Microthorax pusillus (Engelmann, 1862)

Most likely ID: n.a.


Synonym: n. a.


Sampling location: Ulmisried, Simmelried


Phylogenetic tree: Microthorax pusillus



  • shape semicircular, ventral margin slightly concave, dorsal convex
  • flattened laterally
  • length 20–35 µm
  • right side flat, left side with 2 anterior and 3 small teeth
  • right side with 3 rows of cilia interrupted in middle
  • three short praeoral rows of cilia
  • macronucleus spherical in mid-body
  • one spherical micronucleus
  • contractile vacuole above mouth opening
  • extrusomes spindle-shaped, ejected extrusomes anchor-shaped
  • oral apparatus at posterior end of cell
Microthorax pusillus

Microthorax pusillus is a very common ciliate in my sites Ulmisried and Simmelried. There I find this ciliate in the mud or between decomposing plant masses. In my populations some specimens with 40 µm length were about 10 % larger than given by Kahl and Foissner (s. fig. 3 a-b). Although the ciliate is quite small, it is easily recognized by the bean-shaped body and the mouth opening located at the posterior end. On the right side of the body there are 3 rows of cilia, which are interrupted in the middle of the body (s. fig. 4a). Each cilium in these rows originates in a separate depression in the pellicle, creating a typical chain pattern.


Microthorax pusillus feeds on bacteria but also on rhodobacteria. Since the Simmelried is rich in rhodobacteria, I have found several times specimens here, which obviously fed preferentially on pink rhodobacteria (s. figs. 3 a-b and 4 a-b). Thus the specimens also appeared intensely pink at low magnification. At high magnifications, partially decomposed rhodobacteria were then visible in the cytoplasm due to the ongoing digestion process.


Fig. 1 a-c: Microthorax pusillus. L = 31 µm. A freely swimming specimen. CV = contraclile vacuole. Obj. 100 X.


Fig. 2 a-c: Microthorax pusillus. L = 25 µm. Three focal planes of the mouth opening (MO). Obj. 100 X.


Fig. 3 a-b: Microthorax pusillus. L = 40 µm. Two focal planes of the right side of a specimen that fed on rhodobacteria found in March 2003. FP = furrows of praeoral cilia rows, PO = praeoral cilia. Obj. 100 X.


Fig. 4 a-b: Microthorax pusillus. L = 38 µm. Two focal planes of the right side of a second specimen that fed on rhodobacteria found in June 2022. CR = interrupted cilia rows of right side, EX = extrusomes, Nu = nucleus. Obj. 100 X.