Amoeba proteus (Pallas, 1766) Leidy, 1878

Diagnosis: Locomotive cells typically 220–760 µm in length; usually polypodial with one dominant pseudopodium, and commonly showing dorsal and lateral folds or ridges. Resting or well‑fed cells irregularly rounded. Floating forms with long radiating pseudopodia. Cytoplasm granular in appearance, often containing truncated bipyramidal or plate‑shaped crystals. Posterior uroid morulate. Nucleus discoid or ovoid, sometimes with peripheral granules; nucleolar material often scattered.

Dimensions: 220–760 µm (mean 425 µm). Nucleus 22–62 µm (mean 38–47 µm), discoid, often biconcave.

Ecology: Freshwater and wet Sphagnum, occurring in a wide range of water types. Not rare.

Remarks: Although this amoeba has traditionally been described as uninucleate, I have frequently observed multinucleate specimens. In one sample, amoebae with a single nucleus occurred alongside individuals with several nuclei (see graph below).