Morphological variability of raphidophycean algae in the lakes of Lithuania JUDITA KOREIVIENĖ * , JŪRATĖ KASPEROVIČIENĖ AND JŪRATĖ KAROSIENĖ Abstract. Three species of raphidophycean algae were found in 33 Lithuanian lakes (of 102 studied) comprising over 50% of phytoplankton biomass in twelve of them. Algae occurred mainly in small, closed, shallow, sheltered lakes surrounded by swampy banks with pH values from 4.28 to 7.81 and conductivity between 13–166 μS cm –1 . Gonyostomum semen was the most widespread and abundant species forming heavy blooms (biomass > 10 mg L –1 ) in six lakes. Species expressed large size differences and morphological variability. Two other raphidophyceans, Gonyostomum intermedium and Vacuolaria virescens, were rare and not abundant. Morphological and morphometric variability of cells and life cycle details are given for the raphidophycean species from Lithuanian lakes for the first time. Key words: Raphidophyceae, Gonyostomum, Vacuolaria, humic lakes, Lithuania Institute of Botany of Nature Research Centre, Žaliųjų Ežerų Str. 49, LT-08406 Vilnius, Lithuania Introduction The Raphidophyceae is a small group of marine and freshwater unicellular flagellate algae lacking a cell wall, with two apical/subapical flagella differing in function and structure as well as numerous trichocysts or muciferous bodies (Hoek et al. 1995; Pantecost 2002). Most studies of the raphidophytes are concentrated on their harmful effect on fisheries (Marshall & Hallegraeff 1999; Hiroishi et al. 2005). Marine Chattonella and Heterosigma species form toxic summer blooms in coastal waters, resulting in disruption to fish farm operations, whereas the nuisance freshwater species Gonyostomum semen influences the recreational value of lakes by causing itching for swimmers (Cronberg et al. 1988). Species from the genera Gonyostomum, Merotrichia and Vacuolaria are usually found in rather acidic waters of freshwater ponds and pools (Pantecost 2002; Ott & Oldham-Ott 2003). Dorsoventrally flattened cells with apical flagella and trichocysts scattered in the peripheral layer are characteristic of Gonyostomum and Vacuolaria, while unflattened rigid cells with subapical/lateral flagella and trichocysts clustered in the anterior end of the cell are distinctive features of Merotrichia (Popova 1955; Ott & Oldham-Ott 2003). * corresponding author e-mail: judita.koreiviene@botanika.lt Current advances in algal taxonomy and its applications: phylogenetic, ecological and applied perspective (eds K. WOŁOWSKI, I. KACZMARSKA, J.M. EHRMAN & A.Z. WOJTAL), pp. 153–163. W. Szafer Institute of Botany, Polish Academy of Sciences, Kraków, 2012.