Sphaerozosma aubertianum West (1889: 206)

Coesel, Peter & Westen, Marien Van, 2013, Taxonomic notes on Dutch desmids V (Streptophyta, Desmidiales): new species, new morphological features, Phytotaxa 84 (2), pp. 46-54 : 51-53

Sphaerozosma aubertianum West (1889: 206) (Figs. 22–26)

Sphaerosoma aubertianum may be readily confused with S. vertebratum De Brébisson ex Ralfs (1848: 65) . Part of this confusion is due to a deficient and inconsistent treatment of those taxa in the well-known flora by West et al. (1923). Semicells in both species are characterized by some two transversal rows of cell wall pores. Markedness of those pores under the light microscope may vary in dependence of the quantity of hardened mucus produced from them. If there is but little or nothing of that exudate the series of pores may be easily overlooked, but in case there is much of it they give the impression of cell wall granules, suggesting to be an intrinsic part of the cell wall. Obviously, both Ralfs (1848) and West et al. (1923) overlooked the cell wall pores in the nominate variety of S. vertebratum for they do not make mention of it. But later on various experienced desmidiologists did depict them (e.g., Lütkemüller 1902, Coesel & Meesters 2007). Where those pores are very obvious, a separate variety, S. vertebratum var. punctulatum ( West 1891: 353) West & West (1905: 28) was distinguished.

Originally ( West 1889), S. aubertianum was described as being marked by two granules on each side of the semicells. Later on West & West (1896: 230) classified S. archeri Gutwi ṅski (1892: 29), characterized by two horizontal series of comparable granules, as var. archeri in S. aubertianum . Still later, West et al. (1923: 208) reported transitional forms between the nominate variety of S. aubertianum and its var. archeri and suggested those names to be synonymous. In that same flora ( West et al. 1923) the presence of cell wall granules in S. aubertianum is mentioned as one of the main differences with S. vertebratum whose cell wall would be smooth. Inconsistently, West et al. (1923: 209) also referred to Lütkemüller (1910) who stated that true warts do not occur in S. aubertianum any more than in S. vertebratum but that it is the hardened ends of gelatinous strands exuding from the cell wall pores. A more reliable differentiating feature between vegetative cells of those two species could be the shape of the cell sinus and, in connection to that, the outline of the semicells. Whereas in the original description of S. vertebratum the outline of the semicells is described as subreniform (De Brébisson & Godey 1835: 269) and the sinus as a narrow notch on each side of the cell ( Ralfs 1848: 65), the sinus in S. aubertianum is said to be opening widely ( West 1889: 206, West et al. 1923: 207) rendering the outline of the semicells about elliptic ( West et al. 1923: 207). A similar difference is demonstrated in our Figs. 21 ( S. vertebratum ) and 22 ( S. aubertianum ) representing pictures of recent Dutch finds. A more significant difference between S. vertebratum and S. aubertianum is in the shape of the zygospore. Whereas the spore in S. vertebratum is described as smooth-walled ( Ralfs 1848), S. aubertianum is characterized by a spiny one (Gutwiṅski 1892, as S. archeri ). Unfortunately, zygospores are but seldom encountered so in most cases cannot contribute to identification. Regarding S. vertebratum , zygospores are only depicted in Ralfs (1848) and Cooke (1887). Zygospores of S. aubertianum have been represented only a little more, i.e., byGutwiṅski Gutwiṅski (1892), West & West (1896), West et al. (1923), Margalef (1956). Also our own material of S. aubertianum contained quite a number of sporulating cells (Figs. 23, 24).

Finally, an additional morphological feature found in our Drenthian material was the observation of crossing rows of cell wall pores (Figs 23, 24). A similar phenomenon is to be seen in SEM pictures of S. aubertianum in Kirk et al. (1976) and in Šťastný (2010). As this stretched, X-like pattern of pores is a feature that may readily be overlooked it would be interesting to examine in how far this appears to be a taxon-linked characteristic.

Sphaerozosma aubertianum was encountered near Wijster, in the water body ‘Reigerveen’. Initially (April 2011) it was only found in vegetative condition, in another year (July 2012) also with zygospores.