shell hyaline, thin, without xenosomes, sometimes covered with brownish crust
in frontal view spherical or oval, sometimes irregular or angular
in lateral view dome-shaped
shell with two opposite necks
length of shell (inclusive necks) 9.3–13.6 µm
diameter of necks 1.7–2 µm
protoplast fills shell not completely
granofilopodia form a dichotomously branched and anastomizing network
nucleus central, with a single nucleolus
several contractile vacuoles
Ditrema mikrous
So far I could detect Ditrema mikrous only in the Simmelried. Like the similar species Ditrema longicollis, Ditrema mikrous likes to settle on floating coverslips. I could observe both species in parallel on the same coverslips. The main distinguishing feature from Ditrema longicollisis the size. According to De Saedeleer, 1934 (s. literature) Ditrema mikrous with 9, 3 – 13,6 µm length (including the necks) is only half the size of Ditrema longicollis. I can confirm these size differences. In my population the length of the specimens was 12,3 – 13,9 µm which agrees well with the data of De Saedeleer. I could also observe that the shells of some specimens were covered with a brownish crust (s. fig. 3). This was different from the orange-brown or brown discoloration caused by iron deposits that occur in Ditrema longicollis. Since I did not find any Ditrema shells with a length of 14–23 µm (i.e., the gap between the size specification of Ditrema mikrous and Ditrema longicollis), Ditrema mikrous cannot be a smaller variant of Ditrema longicollis, confirming De Saedeleer’s delimitation.