Genus Stygamoeba Sawyer, 1975 emend. Smirnov 1996

Diagnosis: Locomotive form thin and elongate resembling tooth-picks or splinters but capable of temporarily acquiring a forked or branched morphology. Anterior hyaloplasm can form very narrow, lateral hyaline extensions in some moving amoebae. Locomotion by steady cytoplasmic flow although cytoplasmic streaming difficult to observe. In non-directed movement, or when stationary, amoebae can adopt a broad, leaf-like shape, the body of which may be branched. Floating form with radiating pseudopodia in known species. At the ultra-structural level, in the case of the only species studied, the cytoplasm contains dictyosomes and mitochondria with flattened cristae. The cell coat consists of a thin, undifferentiated glycocalyx. Cells uninucleate. No flagellate stage in the life cycle.
Type species: S. polymorpha Sawyer, 1975 .

Stygamoeba regulata Smirnov 1996

Diagnosis: The anterior hyaloplasm occupies 114 to 113 of the total cell length, and sometimes forms very narrow lateral extensions. At the posterior, a bulbous uroid is common. Length of the locomotive form ranges between19-38 µm (average 27 µm), and the breadth between 3.4-5.7 µm (average 4.6 µm); L/B ratio 4-8.6 (average 6.1). Furcation of the anterior end common when actively moving cells change direction of locomotion. Amoebae never considerably branched. Floating form with 3-6 thin, hyaline pseudopodia with rounded ends. Lengths 2-3 times greater than the diameter of the central cytoplasmic mass. Nucleus of vesicular type, spherical or elongate, about 2.5 µm in diameter, no nuclear lamina. Central nucleolus about 1.5 µm in diameter. Thickness of glycocalyx about 13 nm. Electron dense bodies obvious in the cytoplasm. Cysts spherical, with as single wall about 100 nm thick.

Ecology: the upper layer of sediments in Niva Bay, The Sound, Denmark, where salinities were about15‰. Can also multiply in marine water with full salinity around 35‰.

Stygamoeba cauta Lotonin and Smirnov, 2019
Diagnosis: Anterior hyaloplasm occupies up to 1/3 of body length. Cells sometimes with slightly raised bulbous uroid. Locomotive form on average 26 x 4 μm (range 17.5-37.5 x 2.5-7.5 μm), length/width ratio 6.6 (range 3-11). A thin waving subpseudopodium is usually exposed over substratum, able to adhere to substrate and then transform to a leading part of the cell. Floating 28 cell is round and has several (up to 3) thin hyaline protrusions. Nucleus ellipsoidal and vesicular, 29 2-4 μm (average 3 μm). No cysts known.

Type locality. Nivå Bay (Baltic Sea, The Sound, 55°55’41.8″N 12°31’23.0″E), upper layer of the bottom sediments (0-1 cm).