Genus Microgromia Hertwig, 1874
Diagnosis: The shell is hyaline, small, and retort-shaped, exhibiting double-sided symmetry. In some cases, one side may be attached to the substrate. The outline is typically circular but can occasionally appear irregular or angular, with or without a distinct neck. The neck is curved and partially fused with the shell surface, forming an internal septum that features a lateral aperture. The shell is composed entirely of organic material, lacking any embedded xenosomes, and ranges in color from yellow-brown to dark-brown.
The cytoplasm contains an asymmetrical or symmetrical bundle of filopodia, which are granular (extrusomes), extremely thin, and capable of branching and anastomosing to form a network or reticulum. The nucleus is relatively large, globular, and contains a central nucleolus. A single contractile vacuole is present.
Habitat: Both freshwater and marine environments, typically in shallow, iron-rich waters, often found within mucous colonies of cyanobacteria.
Remarks: Martin Kreutz published an excellent paper on this intriguing yet poorly understood group of testaceans (Mikrokosmos, Heft 3, 2012). This work marks the first significant report on the group since De Saedeleer’s publication (Beitrag zur Kenntnis der Rhizopoden, 1934). Kreutz highlights the challenges of identifying species based on De Saedeleer’s descriptions and drawings. Despite these difficulties, he successfully identified five species in a pond in Simmelried, near Konstanz, Germany.
An important diagnostic feature is the presence or absence of an internal septum. However, due to the small size of the structures, this characteristic can be difficult to observe under normal light microscopy. A 100x oil-immersion objective may be necessary to resolve fine details. Interestingly, De Saedeleer (1934) described a new species, Microgromia hertwigia, based on the potential presence of a septum, which he himself never observed!
De Saedeleer (1934) noted that a septum can be oriented either from left to right or vice versa. However, this characteristic is of limited use if the observer cannot determine whether they are viewing the specimen in dorsal or ventral orientation (Kreutz, 2012).
De Saedeleer (1934) placed significant emphasis on the visibility of granular filopodia as a diagnostic feature for species identification. In my opinion, however, this characteristic is not particularly useful for this purpose.
Based on the observations of De Saedeleer (1934) and Kreutz (2012), the following key is proposed:
| – | 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 symmetrical. | M. parvisaepimen |
| – | Shell spherical, somewhat elongated, with a distinct neck. Cytoplasm asymmetrical. |
