Cosmarium fruticosum 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 : 125-126

Cosmarium fruticosum Van Geest et Coesel sp. nov. (Figs. 6, 17, 23)

Diagnosis: Cells about 1.7 times longer than broad with a shallow, narrowly open to almost closed sinus. Outline of the semicells in frontal view elongate trapeziform with slightly convex sides and a more or less truncate apex. Each semicell has 16 almost equally sized marginal crenations. Crenations slightly increasing in width from the sinus onto the apical corner; the two median apical crenations somewhat smaller. Cell wall ornamentation consisting of 2–3 concentric series of submarginal, flattened verrucae. The centre of the semicell is unsculptured, but distinctly punctate. Lateral view elliptic with a shallow, obtusely angled constriction. Chloroplast with a single, central pyrenoid. Dimensions: cell length 33–36 μm, cell breadth 20–22 μm, cell thickness 16–17 μm, breadth of isthmus 11–12 μm.

Type:— THE NETHERLANDS. Amsterdam: rain puddle, 52.38525 N, 4.90167 E, Van Geest , 31 May 2016 (holotype L! Nationaal Herbarium Nederland, Hugo de Vries Lab. no 2018.07, preserved as a fixed natural sample).

Etymology: Latin ‘fruticosu’ means ‘scrub-like’, because of the somewhat scrub-shaped semicells.

C. fruticosum is very similar, probably even identical to C. tetragonum var. heterocrenatum West et G.S.West as depicted in Coesel & Meesters (2007, pl. 68: 20). However, the picture in Coesel & Meesters (l.c.) differs from the original description in West & West (1908: 19, pl. 66: 22) by a higher cell length to breadth ratio, a less truncate apex and a higher number of marginal crenations (16 per semicell versus 14 in West & West’s original description).

For that matter, West & West’s var. heterocrenatum obviously is not related to C. tetragonum as originally described by Nägeli (1849: 119, pl. 7A: 5, under the name of Euastrum tetragonum ), as the latter species is characterized by an undulate, instead of a crenate cell margin. Possibly, Krieger & Gerloff (1965: 210) are right to account var. heterocrenatum to C. notabile Brébisson but as West & West (l.c.) do not provide any information about possible cell wall sculpturing, that recombination is still questionable.

Anyway, our alga under discussion essentially differs from any taxon described both under the name of C. tetragonum and C. notabile . Rather it may be compared with C. speciosum var. slovenicum as described by Pevalek (1925: 292) from the Julian Alps in Slovenia. Apart from the question if we have to do with one and the same taxon indeed we are of the opinion that it is not closely related to C. speciosum P.Lundell and deserves the status of a separate species.’ C. fruticosum was found at location 2, 3 and 4.