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, up to 25 nuclei.
Dimensions: Nicholls: diameter 135-265 µm; my measurements: diameter 114-345 µm. Tests of the population of the Bert Bospad varied in size between 114 and 230 µm!
Ecology: Freshwater; in sediment of mesotrophic ditches and water with Sphagnum, and between Hottonia palustris. Rare. I found this species in ‘t Hol (1982), along the Bert Bospad (2017) and in ditches around the Naardermeer (2016) and Woerdense Verlaat (2019), all in the Netherlands. Ken Nicholls described it from Canada (2005) and it has been found in Northern Germany by Hans Rothauscher (2017, pers. comm.) and in Mecklenburg (Germany) and Central Poland by Ralf Meisterfeld.
Remarks: This species was first recognized by Siemensma, 1982, who only published a drawing with the name Arcella robusta. Later this species was also found by Nicholls in Canada who formally described it as Arcella formosa.