Closterium pronum De Brébisson (1856: 157)

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 : 46-47

Closterium pronum De Brébisson (1856: 157) ( Figs. 1–4 , 13–16 )

In Europe, Closterium pronum is a widely distributed species in various types of acidic to circumneutral water bodies. Yet, up to now zygospores of this species were unknown (e.g., Růžička 1977, Coesel & Meesters 2007, Brook & Williamson 2010).

Admittedly, a single figure of a zygospore said to belong to Cl. pronum has been published by Wade (1952) but as adhering gametangium cells have not been depicted identification of that alga cannot be checked.

Recently (April 2012) an abundantly sporulating population of this species in ‘Alenburg’, an artificial pool on former agricultural soil at the Drenthian village of Smilde, was encountered. Morphological characteristics of vegetative cells ( Fig. 1 ) fulfilled those stated in the above-mentioned authoritative floras. With an average cell length to breadth ratio of 38.9 (n = 20, SD = 4.0) the similarly shaped, but less slender species Closterium idiosporum West & West (1900: 290) was no serious option ( Růžička 1977). Zygospores appeared to be most peculiar in that they, when maturing, developed marked, undulate, longitudinal ridges on the spore wall.

Initially, the young, elliptic zygospore is smooth-walled ( Figs 2, 3 , 13 ). Already in this immature stage in its ectoplast a number of longitudinal series of closely set, fine punctae or striae may be distinguished ( Figs. 2 , 14 ). When maturing, those striae transform into more solid plasma extensions ( Figs. 3 , 15 ) which in their turn immerse in irregular, sclerotic thickenings forming longitudinal, undulate ridges on the outside of the zygospore ( Figs 4 , 16 ). At that time, the original smooth exospore wall has completely disappeared as is the case with the initially adhering gametangial cells.

In formation of the above-described costate zygospores Cl. pronum is rather unique. Only Cl. costatosporum Taft (1949: 212) , a North American species whose vegetative cells are quite different from those in Cl. pronum , exhibits somewhat comparable spores (at least in young stage). The peculiar, undulatecostate zygospores of Cl. pronum constitute an important additional differentiating feature with respect to Cl. idiosporum that, in all stages of maturing, is characterized by smooth-walled (at best punctate) zygospores ( Růžička 1977).

For that matter, considering the original illustration of Cl. pronum by De Brébisson (1856) and those in the floras by West & West (1904), Krieger (1937) and Růžička (1977) we cannot preclude that more than one species is at issue. Our vegetative cells most resembled those figured in Krieger (1937, pl. 13: 5, 6) and might represent a separate species.

Figures

FIGURES 1–4: Closterium pronum. Fig. 1: vegetative cell. Fig. 2–4: three successive stages of zygospore formation (Fig. 4: mature zygospore in frontal and apical view). FIGURES 5–8: Closterium pygmaeum. FIGURES 9, 10: Euastrum pseudotuddalense. FIGURES 11, 12: Cosmarium vossenbergense. Scale bar A (50 µm): Figs 1–4. Scale bar B (10 µm): Figs 5–12.

FIGURES 13–16: LM images of Closterium pronum, representing four successive stages of zygospore formation. FIGURE 17: LM image of Closterium pygmaeum. FIGURE 18: LM image of Cosmarium vossenbergense. In all figures, scale bar = 10 µm.