Polychaos dubium (Schaeffer, 1917)
Diagnosis: large to medium-sized amoeba with an irregular polypodial shape during slow locomotion; palmate form with some large parallel pseudopodia; pseudopodia tend to fuse at their bases as the main cell mass advances; uroid large, fasciculate, sometimes with villi; crystals relatively large, with many variations, usually squared, bi-pyramidal or rod-shaped, like a candlestick; there are also “paired bodies” and irregular-shaped crystal-like inclusions in the cytoplasm; monopodial form during rapid locomotion; single nucleus, granular, usually discoid, sometimes ovoid or spherical, filled with small nucleolar pieces; floating form when disturbed ovoid or spherical, but soon after becoming star-shaped with several radiating thick pseudopodia of unequal length.
Ultrastructure: Surface coat consists of very fine filaments 170-180 nm in length, radiating from the plasma membrane. Filaments are extremely fine and often collapse under inappropriate fixation. This species has fibrous honeycomb-like inner nuclear lamina, 250-320 nm in thickness. Mitochondrial heteromorphism has been noted
Measurements: Monotactic form 250-750 µm (Schaeffer up to 1000 µm); polytactic form 250-400 µm; nucleus 26-70 µm (mean 36 µm); crystals up to 14 µm; contractile vacuole up to 60 µm.
Ecology: Fresh water, both clean and polluted; sediments of ditches and shallow peat bogs, in streams and canals. Omnivorous. I noticed as food: diatoms, purple bacteria, Mayorella-species, Trinema enchelys and Arcella hemisphaerica, Euglena, Chilomonas, Cosmarium and Trachelomonas species, Peridinium cintum and rotifers.
Remarks: A common species in all seasons, but sometimes abundant in floating debris in early spring, just after the ice on the ditches has melted. I have collected it in the Netherlands, England (Cambridge) and Southern Sweden. This species can be confused with Amoeba proteus, but is distinguished by its remarkable large crystals. These crystals are never abundant, giving the amoeba often a clear appearance.
