Arcella formosa  Nicholls, 2005

Diagnosis: Shell resembling a pie, irregular in outline and with an irregular pattern of rib‑like structures and depressions; ribs often darker than the rest of the shell. Multinucleated, with up to 25 nuclei.

Dimensions: Nicholls: diameter 135–265 µm.
My measurements: diameter 114–345 µm. Shells of the population along the Bert Bospad varied in size between 114 and 230 µm.

Ecology: Freshwater; in sediment of mesotrophic ditches, in water with Sphagnum, and among Hottonia palustris. Rare, but widely distributed.

Geographical distribution: I found this species in ‘t Hol (1982), along the Bert Bospad (2017), in ditches around the Naardermeer (2016), at Woerdense Verlaat (2019) and in a ditch near Kamerik (2025), all in the Netherlands, and in a sample from the Odenwald, Germany (2024). Ken Nicholls described it from Canada (2005). It has also been found in northern Germany by Hans Rothauscher (2017, pers. comm.), in Mecklenburg and the Odenwald, also in Germany, and in Central Poland by Ralf Meisterfeld.

Remarks: This species was first recognized by Siemensma (1982), who published only a drawing of it under the name Arcella robusta. Later, Nicholls found the same species in Canada and formally described it as Arcella formosa.