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Clathrella foreli
Clathrella foreli, with nucleus, cell body 36 µm, scales about 16 µm – Naardermeer

Genus Clathrella Penard, 1903

Diagnosis: Cell spherical, surrounded by a number of silicious, hyaline, colorless, bowl-shaped scales, with a minimum of six, loosely attached to each other. Filopodia extend through gaps between the scales. Pseudopodia filose, without granules, branching and when attached to the substrate also anastomosing. One vesicular nucleus.

Dimensions: 4-55 µm.

Ecology: Clathrella species are restricted to oligotrophic and mesotrophic water bodies. They occur not only in peat bogs, but also in streams and venns and lakes and ditches.
Penard found this amoeboid in Lake Geneva (Lac Léman), Switzerland, at a depth of 30-35µand at a single location along its shore. Hoogenraad found this species in 1914 in sediments of a Sphagnum bog, the Wisselsche Venn, in the Netherlands, where it was “very numerous”. His observations agreed well with those of Penard. I have found this species at several locations in the Netherlands and also in North- and South-America. Recently (2014) this species has also been observed in Germany by Eckhard Voelcker and Steffen Clauss (pers. comm.), in Lake Baikal by Cavalier-Smith and Chao (2012), probably in India (Anagha Datar, pers. comm.) and in Japan (see misidentification here). This species also seems to have been seen by Stepanek (1952).
One might think that Clathrella is very rare because in the century of its discovery and description it has only been recorded once (Hoogenraad & De Groot, 1940), but the past years I’ve found this species frequently in the Netherlands and in samples from Peru, Montana and Florida, which shows that this species isn’t very rare, but easily overlooked (Siemensma, 2014).

Remarks: I have found specimens with 6 scales each, building a cube or dice and specimens which are about 4 µm small. Just look at the composition below:

These three specimens were all in the same wet mount (all on scale, photomicrographs  were combined with Photoshop). According to Penard the small ones are young ones (see also his drawings below), but that is hard to accept (Meisterfeld, pers. comm.). I’ve cultured the large form without ever observing smaller forms. I’ve seen samples with only the dice-shaped specimens.
In 2013 Eckard Voelcker made some SEM-photomicrographs from dried shells and later he succeeded in preparing living cells for the SEM which you can see here!
Our conclusion is that there are at least three different species, which we want to describe when we have more information. For the moment I call the different species as follow:
Clathrella foreli: the large form, 25-50 µm;
Clathrella “quadratus”: the cubic form, about 10 µm;
Clathrella “vegrandis”: the smallest form, 4-10 µm.

Clathrella foreli
Drawing Penard in Les Sarcodinés des Grands Lacs, 1905.
Clathrella foreli
Original drawing for the book of Hoogenraadand De Groot (1940), collection Ferry Siemensma
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