Desmid of the month
March 2009

Cosmarium punctulatum

Cells of Cosmarium punctulatum are moderate in size, almost as broad as long and marked by a linear, closed sinus. Semicells are trapeziform with broadly rounded angles and slightly convex lateral sides. By far its most common variety, i.e., var. subpunctulatum, is characterized by a more or less circular group of granules in the centre of the semicell separated from radiate series of intramarginal/marginal granules by an unsculptured zone. The globose zygospores are furnished with furcate spines. C. punctulatum is widely distributed and occurs in both slightly acidic, mesotrophic and alkaline, meso-eutrophic waters. Zygospores are rather rare.

Image © Marien van Westen

Cosmarium punctulatum. Notice the subquadrate cell shape and the granulate outline.

Cell dimensions (L x B): 30 x 28 µm

C. punctulatum cell in somewhat oblique position showing that granules cover most of the cell surface.

Image © Marien van Westen

Image © Henk Schulp

Image: Hanny Kooijman-van Blokland © IBED

Dead, empty cell of C. punctulatum showing the pattern of cell wall granulation (in var. subpunctulatum).

SEM picture of C. punctulatum var. subpunctulatum. Notice the unsculptured zone in between the central group of granules and the radiate series of intramarginal granules.

Zygospore of C. punctulatum. Notice that the spines are furcate at the apex and have a ring of small teeth at the top of their conical base (mouse over).

Image © Henk Schulp (mouse over)