Staurastrum subcomptum Van Westen et Coesel, 2018

Van Westen, Marien C. & Coesel, Peter F. M., 2018, Taxonomic notes on desmids from the Netherlands II, with a description of six new species, Phytotaxa 385 (1), pp. 1-12 : 8-10

Staurastrum subcomptum Van Westen et Coesel sp. nov. ( Figs 3 , 47–52 )

Cells a little longer than broad, rather deeply constricted. Sinus V- to U-shaped. Semicell body vase-shaped, not seldom slightly inflated just above the isthmus and provided with a supraisthmial whorl of downward pointing, acute granules. Apical semicell angles produced to form rather short, slightly inflexed processes. Processes furnished with concentric series of granules and with 4–6 small teeth at the truncate end. Granules on the ventral side of the processes much reduced or even wanting. Cells in apical view mostly 5-radiate (occasionally 4- or 3-radiate), the angles produced to truncate processes. Apex centre without granules, surrounded by a whorl of sometimes emarginate granules.

Dimensions: length 34–40 μm, breadth 30–39 μm, isthmus 7–9.4 μm.

Type:— THE NETHERLANDS. Drenthe: shallow pool in nature restauration area in Dwingelderveld near Dwingeloo, 52.807° N, 6.407° E, among submerged mosses on sandy soil, Van Westen, 5 August 2014 (holotype L! Hugo de Vries Lab 2018.06, preserved as a fixed natural sample).

Differential diagnosis:— Staurastrum subcomptum should be compared with Staurastrum comptum Wolle (1880a: 47 , 1880b: 91, pl. 5E), a characteristic species marked by an elongate, vase-shaped semicell body crowned by a variable number (usually six) relatively short, arm-like, slightly convergent processes. After having been described by Wolle (1880, 1884) from New Jersey, St. comptum was recorded and depicted but a few times, viz. by Irénée-Marie (1952) from eastern Canada, Grönblad (1956) from Massachusetts, and Scott & Grönblad (1957) from Lousiana. Where Wolle (1884: 129, pl. 42: 43–46) and Irénée-Marie (1952: 64, pl. 6: 19) make mention of a series of longitudinal ribs on either side of the isthmus Grönblad (1956, figs 150–151) depicts a supraisthmial whorl of fine granules, just like in our material. Such a form is also provided by Scott & Grönblad (1957, pl. 25: 9–10) under the name of St. comptum var. majus Wolle. However, Wolle (1884: 129) did not explicitly describe a var. majus . Two out of the four figures on Wolle’s plate 42: 43–46 are labeled ‘var. major ’, it is true, but obviously only based on somewhat larger cell dimensions, not on any other morphological feature. We neither agree with Prescott et al. (1982: 163) in labeling the above-mentioned form of Scott & Grönblad (l.c.) as ‘ Staurastrum comptum var. pinnatiforme f. reductum (Scott et Grönblad) comb. nov. ’. First of all, Scott & Grönblad do not mention any taxon ‘ reductum ’, so there cannot be a new combination of that taxon either. Moreover, there is not any indication of accessory processes at the base of the arms (associating it to St. pinnatum Turner ).

In conclusion, in our opinion St. subcomptum is characterized by a supraisthmial whorl of (acute) granules instead of longitudinal ribs. Cells in our type material are mostly 5-radiate (occasionally 6-, 4- or 3-radiate) while Wolle (1880, 1884), Irénée-Marie (1952) and Scott & Grönblad (1957) all make mention of 6-radiate cells. As Grönblad (1956) reports both 6- and 3-radiate as well as dichotypical cells we consider the exact number of processes of minor taxonomic importance. Probably the same holds for the number of teeth at the end of the processes: being three in the original diagnosis by Wolle (1880) and in Irénée-Marie (1952) whereas Grönblad (1956) depicts four terminal teeth and Scott & Grönblad (1957) even some five. As far as could be checked, our cell material is marked by some five or six terminal teeth. In view of the campanulate base of the semicell and the variable number of rather short, truncate processes at the apical angles, St. subcomptum might be related to St. margaritaceum Ralfs but it can easily be distinguished from that latter species by its elongate semicells.

Ecology and geographical distribution: —So far, St. comptum is only known from North America ( Prescott et al. 1982). Most of the records are from northern regions of the USA and Canada.As far as specified, collections originated from meso-eutrophic ponds and ditches ( Wolle 1884, Grönblad 1956, Scott & Grönblad 1957). St. subcomptum on the contrary, is found in oligo-mesotrophic, slightly acidic to acidic environments. In the Netherlands it was encountered at several locations in the province of Drenthe, but probably it is more widespread.

Figures

FIGURES 1–8. 1. Closterium joostenii. 2. Closterium cynthiaforme. 3. Staurastrum subcomptum. 4. Staurastrum exlooënse.5. Cosmarium abbreviatiforme. 6. Cosmarium kouwetsii. 7. Zygospore of Cosmarium spierense with adhering gametangial cells. 8. Cosmarium spierense.

FIGURES 41–52. 41–46. Staurastrum exlooënse (45, 46. SEM microphotographs). 47–52. Staurastrum subcomptum. (fig. 47 and 48 are different views of the same cell. (50–52. SEM microphotographs). Scale bar = 10 μm.