Desmid of the month
March 2010

Cosmarium venustum

Cosmarium venustum is a small-sized, rather inconspicuous species. In rough outline, semicells are trapeziform with an undulation about half-way the lateral sides. The cell wall is smooth but, particularly in empty cells, shows a marked scrobiculation. Usually, of C. venustum its variety excavatum is encountered which is characterized by an extra large pit in the centre of the semicell. In the Netherlands, it is a rather common species in acidic, oligotrophic bog pools.
Cosmarium venustum
Image © Marien van Westen

Cosmarium venustum

Cell dimensions (L x B): 25 x 20 µm
Cosmarium venustum var. excavatum, empty celll
Image © Marien van Westen

Empty cell of C. venustum (var. excavatum). Punctation of cell wall refers to scrobicles.

Cosmarium venustum var. excavatum SEM-image
Image Hanny Kooijman - van Blokland © IBED

SEM picture of C. venustum var. excavatum. Notice pattern of scrobicles and the extra large pit in the centre of the semicell characteristic of this variety.

Cosmarium venustum SEM-image detail
Image Hanny Kooijman - van Blokland © IBED

SEM picture of C. venustum var. excavatum. Detail of central semicell region showing cell wall pores in and next to scrobicles.