So far I have only been able to find a single specimen of Holophrya ovum, which I was able to identify with certainty (s. fig. 1 a-c). In fact, there is a high risk of confusion with specimens of Holophrya discolor, which have phagocytized algae. Foissner et al. (1994) explicitly point out this problem. I find such green specimens of Holophrya discolor very frequently. I can often identify them by their small size (< 100 µm), but estimating the length is difficult, especially at low magnification and roundish specimens. It is therefore possible that I have overlooked other specimens of Holophrya ovum so far.

An important characteristic of Holophrya ovum, apart from the green coloration due to symbiotic algae, is the number of longitudinal rows of cilia and the shape of the extrusomes. The specimens must therefore be examined closely for reliable identification.

My specimen had a length of 155 µm and when slightly squashed I was able to count 23 longitudinal rows of cilia. For the actual number of longitudinal rows, you have to count the lateral rows and those on the opposite side of the body. In my specimen one can assume > 50 longitudinal rows. The decisive feature is the shape of the extrusomes. In Holophrya ovum these are slightly curved, 8 µm long rods. They therefore differ clearly from the extrusomes in Holophrya discolor (2–3 µm, spindle shaped) and Holophrya teres (13–18 µm, thin rods, hard to see). In my specimen I could clearly detect the 8 µm long, slightly curved extrusomes (s. figs. 4, 5 and 7).