Cosmarium neocrenatum Van Westen & Coesel 2021, spec. nov.

Cosmarium neocrenatum Van Westen & Coesel spec. nov. (Figs 5, 12, 13, 14, 31)

Diagnosis: Cells 1.2–1.4 times as long as broad with a moderately deep, linear, largely closed sinus. Semicells in frontal view semi-oblong to semirectangular with a crenate margin. Each lateral side with 3–4 deep crenations (including the basal angle), apex with 4, more flattened, crenations. Semicells in frontal view just above the isthmus with a broad inflation furnished with 4–6 flattened, longitudinal costae, often indistinctly granulate and/or paired with a separate granule near the isthmus. Supraisthmial ornamentation separated from radiate series of intramarginal crenulations by an unsculptured zone. Intramarginal crenulations smooth or furnished with one or two minute granules. Outline of semicells in lateral view about rectangular with broadly rounded angles, the basal part inflated. Outline of semicells in apical view elliptic with subtruncate poles, the central part inflated. Chloroplast with a single, central pyrenoid.

Dimensions: cell length 27–32 µm, breadth 22–25 µm, L/B = 1.2–1.37, thickness ca 16 µm, breadth of isthmus 12–14 µm.

Type:— THE NETHERLANDS. Overijssel: quaking fen in estate Rottige Meente near Nijetrijne, 52.82664° N, 5.906261° E, between mosses, mainly Campylium and Calliergon species, pH 7.2, conductivity 236 µS cm- 1. Van Westen, 27 June 2015 (holotype L! Hugo de Vries Lab 2021.01, preserved as a fixed natural sample and represented by our Fig. 5).

The algal cells depicted in our figs 5, 14 and 31 are much alike those represented in the well-known flora by West & West (1912, pl. 98: 9–10) under the name of Cosmarium crenatum Ralfs. Based on West & West (l.c.), in most later publications, e.g. the floras by Prescott & al. (1981), Förster (1982), Croasdale & Flint (1988), Lenzenweger (1999) and Coesel & Meesters (2007), Cosmarium crenatum is characterized by a supraisthmial series of longitudinal costae or series of granulae. For that matter, it was Nordstedt (1872, p. 29, pl. 6: 7–8) who marked the concept of C. crenatum in this way. However, Ralfs (1848, p. 96, pl. 15: 7) in his original diagnosis neither mentions nor depicts such a supraisthmial ornamentation. On the contrary, he talks about ‘segments, the surface of which is punctate and even’. A form that superficially looks like our C. neocrenatum was described from Greenland by Larsen (1907, p 333, pl 7:11) as C. ordinatum (illeg.) with ‘a series of large dentiform protuberances and a series of five oblong strong protuberances’. In our species the ornamentation consists of sometimes hardly visible flattened granules. So in our opinion he described another species. From Brazil, Grönblad (1945, p. 16, pl. 5: 95) described Cosmarium crenatiforme, in rough outline resembling Ralfs’ Cosmarium crenatum but characterized by some supraisthmial granules joining a series of longitudinal costae. In respect of that latter feature it meets our newly diagnosed C. neocrenatum. Yet, we do not think that we have to do with identical species as Grönblad’s poorly known C. crenatiforme is marked by a distinctly deeper cell sinus than our C. neocrenatum.