Quadrulella symmetrica (Wallich 1864) Kosakyan, Lahr, Mulot, Meisterfeld, Mitchell and Lara (Fig. 3)

1863 Difflugia proteiformis var. symmetrica Wallich, An. Mag. Nat. Hist. xii: 458.

1864 Difflugia pyriformis var. symmetrica Wallich, An. Mag. Nat. Hist. xiii: 232.

1864 Difflugia symmetrica Wallich, An. Mag. Nat. Hist. xiii: 245.

1871 Difflugia assulata Ehrenberg, Abh. Ak. Wis. Berlin: 249.

1871 Assulina assulata Ehrenberg, Abh. Ak. Wis. Berlin: 246.

1871 Difflugia carolensis Ehrenberg, Abh. Ak. Wis. Berlin: 250.

1871 Assulina leptolepis Ehrenberg, Abh. Ak. Wis. Berlin: 246, 274.

1875 Quadrula symmetrica Schulze, Arch. mik. Anat.: 329. (homonym)

Updated description. Test ovoid or pyriform, with a rounded posterior end, laterally compressed towards the pseudostome. Test colourless, composed of square plates, regularly arranged in rows. The plates are smaller near the aperture (4 – 5 µm), then gradually larger (reaching up to 10 – 12 µm) towards the posterior end of the test. Test length = 72 – 85 µm, breadth = 40 – 46 µm. Pseudostome 20 – 23 µm wide, often curved and bordered by a thin organic lip.

Differential diagnosis. Morphologically very similar to Q. variabilis, from which it differs by the dimension of the test and the size of scale plates (L = 72 – 85 µm, maximum plate size 10 – 12 µm in Q. symmetrica versus L = 66 – 69 µm, maximum scale size 7 – 9 µm). It can be discriminated from Q. madibai, which has plates of similar size, based on its less slender and elongated test (L/ B ratio is 2.0 – 2.3 in Q. madibai versus 1.7 – 1.9 in Q. symmetrica). Moreover, the general outline of the test in Q. madibai is globally more tubular and does not present a distinct neck. Our molecular data clearly separate these two species (sequence divergence up to 10%).

Type. Fig. 16 in Wallich, 1863 An. Mag. Nat. Hist. XII.

Notes. Q. symmetrica is found in wet mosses ( Sphagnum or other), water streams, forest litter and soil, from all continents, except Antarctica. Detailed morphological and molecular observations are needed to clarify the true position of all described forms.