Taxonomy
An SEM of Desmodesmus subspicatus showing warts and an opening.
Genus Desmodesmus
The genus Desmodesmus (formerly Scenedesmus) has been known and investigated for nearly 200 years. Turpin made some of the first microscopic observations from collections in 1828. Meyen described Scenedesmus, which included non-spiny and spiny colonies, in 1829.
In 1999, using molecular analysis, the non-spiny and spiny forms were shown to be separate genera. Thus, the spiny forms are now called Desmodesmus sp. (An, Friedl et Hegewald). And the non-spiny forms retain the original name, Scenedesmus sp.
Morphology
The genus Desmodesmus is characterised by flat colonies that are
- straight or slightly curved, with cells in one row
- usually 2-, 4-, or 8-celled, more rarely 16- or 32-, joined laterally and lying parallel to each other
- cells are ovoid to ellipsoid, with rounded apices, and bearing long spines or teeth
- the cell wall is granular, spiny or toothed, with wart-like projections and/or ribs present
- the chloroplast is parietal with a single pyrenoid
The inner or medial cells often bear a single apical spine, whereas marginal or terminal cells bear a single large spine on each apex, and these are diagonal to the long axis of the colony.
There are often 1 to 3 additional lateral spines present on the outermost side of marginal cells. The spines are almost equal in length, although the main spines may be longer.
With light microscopy, the spines are visible, but other features of the wall are only visible with the scanning electron microscope or SEM. The SEM reveals a highly ornamented wall, which has features that can be used for improved taxonomy to distinguish species.
Reproduction
Reproduction is primarily asexual by autospores, which are released by fracture of the lateral cell wall, although there was a report of flagellated cell (gamete?) production.