Triradiate Cosmariahome

Triradiate forms of normally biradiate desmid species may be readily identified if genus characteristics preclude any confusion, e.g in representatives of the genus Micrasterias. Where, however, difference in radiation is the only discriminating feature, such as in many species of the genera Cosmarium and Staurastrum, identification may be much more problematic as is shown in the below examples. In general it holds that, within a population of a given Cosmarium species, 3-radiate specimens are in minority with respect to 2-radiate specimens.

Cosmarium subquadrans var. minor

Sphagnum peatlands bordering the 'Amstelveense Poel', a large pool near the Dutch city of Amsterdam, harbour substantial populations of a small-sized, smooth-walled Cosmarium taxon, identified as C. subquadrans var. minor (Fig. 1).

Fig. 1. Cell of Cosmarium subquadrans var. minor exhibiting normal, 2-radiate symmetry.

Cell dimensions (L x B): 14 x 18 µm

Image © Koos Meesters

Occasionally, at the same site 3-radiate desmid specimens were encountered, initially presumed to represent some unknown Staurastrum species but later on concluded to be a 3-radiate form of Cosmarium subquadrans var. minor (Fig. 2).

Cosmarium subquadrans var. minor, Janus-vorm

FFig. 2. Triradiate cell of Cosmarium subquadrans var. minor suggesting to represent some smooth-walled Staurastrum species.

Cell dimensions (L x B): 14 x 16 µm

Image © Koos Meesters

The supposition that the 2-radiate and the 3-radiate specimens belong to one and the same species is supported by incidental finds of so-called Janus forms combining a 2-radiate and a 3-radiate semicell in one and the same cell (Fig. 3).

Cosmarium subquadrans var. minor, Janus-vorm
Fig. 3. Janus form of Cosmarium subquadrans var. minor combining a 2-radiate and a 3-radiate semicell (see apical views).

Image © Koos Meesters

Additional finds of 3-radiate cells showing a lesser or greater degree of asymmetry (Fig. 4) suggest that in the desmid represented triradiation likely is to be considered the abnormal configuration of a normally biradiate taxon. That means that the taxon in question preferably is accounted the genus Cosmarium rather than Staurastrum.

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Fig. 4. Variety of 2-radiate (nrs 1, 2) and 3-radiate (nrs 3, 4) cells of Cosmarium subquadrans var. minor in which the 3-radiate cells show a certain degree of asymmetry.

Image © Alfred van Geest

Cosmarium subcostatum var. beckii

Recently, in a puddle on a parking in the Dutch city of The Hague, a desmid taxon was encountered agreeing with Cosmarium subcostatum var. beckii as figured in the flora by West & West (1908, pl. 87: 10-12). It belongs to a group of Cosmaria characterized by a granulate cell wall and a crenate outline (Figs. 5 and 6).

Fig. 5. Cell of Cosmarium subcostatum var. beckii in both frontal and apical view. Notice crenate outline.

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Fig. 6. Dead, empty cell of Cosmarium subcostatum var. beckii in both frontal, apical and lateral view. Notice inflated rosette of granules in the semicell centre and linear series of granules radiating towards the marginal crenations.

Among the above-described cells of Cosmarium subcostatum var. beckii some triradiate desmid cells were encountered. Initially those were associated with the genus Staurastrum but on closer examination they were interpreted to be a 3-radiate form of Cosmarium subcostatum var. beckii (Fig. 7).

Fig. 7. Dead, empty cell of presumed 3-radiate form of Cosmarium subcostatum var. beckii. Notice similarity in cell size, cell margin and granulation (e.g. the inflated, granulate semicell centre, particularly to be observed in apical cell view) with biradiate cells of that same taxon.