In the south of the West Siberian Plain, Percursaria percursa has been found, which is a species typical of marine ecotopes and rarely found in the region. P. percursa is rarely found in continental waters. Information on the hydrochemical environment of this species in the Baikal region and other continental regions suggest that P. percursa should be categorized as a euryhaline alga. In the new habitat in the West Siberian Plain, P. percursa has been found in sulfate chloride sodium water with mineralization of 31.8 g/dm³ together with Cladophora glomerata. Considering known publications on the history of ecological environment and the formation of modern aquatic biota in the Caenozoic Era, P. percursa is considered to be a progressive allochthonic element. In the West Siberian Plain, the species is a migrant that has naturalized in brakish-water ecosystems. Such hydrosystems are widespread in the forest-steppe and the steppe belts of the region, therefore P. percursa is likely to be found in other locations.