Pelomyxa binucleata (Gruber, 1884)
Diagnosis: Locomotive form usually broadly oval, but in faster movement more elongated, cigar-shaped. Slow forms with a pronounced hyaline bulbous uroid with conical villi and small lateral hyaline, usually conical, pseudopodia. Numerous cilia are scattered all over the cell surface except the frontal lobopodium, often found in groups of 5-6 cilia and always present in the uroid zone. Slow moving forms often form a strong hyaline lobopodium and start moving in its direction, but almost at once the zone of hyaloplasm shifts to the lateral cell surface, and the vector of movement changes, and so on. Fast moving amoebae form only a narrow zone of hyaloplasm on the anterior body end, without any lateral pseudopodia and with a small uroid, often oriented towards the substrate during movement. Resting forms have a spherical shape, without uroid, but with a zone of hyaline villi on the body surface, among which numerous cilia are visible. Cells of this form have a pronounced layer of peripheral hyaloplasm. Short hyaline pseudopodia of various shape are generated spontaneously all over the surface.
As reflected in the species name, the key character of P. binucleata is the permanent presence of two nuclei in adult individuals. Both nuclei of P. binucleata are spherical with an almost identical structure and size. Most part of the nucleus is occupied by the central nucleolus, which has a complex spatial organization.
Dimensions: Length of slow locomotive forms vary from 200 to 350 µm; fast moving forms reach 400-450 µm in length; spherical cells vary from 150 to 300 µm.
Ecology: Pelomyxa binucleata is one of the most common Pelomyxa species in water bodies. It may be found in almost all kinds of stagnant and slowly flowing waters with sufficient littoral silt sediments. This amoeba is especially numerous in places with a rich aquatic vegetation at small depths (10-60 cm).
P. binucleata are found all year round, being as common in samples from frozen water bodies in the middle and the end of winter as in summer and autumn samples. Adult amoebae are the most numerous.
Remarks: Bright and contrasting coloring is very typical of P. binucleata individuals, regardless of the biotope of provenance. This is associated with a large number of food inclusions, mostly various unicellular chlorophyll-containing microorganisms. Besides, the cytoplasm of P. binucleata often contains bacterial colonies of different shades of red.
Another feature of P. binucleata cells is a very small number of structural vacuoles in the cytoplasm. It is due to this fact that the cytoplasm of these amoebae appears transparent under a light microscope and all their cytoplasmic inclusions, very contrasting. Singular vacuoles and their groups do occur in the cytoplasm, moving freely with its flows, but great care should be taken in drawing homologies between them and the structural vacuoles filling the whole cell volume in other pelomyxoids.
The cytoplasm of P. binucleata always contains numerous small refractive granules with a diameter of about 3 µm. In adult individuals of P. binucleata, glycogen bodies are always present in large numbers. They are spherical, smooth and reach 10-15 µm in diameter.
A characteristic feature of P. binucleata is the presence of very long rod-like prokaryotic endocytobionts of two types. The length of both bacterial species varies in a broad range: from 3 µm to 30 µm and more. However, their diameter is constant, being about 1 µm in one of the species and not exceeding 0.5 µm in the other one.