Europe except the Alps and the northern Scandinavia, to Anatolia; Ireland; NE. Morocco, Algeria (Glöer); N. America. Grazer, on rocky substrate in brackish and freshwater streams and rivers, in calcium-rich environments, also in semi-marine intertidal: baltic marshes, sheltered estuaries in W. Europe…

As far as we know, fluviatilis is the only member of the genus which does not have a conspicuous pseudoapophysis on its operculum (Sand & al., 2020 p.37). Above, a young specimen from Blato na Cetini, a settlement on the Cetina river, downstream the Pranjčevići dam, S. Croatia. Size: 5,7mm. The red dot in the detail shows the position of the pseudoapophysis that is usually found on the opercula of the other members of the genus. In fluviatilis, obviously this piece is absent or vestigial.

In this drainage basin, the species goes rather far inside the valleys. Here, at the re-emergence of the Arbéroue river, at the lower level of the Isturitz-Oxocelhaya cave system, the snails colonize every stones up to the grid, in pure cold freshwater.

Like many freshwater Nerites, Theodoxus fluviatilis often bears eggs capsules on its shell. Each capsule contains about 200 eggs. In a capsule, the first larva to be born eats the others. – On stones in shallow water, Font Estramar, Salses-le-Château, Eastern Pyrenees, S. France. 5,8-7,5mm.

Font Estramar: the river, completely submerged, has an important flow rate (more than 2m³/s); its water is slightly brackish, and warm (17,8°C); it was explored on 2900m long; the deepest point reached inside is at -286m (2020, Xavier Méniscus). Original picture provided by A. Bertrand (FR).

Inside a same population, the species can show rather high degrees of variability in shape, colour and pattern. Above, some specimens from the washing-place located at “Les courtals d’Abal”, at the foot of the ancient settlement of Glabanel village, north of La Palme, Aude, Occitania, S. France.

« Pattern: a fairly wide mesh of white stitches netted of rather fine brownish threads, sometimes going lighter, sometimes darker; stitches narrow to wide, sometimes very irregular, so that the net threads do not show any closed stitches anymore and are very close and almost zigzagging. » – S. Clessin: “Binnenmollusken aus Rumänien”, Malakozoologische Blätter n.s. Bd.8, Cassel 1886, p.55.

« Many specimens were found by the marquis Doria in the lake of Paleaston near Poti. It is somewhat elongated, narrow, regular in shape, solid; its colour is more or less dark olive; the apex is constantly eroded; the left margin is almost straight, a little oblique and greenish white. It is not very different from the common typical Theodoxus fluviatilis: I have a large number of individuals belonging to the latter species from Turin […] Palermo and from many other Italian localities, which diversify more from each other than from the variant of Paleaston lake. » – A. Issel: Catalogo dei molluschi raccolti dalla missione italiana in Persia aggiuntavi la descrizione delle specie nuove o poco note, Torino 1865, p.22-23. The “Paleaston” lake mentioned by Issel is the Paliastomi coastal lagoon, close to Poti, Samegrelo-Zemo Svaneti, W. Georgia.