Arcella prismatica Taylor, Strüder-Kypke and Siemensma, 2025
Diagnosis: Shell polyhedral with 5—8 ribs. Sides steeply angled and often nearly perpendicular to the ventral plane. Dorsal surface usually flat, concave or slightly domed, rarely with a pronounced dome or peak. Aperture circular, with a crenulated edge. Pseudopods barely protruding from the aperture and never extending laterally beyond the outer margin of the shell.
Description. The proteinaceous shell is transparent and colourless when newly formed, then gradually darkening to gold, and eventually dark brown. The ventral margin is usually polygonal, but sometimes nearly circular. The dorsal margin is angular, approximating a polygon, with five to eight edges, which may be slightly convex or markedly concave. In most individuals, the dorsal region has no folds or creases, but is made up of a continuous surface, usually flat, but also sometimes depressed at the centre, or slightly raised. In some cases, the top may be distinctly domed or equipped with a peaked ridge or creases. The dorsal face may be nearly parallel to the ventral one, or slightly sloped from side to side. In lateral view some ribs are visible, mostly parallel, but sometimes converging or tapering distally. The aperture is about a third as wide as the shell and slightly invaginated. The opening consists of 7—10 relatively large crenulations, often somewhat irregular or ragged in appearance. In vivo, the amoeba is usually entangled in filaments of eukaryotic algae or leaves of Sphagnum, often in large numbers. Cytoplasm is often greenish from ingested algae. Pseudopods are bluntly lobose, hyaline, and often numerous, but short.
Dimensions: Accoding to Taylor, Strüder-Kypke and Siemensma (2025): Shell diameter of the ventral surface 81—115 µm; height 60—87 µm (data from 87 shells collected in Mer Bleue and 65 shells collected in Chisasibi, Canada). My measurements on a sample sent to me by Bruce Taylor: Shell diameter 77—103 µm, mean 90.6 µm; height 63—93 µm, mean 76.0 µm; aperture 27—32 µm, mean 29.6 µm (n=16).
Habitat: Freshwater. In surface water, in close association with algae and mixotrophic ciliates. Also from the margins of shallow ponds, in grassy fens, among hummocks of Sphagnum (Canada). Reported from muddy brown water from a relatively fast-moving creek (Beverly, USA).
Geographical distribution: Mer Bleue Bog Conservation Area, Ottawa, Ontario, Canada (2025). Beverley, Massachusetts, USA (iNaturalist, 2025)
References: Taylor, B., Strüder-Kypke, M., Siemensma, F., 2025. A new species of testate amoeba, Arcella prismatica sp. nov. (Amoebozoa: Arcellinida), from peatlands in Ontario and Quebec, Canada. Can Field Nat 138, 1–15. https://doi.org/10.22621/cfn.v138i1.3195.
