T. fluviatilis (Linnaeus, 1758) lives in freshwater and brackish waters up to 17‰, such as the Baltic Sea www.gbif.org/species/2291444 and a brackish loch in Orkney, Scotland. It is unlikely to be confused with Littorina obtusata or L. fabalis as it is easily distinguished by its much larger aperture containing a large, shelf-like septum. Linnaeus named a brackish form of this species as Nerita littoralis. There are no extant type specimens. Many subsequent authors interpreted Linnaeus’ brief description of it, “shell smooth, apex carious [decayed], lips toothless . . . variegated in innumerable ways”, living on “rocky shores of European seas” (but probably from his local shore of the brackish Baltic with abundant T. fluviatilis and no Littorina) to be that of L. obtusata sensu lato, and the name was mistakenly used widely, especially in Britain, in the form Littorina littoralis until the late Twentieth Century in identification guides and on the recording card of the Conchological Society of G.B. & Ireland. (Reid, 1996, pp.202 to 206)
Brackish loch, Orkney, Scotland. September 2016. Leg. S. Taylor.
Full account at: 02 Differentiation L. obtusata/L. fabalis

marinvert.senckenberg.science/image-browse/

For more details and images of Theodoxus fluviatilis, see flic.kr/s/aHskPnSuy7