Cosmarium bipyrenoideum Coesel et Meesters, 2015

Coesel, Peter & Meesters, Koos, 2015, Taxonomic notes on Dutch desmids VII (new species, new names, new record), Phytotaxa 208 (1), pp. 55-62 : 55-57

Cosmarium bipyrenoideum Coesel et Meesters , sp. nov. (Figs. 1–3, 11–15)

Cells in frontal view slightly longer than broad, deeply constricted. Sinus closed to about halfway, then opening widely. Semicells semicircular-reniform with broadly rounded basal angles. Cells in apical view oblong with almost parallel sides and broadly rounded angles. Cell wall smooth, faintly scrobiculate. Chloroplast with two pyrenoids. Dimensions: length 37–45 μm, breadth 35–40 μm, isthmus 8–12 μm, thickness ca. 20 μm.

Type:— THE NETHERLANDS. Flevoland: shallow pool in ‘Kuinderbos’ near Kuinre, 52.782º N, 5.807º E, among aquatic weeds, conductivity 170 μS cm-1, Coesel & Meesters, 5 September 2014, (holotype L! Hugo de Vries Lab 2014.01, preserved as a fixed natural sample).

Differential diagnosis:—At first glance, Cosmarium bipyrenoideum is a little conspicuous, smooth-walled Cosmarium species, superficially resembling common species such as C. phaseolus Ralfs (1848: 106) , C. depressum ( Nägeli 1849: 114) P. Lundell (1871: 38) or C. klebsii Gutwiński (1892: 19) . Most striking difference with those species, however, is the occurrence of two (instead of one) pyrenoids per semicell which is a rather exceptional phenomenon in Cosmarium species with comparable cell dimensions With regard to this characteristic our newly described species agrees with C. regulare Schmidle (1894: 57) as represented in Krieger & Gerloff (1962: 27, pl. 8: 15). However, description and illustration in that latter paper are mainly based on Messikommer (1927: 343) rather than on Schmidle’s (l. c.) original publication. Whereas the apical cell view in Messikommer (1927, pl. 1: 5) is oblong in outline, the original one in Schmidle (1894, pl. 7: 16) is narrowly elliptic, strongly tapering toward the poles. Our taxon under discussion is also fairly similar to a form of C. pseudonitidulum Nordstedt (1873: 16) depicted by Manguin (1936: 327, pl. 5:71). Krieger & Gerloff (1965) described that form as a new variety: C. pseudonitidulum var. rotundatum Willi Krieger et Gerloff (1965: 167) . However, considering the shape of the semicells (trapeziform-rectangular in the nominate variety of C. pseudonitidulum , pyramidal-reniform in var. rotundatum ) it is highly questionable if those taxa belong to one and the same species. Although our newly described species looks much like the form circumscribed by Manguin (l. c.) it is not quite certain whether we are dealing with identic species. Where Manguin’s form shows a somewhat truncate apex, the semicell apex in bipyrenoideum is almost equally convex. Moreover, in apical view Manguin’s cell form is distinctly elliptic versus oblong with about parallel sides in C. bipyrenoideum .

FIGURES 1–10. 1–3. Cosmarium bipyrenoideum . 4. Cosmarium cataractarum . 5–8. Cosmarium distentum . 7–8. Zygospores. 9–10. Euastrum spondylosioides . Scale bars = 15 μm (Figs. 1–8), 10 μm (Figs. 9–10).

Ecology and distribution:— Cosmarium bipyrenoideum was encountered in a shallow sand pit with aquatic weeds such as Typha angustifolia L., Phragmites australis (Cav.) Steud. , and Potamogeton natans L.. These plants together with the bank-dwelling macrophytes Hydrocotyle vulgaris L., Lycopus europaeus L. and Ranunculus flammula L. indicate a meso-eutrophic, more or less neutral habitat. Also accompanying desmid species such as Closterium leibleinii Ralfs , Cosmarium fontigenum Nordstedt , C. regnellii Wille , C. reniforme (Ralfs) W.Archer , Euastrum lacustre (Messikommer) Coesel and Eu. germanicum (Schmidle) Willi Krieger are characteristic of such environmental conditions ( Coesel & Meesters 2007). Presumably, Cosmarium bipyrenoideum has been recorded a number of times earlier under the name of Cosmarium regulare , e.g., by Messikommer (1927) from Switzerland, Homfeld (1929) from Germany, and Lenzenweger (2003) from Austria.