F. gracilis is reported from many parts of Europe, in Sweden even as one of the dominant freshwater diatom taxa.

[2] However, a recent study using both molecular and morphological data has shown that F. gracilis is separated from other similar long Fragilaria taxa, and can be identified by both its rbcL barcode, and by using light microscopy (LM).

In LM, is can be identified by its quite stable character of almost parallel valve sides (in the long cells) and the clear opposite arrangement of the striae giving the impression of regularly arranged parallel lines across the valve in LM.

These characters are also in agreement with Østrup's original description of 1910 [3] of almost linear valves, and Tuji's [4] observation of the type material, describing the striae as “being parallel throughout”, with SEM pictures showing mainly opposite striae.

Note that the term “parallel” refers to the orientation of the striae to each other, while the terms “alternate/opposite” refers to whether the striae on either side of the sternum F. gracilis has been isolated and cultured to clones several times, and the Thonon Culture Collection (TCC) is hosting living strains.