Difflugia gramen Penard, 1902
Diagnosis: test ovoid, smaller specimens nearly globular. Test not compressed, composed of some large and many medium sized mineral grains, mostly particles of quartz or diatom frustules, and between these are smaller ones, usually spherical, cemented by an organic matrix. Surface of test slightly rough. Smooth cement matrix between the particles sometimes with perforations. Color of the matrix ranging from colorless to dark brown. Aperture with three lobes, larger tests sometimes with four lobes. Aperture with small ridge, often ornamented with a row of small granules. Aperture sometimes with an indication of some collar. 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 um 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: Difflugia 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 Difflugia gramen can easily be confused with smaller specimen of Difflugia lobostoma, but Difflugia 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 Difflugia 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.