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Arcella formosa
Arcella formosa, Ø 204 µmdrawing Ferry Siemensma, 1982

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.

Arcella robusta formosa
A. formosa, 258 µm – Bert Bospad, stacked image. Shells can look distorted, but this is a normal shape.
Arcella robusta formosa
A. formosa, 162 µm – living cell with one nucleus arrowed. Note the variety of food particles, e.g. green algae and purple bacteria
Arcella formosa
Arcella formosa
A. formosa, 241 and 121 µm – Bert Bospad
Arcella robusta formosa
A. formosa, small and large specimen, both from the same sample (114 and 235 µm – Bert Bospad)
Arcella robusta formosa
Arcella formosa, 211 µm – Bert Bospad, stacked image
Arcella formosa
Arcella formosa
A. formosa, 147 and 160 µm – Bert Bospad
Arcella formosa
Arcella formosa
A. formosa, 165 and 180 µm – Bert Bospad
Arcella formosa
Arcella formosa
A. formosa, 257 and 172 µm, Bert Bospad
Arcella formosa
Arcella formosa
A. formosa, 177 and 124 µm, Bert Bospad
Arcella formosa
Arcella formosa
A. formosa, 184 and 185 µm, Bert Bospad
Arcella robusta formosa
A. formosa, 132 µm – detritus Naardermeer
Arcella robusta formosa
A. formosa, 132 um side view – stacked image
A. formosa, (l.) 190 µm, Kromme Rade, 2014 and (r.) 177 µm, Bert Bospad, 2016.
Arcella formosa robusta
My first drawings of Arcella formosa, 1979
Arcella formosa
A. formosa, 170 µm – Naardermeer, 2012
Arcella formosa
A. formosa, records world wide
Arcella formosa
A. formosa, records in Europe
Arcella formosa
A. formosa, records in the Netherlands
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