Difflugia gigantea Schlumberger, 1845
Diagnose: test ovoid, elongated, narrowed anteriorly, almost pyriform, sometimes slightly depressed; covered with large grains of sand forming irregular projections; this species resembles Difflugia pyriformis, but it differs in its more elongated form and in its greater size.
Dimensions: Schlumberger: 168 – 483 µm (see remarks about his unit below). My measurements: length 322 – 554 µm.
Ecology: Freshwater, in sediment of mesotrophic waterpools, and between sphagnum.
Remarks: The name “gigantea” was used by Didier Chardez in a publication (1967) where he introduced, in his words, “Difflugia oblonga v. gigantea LEIDY, Thèque semblable à l’espèce, mais plus grande, 450 à 580 µ” (= Test resembling the type, but much larger, 450 to 580 µm), without any further comment or reference to a specific date, description or drawing of Joseph Leidy. This variety was elevated to species status by Ogden and Fairman (1979). In 1984 Chardez, probably unaware of the Ogden and Fairman publication, also ranked his variety to species status, now referring to the test of 580 µm on Plate X, fig. 15 in Leidy, 1879. However, the name gigantea had already been used, and therefore preoccupied, by Schlumberger in 1845, who described his Difflugia gigantea as follows:
Animal with brownish-blue test, as if covered with large grains of sand forming irregular projections; ovoid, elongated, narrowed anteriorly. Length 0.08 to 0.23; greatest width, 0.036 to 0.05. This species resembles Difflugia proteiformis, Ehrb.; but it differs in its more elongated form (narrowed in front, almost pyriform, sometimes slightly depressed) and in its greater size. Pseudostome circular, with irregular edge, gives passage to three or four thick and obtuse cylindrical pseudopodia, thick, which in contraction, are covered with small bulges.
It is not clear which unit of measurement was used by Schlumberger. Perhaps “lines”, as in some older texts. A line is 1/12 of an inch, so .08 would be 169 micrometres. In that case his shells were 168-483 µm long and 75-105 µm broad.
If he used the French “ligne”, based on the French inch, so the conversion is a little different. A “ligne” is 2.2558291 mm, so roughly the shells are 180-518 µm. However, I don’t have any idea whether it is plausible that Schlumberger would be using that unit in 1845. It seems rather late, for someone writing in French!
It is not likely that he used millimeters as measurement, because that would mean that his tests were 80-230 µm, not quite a reason to call this species “gigantea”.
There is no clear match between the shells of Leidy, Chardez and Ogden. Chardez gives two different sizes, the measurements published in 1967 are different from those published in 1984. Ogdens shell strongly resembles test a.
Following dimensions given for D. gigantea sensu Chardez, 1967 and Ogden and Fairman, 1979: Chardez (1967): 450 – 580 µm; Chardez (1984): length 450 – 462 µm; Ogden and Fairman (1979): 341 – 480 µm; Lahr & Lopes (2006): 180 – 280 µm.
The name D. gigantea (Chardez, 1967) Ogden et Fairman, 1979 is not valid.