Monactinus simplex

(Meyen) Corda, 1839

Most likely ID: n.a.


Synonym: Pediastrum simplex, Micrasterias simplex


Sampling location: Pond of the waste disposal company Constance


Phylogenetic tree: Monactinus simplex



  • coenobium star-shaped, flat and single-Iayered
  • diameter coenobium up to 250 µm
  • coenobium of 4, 8, 16, 32, 64, (128) cells
  • cell wall smooth or granulated
  • marginal cells with a single, gradually tapering projection
  • inner cells Y-shaped with intercelluar spaces, sometimes with a single central space
  • marginal cells occasionally bearing tufts of mucilaginous spines
  • one chloroplast, filling the cell
  • single pyrenoid

I found Monactinus simplex in the plankton  of the pond of the waste disposal plant of Constance in September 2023 in large large quantities.


The genus Monactinus is easily recognized because the marginal cells have only one horn-shaped process. Between the inner cells of Monactinus simplex large intercellular spaces are present (s. fig. 1 a). Sometimes the inner cells lacking so that the outer cells form a ring. The cell wall of the specimens in my population was granulated but noch covered with short spines as in the similar form Monactinus simplex var. echinulatum.


Fig. 1 a-b: Monactinus simplex. D = 85 µm. Two focal planes of a coenobium of 16 cells. There are intercellular spaces between the Y-shaped inner cells (a). The surface of this specimen is finely granulated (b). PY = pyrenoid. Obj. 100 X.