Desmid of the month
August 2009

Cosmarium ovale

C. ovale is a most conspicuous Cosmarium species. Its large cells (measuring up to over 200 micrometer) cannot be confused with any other European representative of this genus. Semicells are pyramidal in outline and marked by marginal and intramarginal series of conical granules. The parietal chloroplast consists of some four plates incorporating quite a number of smaller pyrenoids each. In the Netherlands, records of C. ovale are only known from some mesotrophic moorland pools dating from the first half of the last century (all pictures herewith presented are after Czech material).

Image: Cell of Cosmarium ovale. Notice the pyramidal outline of the semicells, the marginal series of conical granules and the large cell size.

Cell dimensions (L x B): 225 x 130 µm

Cosmarium ovale

Optical section of a cell of C. ovale showing parietal chloroplast plates with many pyrenoids. Notice accumulation of crystals in the interior (the vacuole) of the semicells.





Empty semicell of C. ovale showing coarse cell wall pores in the centre and conical granules near the margin.



SEM image of Cosmarium ovale.