Cosmarium vossenbergense Coesel & Van Westen, 2013

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 : 49-50

Cosmarium vossenbergense Coesel & Van Westen , spec. nov. (Figs. 11, 12, 18)

Cells about as long as broad, deeply constricted. Sinus acute-angled, opening widely. Semicells in frontal view ellipsoidcuneate with a dentate-undulate outline. Cell wall in the semicell centre furnished with some 4 granules arranged in a quadrate, at the apex and the lateral sides with a number of sparse marginal and intramarginal granules. Semicells in apical view elliptic with a slight median inflation, an undulate outline and series of intramarginal granules. Chloroplasts furcoid with a single, central pyrenoid. Cell length 16–18 µm, cell breadth 16.5–19.0 µm, cell thickness 8–10 µm, breadth of isthmus 4.8–5.6 µm.

Type: –– THE NETHERLANDS. Drenthe: Wijster, Reigerveen in estate Vossenberg , 14 March 2011, Hugo de Vries Lab. 2012.01 (holotype L!), preserved as a fixed natural sample .

The algal form represented in our Figs 11, 12 and 18 in particular should be compared with three small-sized, ornamented Cosmarium species , viz. C. prominulum Raciborski (1885: 79) described from Poland, C. sculptum Eichler & Gutwiṅski (1894: 169) also described from Poland, and C. spyridion West & West (1895: 64) described from Madagascar. All three species are marked by a widely open sinus and elliptic-hexagonal semicells that are ornamented with a pronounced central tumour and a variable number of smaller granules.

Our alga, characterized by four granules crosswise placed on the central tumour, is most similar to C. spyridion ‘forma B’ in Bourrelly & Couté (1991: 103, pl. 33: 12). It is somewhat less similar to C. spyridion represented in Scott & Prescott (1961: 70, pl. 31: 13) marked by even smaller cell dimensions and a central tumour with five granules placed in a circle. Important, however, are some essential differences with C. spyridion as originally described by West & West (1895: 64, pl. 7: 26). Whereas the figure in West & West (1895) shows a cell with a relatively broad isthmus, an obtuse-angled sinus, low-trapeziform semicells with an obviously smooth-walled central tumour, cells of our algal form are characterized by a relatively small isthmus, a more acute-angled sinus and elliptic-hexagonal semicells with a granulate central tumour. Even greater are the differences with C. prominulum and C. sculptum as described by Raciborski (1885) and Eichler & Gutwiṅski (1894), respectively. Next to a broad isthmus and an obtuse-angled sinus C. prominulum is marked by a highly pronounced central tumour resulting in a rhomboid apical cell view (versus elliptic in our alga). In C. sculptum , also characterized by a broad isthmus and an obtuse-angled sinus, semicells are distinctly elliptic with broadly rounded lateral sides. In conclusion, with respect to the three above-discussed Cosmarium species , our algal form most resembles C. spyridion but in our opinion differs enough to justify its description as a new species.

Cosmarium vossenbergense was encountered (in rather low cell numbers) near the Drenthian villages of Wijster, Uffelte and Doldersum. All records refer to artificial mesotrophic pools on sandy soil created about ten years ago on former farmland for nature restoration purposes. Occurrence of the macrophyte Juncus effusus and occasionally of Typha angustifolia indicated a slight degree of eutrophication. Conductivity values ranged from 22 to 46 µS cm -1 and pH from (4.3) 5.2 to 6.5.