Annulus (Geissleria)

An annulus (in the genus Geissleria) is a group of distinctive striae at the apices of the valve. An annulus has one to four pairs of transapical striae that interrupt the typical striae on each side of the distal ends of the raphe. The annulus may be separated from typical striae by a narrow hyaline area which may have a ring shape. From Latin for ring. Plural is annuli.

The areolae in the annulus may be larger, more elongate, and more densely spaced than the rest of the areolae. Internally in some species, these areolae possess a cluster of silica outgrowths. Annuli may be distinct or barely discernible, depending on the species. For example, the annuli of G. acceptata are difficult to resolve in LM.

This group of striae was named for the hyaline ring around them in Navicula annulata (now Geissleria annulata).

Citations

Kulikovskiy, M., Andreeva, S., Gusev, E. and Annenkova, N. (2014) Phylogenetic position of the diatom genus Geissleria Lange-Bertalot & Metzeltin and description of two new species from Siberian mountain lakes Phytotaxa 177(5): 249-260

Publication Link: 10.11646/phytotaxa.177.5.1
Lange-Bertalot, H. and Metzeltin, D. (1996) Indicators of oligotrophy - 800 taxa representative of three ecologically distinct lake types, Carbonate buffered - Oligodystrophic - Weakly buffered soft water. Lange-Bertalot, H. (ed.), Iconographia Diatomologica. Annotated Diatom Micrographs. Vol. 2. Ecology, Diversity, Taxonomy. Koeltz Scientific Books. Königstein, Germany, 2:390 pp.

annulus-geissleria-lm — A *Geissleria* species in valve view (LM). Arrow indicates one of the stria in this annulus.

annulus-geissleria-cascadensis — Internal view of *Geissleria cascadensis* (SEM). The annulus shown here consists of three striae on either side of the distal end of the raphe. The areolae of this annulus are covered by silica outgrowths. Scale bar = 2 µm.

annulus-geissleria-sem1 — Internal view of *Geissleria acceptata* (SEM). Arrow indicates an areola in this annulus. This areola has silica outgrowths around the opening.