Closterium pygmaeum Gutwi

Coesel, Peter & Westen, Marien Van, 2013, Taxonomic notes on Dutch desmids V (Streptophyta, Desmidiales): new species, new morphological features, Phytotaxa 84 (2), pp. 46-54 : 47

Closterium pygmaeum Gutwi ṅski (1890: 66) (Figs. 5–8, 17)

Closterium pygmaeum is a seldom recorded, poorly-known species, originally described from Poland. The original illustration by Gutwiṅski (1892: pl. 1: 5) copied in Růžička’s flora (1977: pl. 10: 1) is rather poor. The cell figured is characterized by small dimensions (26–29 × 4.8 µm), a low length to breadth ratio (about 6), an almost flat ventral side and rather broadly rounded apices. Both Krieger (1937) and Růžička (1977) think Cl. minutum Roll (1915: 191) described from Russia, and Cl. perminutum Borge (1925: 18) described from Brazil to be identical to Cl. pygmaeum . However, judging from the original figures copied in Růžička (1977: pl. 10: 2, 3) there are substantial differences in cell morphology, in particular the degree of curvature and shape of the apex. By this, an unambiguous concept of the morphology of Cl. pygmaeum is wanting.

Records of Cl. pygmaeum documented with reliable illustrations are very rare. Allorge & Manguin (1941) depict a Cl. pygmaeum cell from the Pyrenees much resembling the figure of Cl. minutum in Roll (1915), i.e., a relatively strongly curved cell with rather narrowly rounded poles. A figure of Cl. pygmaeum in Compère (1977) from Lake Chad, with a cell length of 80 µm and narrowly rounded poles, almost certainly belongs to another species. Figures of Cl. pygmaeum by Bourrelly (1987) from Austria are comparable with that in Allorge & Manguin (1941). Also a figure of Cl. pygmaeum in Bicudo & Castro (1994) from Brazil, though in shape resembling Gutwiṅski’s original figure somewhat better, with a cell length of a good 50 µ m is questionable.

The present authors were happy to encounter in recent years some Dutch populations of a Closterium species whose characteristics well matched with those of Cl. pygmaeum . The finds enabled us to establish the range of morphological features occurring in these populations originating from three different sites. The sites in question are oligo-mesotrophic moorland pools located near Huis ter Heide (province of North Brabant), Assen (province of Drenthe), Mantinge (province of Drenthe) and Wijster (province of Drenthe). Remarkably all locations are in nature-recovered, former agricultural areas. Next to cells that perfectly agreed with Gutwiṅski’s (1892) original description and illustration, so with a more or less straight ventral side (our Figs 5, 6, 7, 17) also cells were encountered marked by a slight curvature (Fig. 8). Cell length ranged from 22 to 30 (38) µm, cell breadth from 4.5 to 5.5 (6) µm, breadth of the apices from 1.5 to 2.5 µm. Chloroplasts (two per cell) contained a single pyrenoid in their axis.

From the above data Cl. pygmaeum comes to the fore as a small-sized, thick-set Closterium species with a straight to slightly curved ventral cell side and rather broadly rounded apices. In view of this characterization, Cl. pygmaeum could be identical to Cl. perminutum indeed, but the identification to Cl. minutum , marked by stronger curved cells with narrowly rounded poles, is still questionable.