Cosmarium biretum. Notice dilate apex of the cell sinus and fine granulation of the cell wall. Outline of the semicells subrectangular to slightly pentagonal.

Cell dimensions (L x B): 47 x 48 µm



Desmid of the month
February 2022

Cosmarium biretum

Cosmarium biretum somewhat resembles C. reniforme. Apex of the cell sinus (at the isthmus) is distinctly dilated contributing to a more or less reniform outline of the semicells. For that matter, shape of the semicell may be rather variable, ranging from kidney-shaped to subrectangular or pentagonal. Another differentiatng feature with respect to C. reniforme is in size and arrangement of the cell wall granules. Whereas granules in C. reniforme are relatively coarse and aranged in most regular, longitudinal series, those in C. biretum are more delicate and less regularly distributed. Finally, semicells in C. biretum are marked by a pronound central inflation which is lacking in C. reniforme. In the Netherlands, C. biretum is of occasional occurrence in meso-eutrophic, pH-neutral to alkaline water bodies. Its smooth-walled, broadly elliptic zygospore has only been found once.

mouse over Cosmarium biretum
mouse over images © Alfred van Geest

Another cell of C. biretum with a more kidney-shaped outline of the semicells.

mouse over: Cell of C. biretum in apical view. Notice prounced median inflation.

Empty cell of C. biretum showing pattern of cell wall granulation.


C. biretum SEM image

Zygospore of C. biretum with adherent empty gametangial cells.