Euastrum spondylosioides 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 : 60-61

Euastrum spondylosioides Coesel et Meesters , sp. nov. (Figs. 9–10, 26–29)

Cells in frontal view longer than broad, with a moderately deep sinus that is (almost) closed near its apex but widely opens to its extremity. Semicells rectangular-trapeziform with concave sides. Basal and apical angles broadly rounded, lateral sides in their mid region almost straight, apex slightly retuse or marked by a shallow, median indentation. Cells in apical view elliptic with somewhat produced poles. Cell wall smooth, finely punctate, occasionally with a distinct pit in the semicell centre. Chloroplast with a single pyrenoid. Dimensions: length 14–17 μm, breadth 9.5–12.5 μm, isthmus ca. 5 μm, thickness ca. 8 μm.

Type:— THE NETHERLANDS. Drenthe: wet cattle trail in ‘Eexterveld’ near Eext, 53.015º N, 6.697º E, Meesters, 1 April 2011 (holotype L! Hugo de Vries Lab 2014.02, preserved as a fixed natural sample).

Differential diagnosis:—The above-diagnosed species resembles given Euastrum Ehrenberg ex Ralfs (1848: 78) species with a more or less cosmarioid cell shape, such as Eu. coeselii Kouwets (1987: 213) , Eu. validum West et G.S. West (1896: 245) , Eu. sublobatum Ralfs (1848: 91) and Eu. brevisinuosum ( Nordstedt 1887: 161) Kouwets (1984: 338) . In particular, the latter species, with its broadly rounded apical angles, retuse apex and broad isthmus has much in common with our species under discussion. Yet, there are also some undeniable differences, the most relevant of them referring to the shape of the apical semicell lobe in vertical view: rectangular in Eu. brevisinuosum versus elliptic in Eu. spondylosioides . Moreover, cell dimensions in Eu. brevisinuosum are almost two times as large as those in our material and semicells in Eu. brevisinuosum are marked by a prominent isthmial protuberance that lacks in Eu. spondylosioides .

Our newly described Euastrum species also roughly resembles some species of the genus Cosmarium , e.g., C. norimbergense Reinsch (1867: 117) or C. decedens ( Reinsch 1867: 114) Raciborski (1889: 80) . Actually, it belongs to a group of Euastrum species that could be assigned the genus Cosmarium as well, compare Euastrum binale var. borgei A.M. Scott et Prescott (1958: 30) . Eventually, our decision to describe it as a species of Euastrum is based on the shallow, median indentation of the semicell apex which is traditionally considered a main characteristic of that genus ( Růžička 1981).

The association with the genus Spondylosium Brébisson ex Kützing (1849: 23) , as expressed in the epitheton of our species, was induced by some two findings. First, Lütkemüller (1893: 539, pl. 8: 1) described Sphaerozosma pulchellum var. austriacum . The filamentous species Spaerozosma pulchellum W. Archer (1858: 253) was transferred to Spondylosium pulchellum (W.Archer) W.Archer in Pritchard (1861: 724). Lütkemüller (l. c.) depicted only some two coupled cells of his newly described variety leaving undecided if they formed part of a longer chain. The cells in question differ from the nominate variety of Spondylosium pulchellum in that the semicell apex (relative to the semicell base) is broader and somewhat dilated. Cells of our newly described Euastrum spondylosioides show that same characteristic only that the semicell apex is even broader, almost as broad as the semicell base. Exact such a cell form as ours was represented by Tomaszewicz & Hindák (2008: 291, figs 5, 6) from western Slovakia. They depicted short cell chains varying from two to four cells associating the genus Spondylosium . Tomaszewicz & Hindák (l. c.) labeled the form in question ‘ Euastrum sublobatum morpha’ but in our opinion it should not be accounted that species as it differs in various respects (cell outline, dimensions) essentially from Eu. sublobatum as originally described in Ralfs (1848: 91).

Possibly, our Eu. spondylosioides is related to Spondylosium pulchellum , a filamentous species that is known to desintegrate very easily in separate cells. Eu. spondylosioides cells in our sample were encountered only rather incidentally so it is conceivable that short chains would be found at higher population densities.

Ecology:— Euastrum spondylosioides was encountered in a wet cattle trail on sandy moorland soil.Accompanying desmid species such as Cylindrocystis brebissonii De Bary , Mesotaenium macrococcum (Kützing) J.Roy et Bisset , Mesotaenium minimum Cushman and Roya obtusa (Brébisson) West et G.S.West indicate a more or less ephemeral character of the aquatic conditions. This is in striking agreement with the find of the Spondylosium -like Euastrum sublobatum ‘morpha’ in Slovakia, i. e. ‘a sand puddle on the shore of a gravel pit, together with Cylindrocystis brebissonii De Bary and Mesotaenium endlicherianum Nägeli’ (František Hindák, pers. comm., 13 March 2002). Recently, a colleague of ours, Marien van Westen, informed us to have found the taxon in question too, viz in an ephemeral puddle on loamy soil at the estate of Vossenberg, near the village of Wijster, ca. 20 km southwest of our find at Eext. He also encountered but a few, separate cells ( Figs. 28–29 ). In addition to that, the species in question was also found by another Dutch colleague on wet, sandy soil in a nature development area near Tilburg (province of Noord Brabant) so, presumably, it is not an extremely rare taxon.

Figures

FIGURES 26–29. Euastrum spondylosioides. 28–29 courtesy M. van Westen. Scale bar = 10 μm.