Difflugia bryophila (Penard, 1902) Jung, 1942
Diagnosis: Shell transparent, colorless; pyriform or elongated ovoid with the sides usually tapering evenly to the aperture; shell composed of a mixture of small to medium pieces of quartz and occasionally fragments of diatom frustules and some small siliceous plates; open network of organic cement. Aperture circular and surrounded by small pieces of quartz.
Dimensions: Ogden 83-141 µm. My measurements: 78-138 µm.
Remarks: Larger shells resemble those of Difflugia lacustris. They are sometimes hard to distinguish from each other. In the spring of 2012 I had an interesting sample from a small floating piece of debris, coming from the bottom of a shallow ditch along the Naardermeer. The sample contained a huge collection of shells, most empty, but the smaller ones were inhabited. While all shells were present in some cubic centimeters of debris, it is likely that they were all related.
I measured the length of 469 shells. The results are given in the graph below. It is remarkable that there are at least two populations. One possibility is that there are smaller shells in spring and larger shells later in the year. That could explain that in my sample (spring) only smaller shells were inhabited. A second explanation could be that there are at least two different species, but these cannot be distinguished light microscopically.