Difflugia geosphaira Ogden, 1991
Diagnosis: shell ovoid, circular in cross section, with a well defined apertural rim; walls constructed of an organic matrix with siliceous particles added; aperture large, circular; cytoplasm does not fill casing, nucleus with many peripheral nucleoli, two or three contractile vacuoles, peroxisomes and symbiotic bacteria present; lobose pseudopodia.
Dimensions: 45-62 µm long. My measurements: 56-93 µm long.
Ecology: edge of drainage dyke, Pedwell, King's Sedgemoor, Somerset, England. I found this species around the Naardermeer, the Netherlands.
Remarks: Ogden, 1991: Active individuals usually extend one large lobose pseudopod from which one or two smaller branches may develop near the point of extrusion. Such pseudopods may extend for one and a half times the body length, about 80 um, and may be up to 10 µm in diameter. Bands of filaments are often seen running along the length of large pseudopods; their appearance corresponds to the dimensions of microtubules but they could equally be bands of microfilaments. The cytoplasmic body occupies about two-thirds of half of the anterior shell volume around the aperture in moving animals, the posterior portion of the shell being unoccupied. A central nucleus and two to three contractile vacuoles are clearly visible in the cytoplasm. Pseudopodia in the vicinity of the aperture consist of a range of cytoplasmic strands which vary in size from 0.5 um to about 5 µm in diameter.
Due to the confusion surrounding earlier descriptions of globular forms of Difflugia, the species was previously considered by Ogden (1988) to represent Difflugia globulosa sensu Penard, 1902, based on general size, shape and composition of the shell. Nevertheless, it was noted then that there were differences such as the presence of a distinct organic apertural collar and the regular size of the body and aperture. Further investigation of the cytoplasmic features of these specimens shows that they also differ from the earlier forms reviewed by Penard (1902) and Cash & Hopkinson (1909), in one important feature, the shape and structure of the nucleus. These earlier authors described most globular forms as having a single nucleus with a large central nucleolus. One spherical species, Difflugia subequalis, was described by Penard (1910) to have a single nucleus and several nucleoli. Nevertheless this species has substantially different measurements, on average a body length of 81 µm compared with 55 µm, a breadth of 88 µm compared with 52 µm and an aperture diameter of 53 µm compared with 23 µm. In addition aperture was not perpendicular with axis of the shell, hence the specific name. The specimens examined here are now considered to be sufficiently distinct to justify designation as a new species, the diagnostic features of which are: a distinct organic apertural collar, regular body size and aperture, and a single nucleus with several, mainly peripheral, well defined nucleoli.
This species resembles Parmulina louisi Chardez & Beyens, 1988.