Initial Observations
When scanning across a slide from a moss squeeze, I often encountered a transparent sac with what looked like a row of 5-6 'teeth' or a 'crown' at one end. The contents of the sac varied from a lot of dark particles to barely anything visible. Figure 1 shows an example where two of these sacs are visible amongst organic material, filamentous algae and diatoms. They don't appear to move and rarely appear to interact with other organisms or debris.
Structure
Almost every moss squeeze slide I have studied has several of these transparent sacs. They are usually a similar size at about 80-100 μm, oval with a flattened end. At low magnification (x40) using phase contrast lighting, they stand out because of their shape. Under normal bright lighting, they barely show up as they are fairly transparent.
At medium magnification (x100), they appear as a horse-shoe shape, with jagged 'teeth across one end' (Figure 2). The insides show up, but vary considerably between each sac. Whether this is what they have eaten or if it is some internal organ or organelle , it is hard to distinguish.
At high magnification (x400), various features start to emerge. Almost certainly there is a rigid structure (Figure 3), which has some kind of pattern or uniform framework (Figure 4). There are 'teeth-like' spikes at the end (also Figure 5) and sometimes some form of spikes or hairs over the body (Figure 5).
Behaviour
From my observations they are always stationary. The 'sacs' don't seem to 'do' much, but there are a few moments I have observed them when they might be 'doing' something.
Eating? In Figure 5, there appears to be some debris around the 'teeth' end. I wonder if it is being eaten?
Reproducing? I saw two of the transparent sacs attached to one another (Figures 6 & 7). I wondered if they were mating or dividing in some way.
Scientific notes
It took me a while to identify these organisms. It was hard to determine if the organism was multicellular or single celled. I wondered if the internal structures were organs. They were definitely animal, rather than plant, because few seemed to be green in colour. As I didn't know what group they were in, I found it hard to use a key and relied on doing a Google image search for 'freshwater microorganisms', which took a long time to find a match. There were several dead-ends, including looking at rotifers [1]. Finally I stumbled across an electron micrograph of a 'shell' that looked a similar shape to my transparent sacs.
This helped me realise that this organism was a type of amoeba, known as Euglyphida. These are single celled organisms that make a shell or 'testa' to surround themselves. The patterned-net I had seen under high magnification is in fact a shell of plates or scales, called a testa. Because of this, these amoeba are known as 'testate omoeba' [2]. On looking for scientific papers, I found one [3] that revealed the following key points about these organisms:
References
[1] http://www.microscopy-uk.org.uk/mag/wimsmall/extra/rotif.html
[2] http://eol.org/pages/2911370/overview
[3] Tsyganov, A. N., Shatilovich, A. V., Esaulov, A. S., Chernyshov, V. A., Mazei, N. G., Malysheva, E. A., & Mazei, Y. A. (2017). Morphology and phylogeny of the testate amoebae Euglypha bryophila Brown, 1911 and Euglypha cristata Leidy, 1874 (Rhizaria, Euglyphida). European journal of protistology, 61, 76-84.
When scanning across a slide from a moss squeeze, I often encountered a transparent sac with what looked like a row of 5-6 'teeth' or a 'crown' at one end. The contents of the sac varied from a lot of dark particles to barely anything visible. Figure 1 shows an example where two of these sacs are visible amongst organic material, filamentous algae and diatoms. They don't appear to move and rarely appear to interact with other organisms or debris.
 |
| Figure 1. Transparent sacs (Euglyphida) at x 40 in a moss squeeze. ©2018 Andy Chandler-Grevatt. |
Almost every moss squeeze slide I have studied has several of these transparent sacs. They are usually a similar size at about 80-100 μm, oval with a flattened end. At low magnification (x40) using phase contrast lighting, they stand out because of their shape. Under normal bright lighting, they barely show up as they are fairly transparent.
At medium magnification (x100), they appear as a horse-shoe shape, with jagged 'teeth across one end' (Figure 2). The insides show up, but vary considerably between each sac. Whether this is what they have eaten or if it is some internal organ or organelle , it is hard to distinguish.
 |
| Figure 2. Transparent sacs (Euglyphida) at x100 in a moss squeeze. ©2018 Andy Chandler-Grevatt. |
 |
| Figure 3. Transparent sacs (Euglyphida) at x 400 in a moss squeeze. ©2018 Andy Chandler-Grevatt. |
 |
| Figure 4. Transparent sacs (Euglyphida) at x400 in a moss squeeze. Notice detail of 'shell' scales and 'teeth'. ©2018 Andy Chandler-Grevatt. |
 |
| Figure 5. Transparent sacs (Euglyphida) at x400 in a moss squeeze. Prominent 'spikes' on shell. ©2018 Andy Chandler-Grevatt. |
Behaviour
From my observations they are always stationary. The 'sacs' don't seem to 'do' much, but there are a few moments I have observed them when they might be 'doing' something.
Eating? In Figure 5, there appears to be some debris around the 'teeth' end. I wonder if it is being eaten?
Reproducing? I saw two of the transparent sacs attached to one another (Figures 6 & 7). I wondered if they were mating or dividing in some way.
 |
| Figure 6. Reproducing transparent sacs (Euglyphida) at x100 in a moss squeeze. ©2018 Andy Chandler-Grevatt. |
 |
| Figure 7. Reproducing transparent sacs (Euglyphida) at x400 in a moss squeeze. ©2018 Andy Chandler-Grevatt. |
Scientific notes
It took me a while to identify these organisms. It was hard to determine if the organism was multicellular or single celled. I wondered if the internal structures were organs. They were definitely animal, rather than plant, because few seemed to be green in colour. As I didn't know what group they were in, I found it hard to use a key and relied on doing a Google image search for 'freshwater microorganisms', which took a long time to find a match. There were several dead-ends, including looking at rotifers [1]. Finally I stumbled across an electron micrograph of a 'shell' that looked a similar shape to my transparent sacs.
This helped me realise that this organism was a type of amoeba, known as Euglyphida. These are single celled organisms that make a shell or 'testa' to surround themselves. The patterned-net I had seen under high magnification is in fact a shell of plates or scales, called a testa. Because of this, these amoeba are known as 'testate omoeba' [2]. On looking for scientific papers, I found one [3] that revealed the following key points about these organisms:
- They eat bacteria
- They are common in soil and in moss.
- Mostly identified by shape and arrangement of the shell plates.
- There are also differences in the size of the 'mouth' and the scales around it.
- They have spines on the outide of their shells. The position and distribution of the spines may help identify different species.
I will further my research into these fascinating Transparent Sacs (Testate Amoeba)
References
[1] http://www.microscopy-uk.org.uk/mag/wimsmall/extra/rotif.html
[2] http://eol.org/pages/2911370/overview
[3] Tsyganov, A. N., Shatilovich, A. V., Esaulov, A. S., Chernyshov, V. A., Mazei, N. G., Malysheva, E. A., & Mazei, Y. A. (2017). Morphology and phylogeny of the testate amoebae Euglypha bryophila Brown, 1911 and Euglypha cristata Leidy, 1874 (Rhizaria, Euglyphida). European journal of protistology, 61, 76-84.
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