In my sampling sites Holophrya ovum is not very common. In the most cases I found several specimens in old samples with decaying plants. There is a high risk of confusion with specimens of Holophrya discolor, which have phagocytized algae and appears also green. 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.

The specimens of my population had a length of approximately 90–100 µm and when slightly squashed I was able to count 23 longitudinal rows of cilia on one half of the body (s. fig. 5). For the actual number of longitudinal rows, the lateral rows and those on the opposite side of the body have to be considered. Thus, the number of longitudinal rows of cilia in the specimens of my population was about 50. An important feature is the shape and length of the extrusomes. In Holophrya ovum these are thin, slightly curved rods with a length of 8–9 µm (s. fig. 9). They differ clearly from the extrusomes in Holophrya discolor (2–3 µm, spindle shaped) and Holophrya teres (13–18 µm, thin rods). In my specimen of Holophrya ovum I could clearly detect 9 µm long, slightly curved extrusomes (s. fig. 9). The rods of the basket are clasp-shaped and appear as double-rods (s. fig. 10).  

The symbiotic algae of Holophrya ovum are from Chlorella type with a own nucleus. The species was identified as Chlorella faginea by Sud (1969).