Netzelia gramen (Penard, 1902) Gomaa et al., 2017
Diagnosis: shell ovoid or spherical. The shell surface is rough and composed of a mixture of small to medium pieces of quartz, the particles are bound together with an organic cement. Shells with siliceous plates were also reported by Cash et al. (1919). The aperture is tri-lobed and is surrounded by slightly raised collar of small particles which are cemented together. Nucleus spherical with central nucleolus. Zoochlorellae may be present.
Dimensions: Penard (1902) 50-80 µm; Cash mean 70 µm; Hoogenraad & De Groot (1940) mean 70 µm; Ogden (1980) 89-117 µm for population A and 61-97 µm for population B; Stepanek & Jiri (1958) and Vucetich (1973) 71-92 µm. My measurements 54-126 µm, mean 86 µm. Within one cultured population size can be very different, varying from 54-105 µm, mean 72 µm.
Ecology: Difflugia gramen is recorded for all continents. Species can be found in plankton, periphyton and in sediment. It forms gas bubbles to float.
Remarks: Netzelia gramen was first described by Penard (1902). See his original drawing below. Hoogenraad & De Groot (1940) noticed that nearly all observed specimens had zoochlorellae. I have found these species mainly without zoochlorellae.
Based on the shape and structure of the test Netzelia gramen can easily be confused with smaller specimen of Difflugia lobostoma, but Netzelia gramen has a vesicular nucleus while D. lobostoma has a nucleus with many globular nucleoli. This is a key character, described by Penard (1902). As most tests in samples are empty, this key character is not very useful in practice. You have to press the cover glass to break the test and if you are lucky you can see the nucleus, which makes identification more easy. I have found both species together in samples, where Netzelia gramen was usually < 120 µm and Difflugia lobostoma > 120 µm. Lahr & Lopes (2006) give a ratio from 80-160 µm, mean 108 µm, for this species in a river in Brazil.