Genus Microgromia

Microgromia   Microgromia
Shell, arrow indicates septum    Microgromia haeckeliana, 16.9 µm
 
Genus Microgromia Hertwig, 1874
Diagnosis: Shell hyaline, small, retort-shaped, double-sided symmetrically, sometimes with one side fastened to the substrate; outline circular, sometimes irregular or angular, with or without a distinct neck. The neck is curved and partially fused with the shell surface, thus forming an internal septum with a lateral aperture. Shell composed of an organic material without any embedded xenosomes; color usually yellow-brown till dark-brown. Cytoplasm with an asymmetrical or symmetrical bundle of filopodia which are granular (extrusomes), very thin, and are able to branch and anastomose, thus forming a network or reticulum. Nucleus relatively large, globular with a central nucleolus. One contractile vacuole.
Ecology: freshwater and marine; shallow, ironrich water; in mucous colonies of cyanobacteria.

Remarks: Recently Martin Kreutz published an excellent paper on this interesting but poorly known testacean group (Mikrokosmos, Heft 3, 2012), the first report after the publication of De Saedeleer (Beitrag zur Kenntnis der Rhizopoden, 1934). Kreutz notices the difficulty of identification species using the descriptions and drawings of De Saedeleer. Nevertheless he succeeded in the identification of five species in a pond in Simmelried, near Konstanz, Germany.
An important diagnostic characteristic is the presence or absence of an internal septum. However, because of the small dimensions of the scales, this characteristic is sometimes hard to observe under normal lightmicroscopical conditions. An 100X oil-immersion objective maybe very helpfull to resolve fine details. It is curious that De Saedeleer based his description of a new species, Microgromia hertwigia, on the potential presence of a septum, which he never could observe!
According to De Saedeleer a septum can be oriented from left to right or vica versa. But this characteristic isn't very usefull if an observer cann't see if he is looking at a specimen in dorsal or ventral view (Kreutz, 2012).
De Saedeleer attaches great value to the visibility of the granular filopodia to identify species, but in my opinion this is not a useful characteristic.
 

Based on the observations of De Saedeleer and Kreutz, I propose the following key:

Shell nearly spherical, with a distinct, often thickened neck. Septum clear, usually with a shorter kind of septum at the base of the neck, thus forming a V-shape. Cytoplasm asymmetrical. M. longisaepimen
Shell nearly spherical, often angular, with a distinct neck and a rather curved but well developed septum. Cytoplasm asymmetrical. M. haeckeliana
Shell nearly spherical, without neck, with a very small aperture. Septum short. Cytoplasm nearly symmetrial. M. parvisaepimen
Shell spherical, somewhat elongated, with a distinct neck. Cytoplasm asymmetrical. M. minor
Microgromia spec.
Microgromia
Microgromia spec., 13.6 µm, pseudopodial network about 200 µm large - Austria, Sphagnum
Microgromia spec BB
Microgromia spec. 13 µm - Bert Bospad
Microgromia longisaepimen
Microgromia spec., with pseudopodial network (granuloreticulum)
Microgromia longisaepimen
Microgromia spec., with extended pseudopodial network (granuloreticulum)