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.