Guide to Geissleria

Geissleria valves are elliptic to linear-elliptic, becoming obtuse, broadly rounded or rostrate at the ends. The striae are comprised of fine lineolate areolae; areolae are 50-80 in 10 µm.

Annuli are present at the poles. Depending on the species, annuli may be distinct or barely discernible. An annulus is a group of one to four transapical striae that interrupt the typical striae on each side of the distal raphe end. These striae may have areolae that are larger, more elongate, and more densely spaced. Internally, in some species, the areolae in the annulus possess a cluster of silica outgrowths.

The raphe is straight and filiform, with straight proximal ends. Terminal raphe ends are deflected. An isolated puncta may be present, or it may be absent, in the central area.

Geissleria includes species that were formerly included in the 'Annulatae' section of Navicula, including Navicula paludosa Hust. and N. similis Krasske. Geissleria species are found in oliogotrophic to eutrophic waters, but are locally rare in abundance. The genus includes over 70 taxa, which are widely distributed across Europe, Asia, South America, North America and the Maritime Antarctic and sub-Antarctic. Geissleria species have been recorded in a range of freshwater habitats, including rivers, lakes, springs, splash zones, and soils (Novais et al. 2013). Species level ecological preferences may be more restricted.