Cosmarium arcuatiporosum 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 : 112

Cosmarium arcuatiporosum Van Westen et Coesel spec. nov. ( Figs 3 , 16, 17, 18, 19 , 27 ).

Diagnosis: cells about as long as broad with a deep, median constriction. Sinus narrowly linear with a dilated extremity. Semicells in frontal view semicircular-trapeziform, the outline in the upper part of the sides often not quite firm. Cell wall smooth. Cell wall pores within the margin confined to two, disjunct zones: a single, arch-shaped series of pores just above the isthmus and some two, transversal series just below the apex. Semicells in lateral view subcircular, in apical view elliptic, in the middle of each side a little bit inflated. Chloroplast with a single, central pyrenoid. Dimensions: cell length 19–25 µm, cell breadth 17–22 µm, cell thickness 10–12 µm, isthmus 6–9 µm. Zygospores globose, furnished with furcate spines. Diameter without spines 22–24 µm, with spines 31–34 µm.

Type:— THE NETHERLANDS. Drenthe: shallow pool in estate Dwingelderveld near Lhee, 52.812811° N, 6.240110° E, between Sphagnum and filamentous algae, pH 5.6, conductivity 27 µS cm-1. Van Westen, 07 May 2018 (holotype L! Hugo de Vries Lab 2019.03, preserved as a fixed natural sample).

Differential diagnosis:—In cell shape, C. arcuatiporosum somewhat resembles C. subtumidum Nordstedt and to a lesser extent C. phaseolus Ralfs. Apart from the fact that those latter species have larger cell dimensions, the main difference is found in the pattern of cell wall pores. Nordstedt (in Wittrock & Nordstedt 1878: 72) for C. subtumidum explicitly mentions a punctate cell wall. Ralfs (1848: 106, pl. 32: 5) describing C. phaseolus does not say anything about pores but depicts them as coarse dots all over the cell wall. So, neither of those species is characterized by the most peculiar pore pattern shown in our newly described species, a pattern for that matter that we do not know either of any other Cosmarium species. Confusion with similar looking species is possible, especially when the characteristic arc of pores above the isthmus is not clearly visible.

This species was found at only two locations in the province of Drenthe, between Sphagnum in a shallow artificial pool on loamy, sandy soil that was dug for nature restoration purposes. Fortunately, this species was also found sporulating in a sample taken in 1988 from a pool from the west of Ireland (Frans Kouwets, personal communication), that was kindly put at our disposal. Zygospores ( Figs 3 , 19 ) are depicted after Irish material.

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.