Desmid of the month
April 2005

Cosmarium ralfsii

Although destitute of any cell wall ornamentation, Cosmarium ralfsii, in particular the variety montanum, can be readily distinguished by its large cell dimensions and characteristic shape. C. ralfsii var. montanum differs from the nominate variety in that the basal semicell angles are somewhat produced (giving the impression of being slightly tipped up).

In the Netherlands C. ralfsii is very rare and indicative of acidic, oligotrophic habitats where there is a slight input of minerals from the subsoil or the surroundings. It goes as an ecologically most demanding species, indicating a high conservation value.



Cosmarium ralfsii var. montanum as recently sampled from a Dutch moorland hollow. Notice the produced basal angles of the semicells.

Cell dimensions (L x B): ca 110 x 100 µm

Cosmarium ralfsii var. montanum


Another cell of C. ralfsii var. montanum (mouse over)


Image © Koos Meesters (nafter Belgian material)