This testate amoeba has contracted its pseudopods inside its shell and remains still.
Commentary by Prof. Yuji Tsukii, Hosei University The shell surface of the Arcella (testate amoeba) is generally smooth, but in this video, various particles seem to be attached to the surface. Also, the position of the pseudostome (opening) in this organism is not at the exact center of the body. If it were, the organism might be a different kind of testate amoeba other than Arcella.
After being collected from a paddy field, the sample was put on 1% agar plate with a drop of distilled water and kept for a few days.
Based on the color and shape of the test (shell) in this video, it looks like it belonged to an Arcella, but the shell is empty now.
Commentary by Prof. Yuji Tsukii, Hosei University For identification of Arcella to the species level, the side view of its test (shell) must be examined. The key characteristics are: (a) the ratio of the test diameter to height; (b) the ratio of the test diameter to the pseudostome (mouth) diameter; and (c) whether the test surface is smooth or depressed. Identification is difficult because this video does not show the side view of the test. But since several parts of the test appear depressed, the organism is most likely Arcella gibbosa.
After being collected from a paddy field, the sample was put on 1% agar plate with a drop of distilled water and kept in the dark for 4 days.
In the first scene of this video, a testate amoeba Arcella is moving slowly among aggregates. In the second scene, the amoeba notices a green organism that comes near. In the last scene, the Arcella has caught the green organism and is consuming it. Unfortunately, the video did not film the moment of capturing the organism.
Commentary by Prof. Yuji Tsukii, Hosei University This large amoeba with a disk-shaped shell (test) and thick pseudopodia is a testate amoeba called Arcella.
The green organism that approaches belongs to genus Trachelomonas,
because it is surrounded by a shell (lorica) and exhibits characteristic
locomotion. Trachelomonas is also characterized by a red eyespot in its
cytoplasm.
Newly formed shells of trachelomonas are transparent and gradually
become reddish-brown. As this organism has a transparent shell, it has
probably just divided.
The organism might be Trachelomonas crispa, but we cannot see the long, spiky spines surrounding the opening where the flagellum extends, which are a characteristic feature of Trachelomonas crispa.
Another possibility is that the organism belongs to genus Strombomonas.
However, the opening (outlet) for the flagellum of Strombomonas is
characterized by a narrower structure like a bottleneck. Therefore the
organism may be Trachelomonas rather than Strombomonas.
