Arcella leidyana Deflandre, 1928
Emended diagnosis (not yet formally published): Taylor B, Porfirio‑Sousa, Jones, Seaquist, Siemensma, Taylor E & Tice, 2026

Diagnosis: Shell polyhedral, with 6–10 ribs (usually 8); dorsal surface faceted, forming a peak, gable or angular dome. In apical view, shell is polygonal or nearly round. In lateral view, approximately house-shaped, aborally peaked with nearly parallel sides almost perpendicular to the ventral plane (mean angle 89°). Aperture notably wide and shallowly invaginated; central, approximately circular, with subtle crenulations forming a wavy margin around the raised lip. The ratio of aperture to ventral diameter ranges from .38 to .47 (mean .42).

Dimensions: according to Deflandre (1928): shell diameter 52—76 µm; height 100—130 µm; aperture 20—30 µm.
Van Oye (1926): shell diameter 160 µm; height 200 µm.
Taylor et al., 2026: shell diameter 116–153 μm (mean 134 μm, n=47); height 103–134 μm (mean 116 μm); aperture diameter 41–68 μm (mean 58 μm); aperture height 10–20 μm (mean 14 μm).

Ecology: Freshwater.

Geographical distribution: Deflandre (1928)

Differential diagnosis: Arcella leidyana is set apart from congeners by molecular markers in the mitochondrial COI gene. Morphologically, it differs from A. spectabilis in its profile, with nearly parallel lateral walls rising to a peaked apex, and in the shallow invagination of its very broad aperture. It is readily distinguished from A. prismatica by the dorsal (apical) surface of the test, which is never flat, and by the aperture, which lacks strongly defined lobes. Its overall form resembles that of A. conica Playfair, 1918, described by its author as having “the shape of a marquee tent”; however, A. conica possesses a compact, smoothly rounded aperture without crenulations, and is a much smaller species (diameter 68–100 µm).

References: