Annulus

An annulus is a hyaline ring on the valve face of radial centric and some bipolar and multipolar centric diatoms. The plural is annuli.

This ring is the first silica formed during valve morphogenesis and if visible or well developed (depending on the genus) may not be visible in later stages of cell development.

In some bipolar centric groups (e.g., the Ardissoniales) the annulus lies near the valve margin. During morphogenesis, the annulus produces striae on both sides apically and lengthens the striae on both sides transapically.

The annulus is homologous to the central nodule in pennate diatoms.

Citations

Houk, V., Klee, R. and Tanaka, H. (2014) Atlas of freshwater centric diatoms with a brief key and descriptions Part IV. Stephanodiscaceae B: Stephanodicsus, Cyclostephanos, Pliocaenicus, Hemistephanos, Stephanocostis, Mesodictyon & Spicaticribra. In: Poulícková, A. (ed.): Fottea 14 Supplement, 529 pp.
Kaczmarska I., Ehrman J.M., Davidovich N.A., Davidovich O.I. and Podunay Y.A. (2020) Valve morphogenesis in selected centric diatoms. Botany 98(12): 725–733.

Publication Link: 10.1139/cjb-2020-0088
Mann, D.G. (1984) An ontogenetic approach to diatom systematics. Proceedings of the 7th International Diatom Symposium. Koeltz, Koenigstein, pp. 113–144.
Medlin L.K., Sato S., Mann D.G. and Kooistra W.H.C.F. (2008) Molecular evidence confirms sister relationship of Ardissonea, Climacosphenia, and Toxarium within the bipolar centric diatoms (Bacillariophyta, Mediophyceae), and cladistic analyses confirm that extremely elongated shape has arisen twice in diatoms. Journal of Phycology 44(5): 1340–1348.

Publication Link: 10.1111/j.1529-8817.2008.00560.x

Annulus — The annulus is the very faint inner ring of silica in this *Stephanodiscus*.

Actinocyclus subtilis annulus BA1 st1383 0170 1 — The annulus is the hyaline ring around the center of this *Actinocyclus subtilis* (SEM). Arrows indicate directions of growth during morphogenesis. Scale bar = 10 µm.