Paramphitrema spec. or Artodiscus saltans???
I found this peculiar species in a sample from the Spiegelplas, end March 2013. The shell is elliptical, spindle-shaped, as in other species of Paramphitrema, in cross section compressed or circular, with two apertures at opposite poles, more or less ending in two very short tubes. The shell is a flexible membrane, covered with (presumably) minute mineral particles. At each pole about four filopodia are stretched out, each thin and bending and moving like a flagellum. Plasma fills the shell completely. Nucleus could not be observed, due to the many food inclusions.
The cell can transform into a more spherical cell with more than two openings, thus resembling exactly Artodiscus saltans! I've observed this transformation several times. Also the reverse transformation has been observed.
Dimensions: my measurements of the Paramphitrema-form: 20-29 µm (n=5). .
Ecology: Clear freshwater. In sediments of a very shallow sandy shore of the Spiegelplas, the Netherlands.
Remarks: I have seen more than twenty specimens, both in the Artodiscus-form and the Paramphitrema-form. They were constantly moving and tumbling. Therefore it was impossible to get all filopodia at the same moment visible in the photomicrographs.
Cells stick firmly with filopodia to the substrate. When I tried to pick up cells for DNA-sequencing, everything went into the pipette, except the Paramphitrema-cells. When I tried to loosen cells from the substrate with a fine needle, they sticked immediately to the needle. Very frustrating!
Further investigations are needed to clear its taxonomic status.