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…

Th. fluviatilis 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 many other members of the genus. In fluviatilis, this piece is very often vestigial, and is located in the continuation of the “rib shield” (the “string” of the bow, often slightly thickened).

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, only one egg contains an embryo; the rest are used as food storage. – On stones, Font Estramar, Salses-le-Château, Eastern Pyrenees, S. France. 5,8-7,5mm.

Font Estramar: the cave-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.

The operculum of fluviatilis is usually very coloured. The marginal conchiolin layer (Cl) is reddish and rather translucent, often large; the nucleus (Nu) where it originates is often deeply carved out in the calcareous base and is also saffron-reddish, while the apophysis (Ap), spectacularly curved, is orange to yellow. The anterior marginal part of the callus (Ca) is not merging with the lower part of the anterior abutment (for fluviatilis it is named “rib shield” in Glöer), because the latter is very reduced longitudinally.

The arcus, or outer rib (O), of the apophysis is strongly curved, cylindrical in section, thin at its base and often very fat near the summit. Notice the wide calcareous velum that rigidifies the arched apophysis. In their article of 2010, Sands & al. name it a “Rib Pouch” (RP), probably because it is often strengthened by a thickening that looks like the string of a bow: the “Rib Shield” (RS). All these elements vary in shape and in proportion. What remains constant is the kind of feather formed by the apophysis and its membrane, and above all the fact that the “Rib Shield”, which is the anterior abutment of the apophysis, starts very high near the summit and does not end near the anterior callus (C) like in so many species, but divides the velum in two parts, so that the extension (E), or anterior root, is not merging with the callus but is markedly separated from it. This is a specific feature. Like in the other species of Theodoxus, this “Rib Shield” has a very variable profile, especially near its root.

The anterior root refers to what seems to act like the “pseudoapophysis” seen in other species by Peter Glöer and the other major authors who wrote on this topic. In contrast to the other members of the genus in the area, the anterior abutment (“Rib Shield”) clearly divides the velum in two parts.

The extension E, or root, of the anterior abutment, reaches its maximum size relatively young, after which it does not grow anymore, while the apophysis and the anterior marginal callus continue to develop. At adult stage, E is relatively less present than in younger stages. – Vrana lake, Zadar area, W. Croatia.