Staurastrum quadriradiatum Coesel et Van Westen, 2014

Staurastrum quadriradiatum Coesel et Van Westen , spec. nov. ( Figs. 8–10 View FIGURES 8–12 , 20, 21 View FIGURES 13–21 )

Cells in frontal view about as long as broad, deeply constricted. Sinus widely open, more or less rectangular. Semicells 3- angular in outline. Semicell body cup-shaped, the apical angles produced to form slender , divergent processes. Processes with hardly noticeable undulate/dentate margin. Cells in apical view 4-radiate with concave sides. Dimensions: length 20–23 µm, breadth 22–27 µm.

Type:— THE NETHERLANDS. Drenthe: Assen , pool in district Marsdijk, 53.018° N, 6.600° E, pH 7.5, conductivity 80 µS cm -1, Van Westen, 23 May 2008 (holotype L! Hugo de Vries Lab 2013.03, preserved as a fixed natural sample) GoogleMaps .

Differential diagnosis: — Staurastrum quadriradiatum is a small-sized Staurastrum species that can be compared with St. minimum Coesel (1996: 23) . Both species are very delicate in their morphology and display a corresponding cell shape. Light microscopically, almost no trace of ornamentation is to be seen. The most striking difference is in the number of processes: three in St. minimum , four in St. quadriradiatum ( Figs. 8, 9, 10 View FIGURES 8–12 ). As in other 4-radiate Staurastrum species , cell morphology is differently expressed depending on the position of the cell observed. Usually cells are seen when resting with one of their processes on the substrate (position 1, see Coesel & Meesters 2013: 42). Their appearance is different when resting on two processes (position 2), compare our Figs. 9 and 10 View FIGURES 8–12 .

Another differentiating feature with respect to St. minimum could be in the ornamentation of the processes. While the processes in St. minimum are tipped with small but distinct dentations, those in St. quadriradiatum are seemingly without ornamentation at the top. Only SEM pictures of St. quadriradiatum reveal that the extremities of the processes are not perfectly smooth ( Figs. 20, 21 View FIGURES 13–21 ).

Finally, but certainly not the least important difference, is in the ecology of the two species. Whereas St. minimum is described from acidic, oligotrophic moorland pools ( Coesel 1996), St. quadriradiatum was abundantly encountered in an artificial, eutrophic pool, the biocenosis of which was dominated by zooplankton and chlorococcalean green algae. Later on, this species was also found on three other localities in Drenthe. One of these was a former, shallow sand pit filled with water. The other locations were shallow, sandy pools at the margin of the built-up area of villages. All locations were used for recreational purposes.