 Zettler and colleagues (2004) showed that in the outer coastal waters of the Baltic Sea, the nearly black and often corroded shell form of "Theodoxus fluviatilis" is predominant, whereas in the inner (sheltered) parts of coastal waters, yellowish-green forms prevail.Glöer and Pešić (2015) observed that specimens from a darker stony substrate were black or dark brown.Shells of specimens of "Theodoxus fluviatilis" from Northern Europe are ornamented with a pattern of white, drop-like spots on a dark or red background.