Fragilaria gracilis is a species of freshwater pennate diatoms. F. gracilis is reported from many parts of Europe, in Sweden even as one of the dominant freshwater diatom taxa.
Despite its frequent occurrence, there has been some confusion about the identification of this taxon. 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 ÃÂstrup's original description of 1910 of almost linear valves, and Tuji's observation of the type material, describing the striae as âÂÂbeing parallel throughoutâÂÂ, with SEM pictures showing mainly opposite striae.
The valve shown in the original picture of ÃÂstrup (1910, Tab V, Fig. 117) has a length of 43 and a width of 2.1 üm, with 25 striae per 10 üm. The microphotographs from the lectotype slide, coll. ÃÂstrup 1342, given in Krammer and Lange-Bertalot (1991) and Tuji (2007), show lengths of 28-54 üm, widths of 2âÂÂ2.7 üm, and 18âÂÂ24 striae per 10 üm. Tuji (2007) describes the striae as âÂÂbeing parallel throughoutâÂÂ, with SEM pictures showing opposite striae with some irregular parts where striae are alternate. These alternate parts are however not common, even if Lange-Bertalot and Ulrich (2014) define the striae as âÂÂopposite or alternatingâÂÂ. Note that the term âÂÂparallelâ refers to the orientation of the striae to each other, while the terms âÂÂalternate/oppositeâ 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.