Thecamoeba sphaeronucleolus (Greeff, 1891) Schaeffer, 1926
Diagnosis: Surface strongly wrinkled in stationary amoebae, becoming somewhat smoother in locomotion, occasionally with several longitudinal dorsal folds in rapidly advancing amoebae: outline in locomotion often oblong or oval, sometimes broad with expanded anterior hyaline region, sometimes with knob-like uroid; hyaloplasm usually an antero-lateral crescent, in broader active forms sometimes spreading and flattening into a rather extensive sheet; nucleus usually with a single central nucleolus of coarsely granular, non-homogeneous texture, sometimes partly or completely fragmented into two or three lobes, which are never peripherally arranged; a single contractile vacuole, with diameter to 21 µm.
Dimensions: Page, 1977: length ca. 65-140 µm (mean 98 µm), length: breadth ratio 1.0-2.0 (mean 1.4); nucleus 11.0-15.5 µm.
Ecology: fresh water. Feeding on bacteria.
Remarks: Greeff did not publish any illustrations of this species, and his statement that this scarcely reaches half the size of his Amoeba similis leaves open to serious doubt that the species ordinarily identified as T. sphaeronucleolus is the one which Greeff had in mind. His description. including its attempt to ‘unite ‘ two previously named species under this name, is very unsatisfactory. Nevertheless, there seems to have been among workers who have used the name good agreement on the amoeba intended, namely the one described by Penard (1902) under that name, and that is the species here considered to be T. sphaeronucleolus. Greeff (1891) spelled the specific epithet ‘ sphaeronucleosus’ but most subsequent workers have followed Penard, using ‘sphaeronucleolus‘.
It should not be difficult to distinguish T. sphaeronucleolus from the two most similar rugose species, T. verrucosa and T. terricola, on the basis of nuclear structure alone, without considering size. It should likewise not be difficult to distinguish from T. quadrilineata, a distinctly smooth Thecamoeba with a nucleolus of homogeneous texture. Probably some of the ‘ verrucosa-like ‘ amoebae in the literature belonged to this common species. (Page, 1977)