Arcella formosa - Microworld

Arcella formosa, Ø 204 µm – drawing 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 formosa, 258 µm – Bert Bospad, stacked image. Shells can look distorted, but this is a normal shape.

Arcella formosa, 162 µm – living cell with one nucleus arrowed. Note the variety of food particles, e.g. green algae and purple bacteria

Arcella formosa, 241 and 121 µm – Bert Bospad

Arcella formosa, small and large specimen, both from the same sample (114 and 235 µm – Bert Bospad)

Arcella formosa, 211 µm – Bert Bospad, stacked image

Arcella formosa, 147 and 160 µm – Bert Bospad

Arcella formosa, 165 and 180 µm – Bert Bospad

Arcella formosa, 257 and 172 µm, Bert Bospad

Arcella formosa, 177 and 124 µm, Bert Bospad

Arcella formosa, 184 and 185 µm, Bert Bospad

Arcella formosa, 132 µm – detritus Naardermeer

Arcella formosa, 132 um side view – stacked image

Arcella formosa, (l.) 190 µm, Kromme Rade, 2014 and (r.) 177 µm, Bert Bospad, 2016.

My first drawings of Arcella formosa, 1979

Arcella formosa, 170 µm – Naardermeer, 2012

A. formosa, records world wide

A. formosa, records in Europe

A. formosa, records in the Netherlands

Ferry Siemensma
Created February 22, 2019
Last updated May 28, 2020

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