Cosmarium distentum (G.S.West) 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 : 58-59

Cosmarium distentum (G.S.West) Coesel et Meesters , comb. et stat. nov. (Figs. 5–8, 20–25)

Basionym: Cosmarium laeve var. distentum G.S.West, Ann. S. African Mus. 9: 86, fig. 48, 1912.

Of Cosmarium laeve Rabenhorst (1868: 161) a considerable number of varieties have been described ( Krieger & Gerloff 1969). Considering their cell shape, some of those will belong to quite different species. As Rabenhorst (l. c.) did not provide any illustration there is uncertainty about the detailed shape of the nominate variety. The first picture of C. laeve was published in Nordstedt & Wittrock (1876: 29, pl. 12: 4) displaying a cell that is about 1.5 times as long as broad. Semicells in frontal view are somewhat sexangular with the greatest breadth in the basal part of the semicell. In apical view semicells are evenly elliptic and the cell wall looks coarsely punctate (possibly scrobiculate?). A roughly similar conception of C. laeve is to be found in the well-known flora of West & West (1908: 99, pl. 73: 8–17). In addition to that, the latter authors describe the zygospores of this species to be angular-globose, smooth, angles slightly thickened ( West & West 1908: 100, pl. 73: 18–19).

Cosmarium laeve var. distentum differs from the above-described diagnosis of C. laeve Rabenhorst (1868: 161) var. laeve by a lower cell length to breadth ratio (1.2 versus 1.5 in the nominate variety), a finely punctate cell wall and, most important, a prominent tubercle in the centre of the semicell (best to be observed in apical view). Cells perfectly fitting the diagnosis of C. laeve var. distentum as given by West (1912: 86, pl. 2: 48) were encountered by the second author of the present paper in a sample from a cattle-trough at Bath (province of Zeeland), in 2000. The sample in question represented an almost exclusive bloom of this taxon holding next to the vegetative cells numerous zygospores. Without exception, those zygospores were smooth-walled and globose-oval in shape (Figs. 7–8, 24–25). In this respect they differ from the zygospores of C. laeve described as angular-globose by West & West (1908: 100, pl. 73: 18–19). For that matter, it is somewhat questionable whether the pictures of the zygospores in West & West (l. c.) relate to ‘true’ C. laeve . The latter authors describe those spores from rocks in Thailand ( West & West 1901: 171, pl. 2: 14–16) whereas most records of C. laeve are from neutral-alkaline, more or less eutrophic water bodies ( Krieger & Gerloff 1969). Vodenicharov (1987) as well as Hindák & Hindáková (1994) depicted smooth-walled, more or less global zygospores of C. laeve comparable with those represented in our Figs. 7–8, 24–25. Overall, C. laeve comes to the fore as a polymorphic taxon that better be split up in a number of individual species. In that context, we herewith propose to render C. laeve var. distentum the status of a separate species.

C. distentum should be compared with C. subphaseolus Kouwets (2001: 44) , a species much similar in frontal outline but lacking the prominent central tubercle. In addition, C. wembaerense Schmidle (1898: 32) may be taken into consideration. However, semicells in that latter species also lack a distinct central protuberance. Moreover, C. wembaerense is characterised by tuberculate zygospores.

In the Netherlands, next to the find site at Bath, C. distentum was also encountered in cattle-troughs at Elsloo- Terhagen and near Harfsen. All records are from eutrophic water bodies holding but few desmid species and C. distentum always occurred in high cell numbers. Remarkably, a cattle-trough as habitat (the find site of our conjugating C. distentum ) is also mentioned in the original description of C. laeve var. distentum from South Africa by West (1912) and recently there was a reference to a blooming population in an animal drinking through at Boxgrove, in England (M.D. Guiry, in Guiry & Guiry 2014). So likely, C. distentum has a wide geographical distribution, particularly flourishing in instable habitats where there is but little competition by other algal species.