Cosmarium thwaitesii (var. penioides).

Cell in frontal view,

Cosmarium thwaitesii
image © Alfred van Geest

Cell in lateral view,

Cosmarium thwaitesii
image © Alfred van Geest

Notice elongate, rectangular cell outline. Notice also that cell breadth in lateral view is smaller than in frontal view indicating the biradiate nature of this taxon.
Cell dimensions (L x B): 66 x 32 µm



 


Desmid of the month
August 2026

Cosmarium thwaitesii


Cosmarium thwaitesii might be confused with Actinotaenium americanum (Desmid of July 2026). Both species are characterized by an elongate rectangular cell outline and a very shallow, V- shaped sinus. However, whereas Actinotaenium americanum is an omniradiate taxon (so circular in apical cell view, semicells with a single, central pyrenoid). Cells of Cosmarium thwaitesii are biradiate and marked by two pyrenoids in each semicell. In the Netherlands, Cosmarium thwaitesii is of occasional occurrence in various mesotrophic water bodies.

Cosmarium thwaitesii
image © Roland Luts

Cosmarium thwaitesii (var. penioides). Notice chloroplast with two pyrenoids.

Cosmarium thwaitesii
image © Roland Luts

Cosmarium thwaitesii (var. rotundatum?). A form with more convex semicell sides.