T. fluviatilis lives in freshwater and brackish waters, generally to 2.5‰, but up to 17‰. It is abundant in parts of the Baltic Sea www.gbif.org/species/2291444 , and also occurs in the Black Sea, the Netherlands coast and the two Scottish sites icnluding a brackish loch in Orkney. Linnaeus named three forms of Theodoxus fluviatilis as separate species according to their habitats; Nerita fluviatilis (river), N. lacustris (lake) and N. littoralis (seashore). There are no extant type specimens of form N. littoralis, and many subsequent authors interpreted Linnaeus’ brief description of it, “shell smooth, apex carious [decayed], lips toothless . . . variegated in innumerable ways”, 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.)
For labelled features, see 1Tf 01 Theodoxus fluviatalis (Linnaeus, 1758). River Blackwater, Essex, England. 22nd June 2016. Leg. S. Taylor.