Study of a rediscovered large freshwater multinucleate amoeba Chaos illinoisense Kudo, 1950)

A large freshwater multinucleate amoeba Chaos illinoisense (Amoebidae), previously only recorded in natural collection by Kudo (1950, 1951) from a small pond in Illinois, Central North America, has been rediscovered in backwaters of the River Luga, North-West Russia. Amoebae were cultured, cloned, and studied using lightand electron-microscopic techniques. This species is the only currently known cyst-forming representative of the genus. Numerous cytoplasmic crystals is a conspicuous characteristic of the species; they appear in two forms: platelike and bipyramidal. Trophozoites contain several hundred spherical nuclei of granular type. The fibrous inner nuclear lamina with loosely organised honeycomb-like structure was revealed as well as distinct mitochondrial heteromorphism. Two-layered cell surface coat with filamentous appearance is demonstrated. Current justification of the specific diagnosis is presented.