Cosmarium sexnotatum Gutwinski 1893
Most likely ID: n.a.
Synonym: n.a.
Sampling location: Lake Constance
Phylogenetic tree: Cosmarium sexnotatum
Diagnosis:
- cell broadly oval
- semi-cells almost oval, with convex sides
- apices slightly convex or flat, slightly wavy
- length 18–29 µm
- sinus slightly widened
- central field with 3 parallel, elliptical ridges
- below the ridges a transverse row of tubercles
- two semi-circle rows of warts parallel to margin of semi-cells
- one chloroplast with one pyrenoid per semi-cell
In August 1994 I found Cosmarium sexnotatum in the plankton of Lake Constance. This is an unusual location, as this alga is actually found in bogs and alpine ponds. However, it may have been an input by tributaries. I have not found Cosmarium sexotatum again since 1994.
With a length of about 25 µm Cosmarium sexnotatum is a rather small species within the genus Cosmarium. However, it can be identified quite easily by the three typical parallel ridges in the central fields of the half cells (s. fig. 1 a). Sometimes these ridges can consist of two parts. The similar subspecies Cosmarium sexnotatum var. tristriatum is more compact and has only one semi-circular row of tubercles on the front side of the semi-cells (Cosmarium sexnotatum has two rows). In addition, the semi-cells of Cosmarium sexnotatum var. tristriatum are trapezoidal or almost rectangular in shape.
Fig. 1 a-b: Cosmarium sexnotatum. L = 25 µm. Two focal planes of a specimen found in August 1994 in Lake Constance. Note the three parallel ridges in the central fields of the semi-cells (a). Obj. 100 X.