Cosmarium galeatum Van Geest et Coesel, 2019

Geest, Alfred Van & Coesel, Peter, 2019, Some new and interesting desmids (Streptophyta, Desmidiales) from ephemeral puddles in the urban and industrial areas of Amsterdam (Netherlands), Phytotaxa 387 (2), pp. 119-128 : 125

Cosmarium galeatum Van Geest et Coesel sp. nov. (Figs. 5, 15, 16)

Diagnosis: Cells about 1.8 times longer than broad, with a shallow, very narrowly open to almost closed sinus. Outline of the cells in frontal view oblong with almost straight sides and a very slight constriction. The cell wall of a semicell has 18 crenations, slightly increasing in width from the sinus onto the apex. Every crenation is part of costa that extends from the side to halfway the center of the semicell and is furnished with 4–5 tubercles. The centre of the semicell is unsculptured, but distinctly punctate. Lateral view elongated oblong and with a shallow wide invagination. Apical view elliptic. Chloroplast with a single, central pyrenoid. Dimensions: cell length 42–47μm, cell breadth 23–25μm, cell thickness 18–19μm, breadth of isthmus 12–14μm.

Type:— THE NETHERLANDS. Amsterdam: rain puddle, 52.33838 N, 4.91980 E, Van Geest , 18 November 2015 (holotype L! Nationaal Herbarium Nederland, Hugo de Vries Lab. no 2018.08, preserved as a fixed natural sample).

Etymology: Latin ‘galeatum’ means ‘helmeted’, because of the helm-shaped semicell outline.

C. galeatum should be compared with Cosmarium speciosum var. simplex Nordstedt (1872: 31) and Cosmarium notabile var. transiens Insam & Krieger (1936: 103) . Apart from the slightly smaller length, the width of our specimens is considerably smaller than that of C. speciosum var. simplex , resulting in a distinctly greater length/width ratio (1.83 in our fig. 5 vs. 1.53 in Nordstedt’s fig. 6:12). Also the outline differs considerably. Nordstedt’s alga has a semielliptical outline that attenuates rapidly from the isthmus to the top, while our specimens are more elongated with almost straight sides. C. notabile var. transiens is similar in shape to C. galeatum but differs considerably in lengthwidth ratio, viz. 1.42 in Fig. 4:18 of Insam & Krieger against 1.83 in our fig. 5. In addition, the cells of C. notabile var. transiens are more than 10 μm shorter and it has 14 (at most 16) crenations per semicell. For both taxa also applies that they are described as a variety of species with which in our opinion they have little to do. The complete absence of a submarginal sculpture in C. speciosum var. simplex , which is so characteristic of the nominate variety of C. speciosum , is in our view reason enough to think of a different species. The same applies to C. notabile var. transiens , because form and ornamentation differ too much from the concept of C. notabile as described earlier in this article. C. galeatum was found at locations 4, 7 and 9.