The paper deals with rare in Europe freshwater algae species Thorea hispida (Thore) Desvaux 1818 recorded for the first time in Lithuania. Red algae were investigated in the river Šešuvis, the south-western part of Lithuania (Tauragė district) in 2009–2011. T. hispida grew attached to markedly heterogenous submerged substrata: stones, gravel, tree branches and artificial substrata. T. hispida assemblages were associated with macroalgae Cladophora glomerata (L.) Kütz., Vaucheria sessilis (Vaucher) DC and several hydrophytes species, Potamogeton pectinatus L., P. perfoliatus L., Fontinalis antipyretica Hedw., Amblystegium riparium (Hedw.) Brunch et Schimp. Environmental factors related to water quality were not the most important pressures influencing species population development, although the lower water temperature (9.43–12.30(–19.50) ºC) and higher conductivity (530.0–568.0 µS cm-1) values were not appropriate for typical habitats. The lower Thorea hispida population seems to be determined mainly by the increase of the river Šešuvis water level caused by frequent floods during the second year of observations. This paper provides information on the morphology of T. hispida and illustrations of the most important morphological features.
Keywords: Thorea hispida, freshwater red algae, the river Šešuvis, Lithuania.