Cosmarium mandosii Van Westen et Coesel, 2020

Van Westen, Marien C. & Coesel, Peter F. M., 2020, Taxonomic notes on desmids from the Netherlands III, with a description of five new species, Phytotaxa 443 (1), pp. 107-115 : 113-114

Cosmarium mandosii Van Westen et Coesel spec. nov. ( Figs 2 , 23, 24, 25 , 26 )

Diagnosis: Cells about as long as broad, with a deep median constriction. Sinus linear, closed for the greater part. Semicells in frontal view elliptic-oblong. Semicell centre furnished with two pairs of longitudinally disposed nodules, each nodule being set with a variable number of granules. Below the central ornamentation a transversal series of some four supraisthmial granules. Margin of the semicell as well as the zone between margin and midregion furnished with evenly distributed, small, sharp granules. Semicells in lateral view about circular, in apical view elliptic with on either side two prominent, median nodules. Chloroplast with a single, central pyrenoid. Dimensions: cell length 20–25 µm, cell breadth 19–24 µm, cell thickness 12–14 µm, isthmus 7–9 µm.

Type:— THE NETHERLANDS. Province of Noord Brabant: ‘Leemkuilen bij Udenhout’, 51.60269N, 5.1783E. Loamy bank of large pool with Ranunculus aquatilis , Crassula helmsii and Nymphoides peltata . Maarten Mandos, 24 July 2018 (holotype L! Hugo de Vries Lab 2019.05, preserved as a fixed sample).

Etymology: named after Maarten Mandos who collected this species and brought it to our attention.

Differential diagnosis:— Cosmarium mandosii should be compared with C. vogesiacum Lemaire and C. polonicum Raciborski , two species that are highly similar and consequently often mutually confused. For a critical discussion of those species including their various infraspecific taxa, see Ducellier (1916) and Laporte (1931). Both species are primarily characterized by a couple of elongate nodules next to each other in the centre of the semicell, particularly well to be distinguished in apical view. Described as first of these two was C. vogesiacum by Lemaire (1883: 20, pl. 1: 4) from the French Vosges. Hardly a year later, C. polonicum was described by Raciborski (1884: 12, pl. 1: 2) from the neighbourhood of Krakow in Poland. In his description of C. polonicum, Raciborski does not refer to Lemaire’s publication, so probably he was not aware of it. Judging from the accompanying illustrations, C. polonicum mainly differs from C. vogesiacum by more numerous and more delicate marginal and intramarginal granules. In view of that, our C. mandosii should be compared in particular with C. polonicum .

Most records of C. polonicum in literature refer to its var. alpinum Schmidle (1895: 457 , pl. 15: 21) differing from the nominate variety by somewhat more pronounced central nodules but in our opinion with little taxonomic relevance. Soon after Schmidle’s var. alpinum, Borge (1906: 40 , pl. 2: 29) described C. polonicum var. quadrinodosum differing from var. alpinum in that each of the two parallel central nodules is transversally split into two smaller entities. In its turn var. quadrinodosum much resembles C. polonicum var. quadrigranulatum Gutwiński (1890: 71 ; 1892, pl. 2: 31) which is marked by some four granules at each of the two central nodules. Depending on markedness, number and granulation of the central nodules, Ducellier (1916) distinguished in C. polonicum some seven different forms that partially could be linked to formally described infraspecific taxa. None of those forms agrees with the central ornamentation pattern as found in our C. mandosii . Most resembling is Ducellier’s form ‘g’ corresponding with C. polonicum var. quadrinodosum Borge , marked by 2x2 nodules in the semicell centre but lacking the superimposed granulation at each nodule characteristic of our C. mandosii . As Ducellier incidentally encountered cells in which one of the semicells belonged to another ornamentation form than the other one may question whether the above described difference in granulation between our alga and the various forms of C. polonicum justifies the description of a separate species. Presumably a more relevant difference is found in the overall cell shape. Where C. polonicum (just like C. vogesiacum ) is characterized by cells that are a bit longer than broad with about trapeziform semicells, cells of C. mandosii are about as long as broad and have elliptic-oblong semicells rendering it an appearance distinctly differing from that in C. polonicum and C. vogesiacum . For that matter, a similar difference in semicell shape can be assessed when comparing C. mandosii with C. quadriverrucosum var. supraornatum described by Skuja (1949, p. 138, pl. 31: 13) from Burma. That latter taxon superficially resembles our C. mandosii but is additionally marked by four instead of two series of longitudinally arranged nodules, nodules that furthermore are destitute of any superimposed granules. Also the apical pore pattern of C. mandosii is different than depicted by Skuja.

The great resemblance of C. mandosii to C. polonicum renders mutual confusion plausible. Indeed, C. polonicum as represented by Lenzenweger & Wertl (2001, pl. 3: 4) from Austria obviously refers to our newly described species. Besides, records from Czech Republic (Jan Šťastný, personal communication) and from France (Frans Kouwets, personal communication) indicate that C. mandosii is rather widely distributed in Europe.

Figures

FIGURES 1–6. Drawings 1. Cosmarium logiensiforme. 2. C. mandosii. 3. C. arcuatiporosum (with zygospore). 4. Euastrum tumoriferum. 5. Eu. dubioides. 6. Eu. neogutwinskii (with zygospore).

FIGURES 16–25. LM photographs. 16–19. Cosmarium arcuatiporosum (19. zygospore). 20–22. C. logiensiforme. 23-25. C. mandosii. Scale bar = 10 µm.

FIGURES 26–34. SEM photographs. 26. Cosmarium mandosii. 27. C. arcuatiporosum. 28. C. logiensiforme. 29. Euastrum tumoriferum. 30 Eu. neogutwinskii. 31.Eu. dubioides. 32. Eu. dubium var. dubium. 33 Eu. dubium var. ornatum. 34. Eu. denticulatum. Scale bar = 10 µm.