish populations ofP. hauckianumbelong to different, often unknown species. The presence ofPlanothidium hauckianumis limited to karstic spring areas, most likely showing a very restricted biogeographical distribution.