Acropisthium mutabile is described by Kahl as a very common species. I could find Acropisthium mutabile only a few times in Ulmisried and Simmelried. It is remarkable that I found the specimens exclusively in the cold season between end of October and mid of March. I could not find them in the summer months.

Although Acropisthium mutabile is mostly smaller than 100 µm, the species can be easily identified by the dome-shaped to snout-shaped anterior end and the pointed posterior end. The mouth opening is lined with rod-shaped, 6.6–6.9 µm long extrusomes (s. figs. 2 and 3a-b). The contractile vacuole is located sub-terminally (s. fig. 3a). In most specimens the macronucleus was long oval and slightly curved. I could recognize a dorsal brush comprising three rows and can confirm Kahl’s description that only one of the rows bears bristles, while the other two consist of small cusps or tubercles. According to Kahl, the diet should consist mainly of small algae. However, I could not find any phagocytosed algae in any of the specimens I examined. Although the contents of the food vacuoles are difficult to interpret, I consider their contents to be rather phagocytosed small ciliates or flagellates (s. figs. 2 and 3b).