Bereczkya minuta Kiss and Török, 2009
Diagnosis: Shell spherical, but often asymmetric because of swellings and depressions, with an ectoplasmic rim about 0.8-2.3 µm diameter; shell agglutinated with refractile mineral xenosomes and other flat or irregular particles. The xenosomes cover the test typically in a single layer, but the covering may be thick and irregular, or absent almost completely.
Dimensions: Shell length 3.5-8 µm.
Ecology: Freshwater; River Danube, Hungary, and Wormerveld, Netherlands.
Remarks: The minute spherical cells are enclosed in a thin, rigid organic test with agglutinated particles. The length of the cell varies between 2.7-6.2 m, the whole organism with the test is 3.5-7.8 m. The shape of the cell is nearly spherical, but in most specimens it is somewhat asymmetric: Swellings and depressions alter the shape to irregular or reniform. The cytoplasm fills the test entirely in all specimens. The test has a colorless inner organic layer, which usually becomes thinner towards the aperture.
The xenosomes are quartz particles with various shapes, mainly flattened plates, or small spheres. A particle type of relatively large (up to 1 m), wrinkled, ovoid knobs is also characteristic in some specimens. The xenosomes attach only superficially to the organic layer. They are arranged rather irregularly. They usually form a single layer, but thicker aggregation of particles is also observed. Sometimes the xenosomes are nearly completely absent. All the cells have a collar-like ectoplasmic rim (0.8-2.3µdiameter) with a central depression, from the middle of which the pseudopodial stem originates. This ectoplasmic rim is well visible, and thus it might be easy to confuse with the unpronounced aperture of the test, which surrounds the rim. The aperture is a wider irregular rigid hole on the thin test wall. In the anterior part of the cytoplasm, adjacent to the ectoplasmic rim, one-two larger, and a few smaller contractile vacuoles are present. The nucleus is peripheral or eccentric; it is situated in the middle or posterior region of the cell. Its shape is ovoid, reniform, or somewhat flattened with acute edges. The nucleoplasm is filled with about 4-8 straight or curved rods, there is no central nucleolus. Usually 1-4 typical filopodia are produced from the pseudopodial stem. The filopodia may be straight or curved, and they may branch perpendicularly or at an acute angle to each other. Their tips are rounded. Their length may reach up to 6 X cell length. A flattened lamellipodium-like cytoplasmic extrusion may also be produced with thin acute subpseudopodia on its edge. Sometimes specimens attach to each other.
The cells can move very slowly by gliding on the surfaces of extended filopodia. A relatively faster, but temporary movement is achieved, when the cell retracts a filopodium, which is attached to the surface on its distal part. In this case, the whole cell is pulled along by the filopodium. The longitudinal cell axis is more or less perpendicular to the surface during the movement. (All after Kiss et al, 2009)