Difflugia louisi (Chardez & Beyens, 1988) n. comb.
Basionym: Parmulina louisi Chardez & Beyens, 1988
Synonym: Difflugia geosphaira Ogden, 1991
Diagnosis: Shell transparent, micro-perforate, ± soft with a somewhat dark, brownish-green tone. Shape almost hemispherical in lateral view. Shell constructed of an organic matrix with siliceous particles added. Large circular aperture, truncated at right angles, and surrounded by a well-developed rim, mostly more darkly colored than the other parts. Cytoplasm limpid, enclosing a spherical nucleus of 6.5 – 7 µm, with many peripheral nucleoli. Two or three contractile vacuoles, peroxisomes and symbiotic bacteria present. Encystment normal, cyst globular with a distinct cyst membrane, ± smooth or irregularly toothed. In general, only a few broad, stumpy pseudopodia are evident; sometimes extending on the outer surface of the shell.
Dimensions: Chardez and Beyens (1988): Length 46-50 µm, width 44-50 µm, aperture 28-30 µm (n = 15); Ogden (1991): 45-62 µm long. My measurements: 56-93 µm long. In a culture dish measurements were: length 54-60 µm; width 50-56 µm, aperture 24-30 µm; collar 2.8-4.4 µm high.
Ecology: Chardez and Beyens: in mosses like Scorpidium, arctic Canada; Ogden: edge of drainage dyke, Somerset, England. I found this species in sediment of ditches and ponds around the Naardermeer and in Crailoo, both in the Netherlands.
Remarks: Chardez and Beyens (1988): “This new species resembles Difflugia subaequalis Penard, but can clearly be distinguished by the structure of the shell and the smaller dimensions. The nature of the shell approaches that of Parmulina obtecta Gruber, from which it is easily differentiated by the shape of the aperture and more particularly by the conspicuous rim. This rim gives the otherwise ± soft shell a more rigid character, which is missing in Gruber’s species. Parmulina obtecta Gruber has a thin and flexible rim, able to retract and close the aperture.”
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.