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 longicollis is 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 coloration caused by iron deposits that occur in Ditrema longicollis. Since I have not find any Ditrema shells with a length of 14–23 µm (i.e., the gap between the size range of Ditrema mikrous and Ditrema longicollis), Ditrema mikrous cannot be a smaller variant of Ditrema longicollis, confirming De Saedeleer’s delimitation.

More images and information of Ditrema mikrous: Ferry Siemensma-Microworld-Ditrema mikrous