Distribution and ecological preferences of Ovalona karelica (Stenroos, 1897) (Crustacea, Branchiopoda, Cladocera) in Volga River Basin (European Russia)

The distribution and ecological preferences of the rare water flea <i>Ovalona karelica</i> (Stenroos, 1897) (Crustacea, Branchiopoda, Cladocera) were analysed for the first time for European Russia. The species was found in different types of waterbodies, such as floodplain lakes of the Kerzhensky State Nature Biosphere Reserve, Nizhny Novgorod City and mouths of rivers inflowing into reservoirs. <i>Ovalona karelica</i> inhabits waters with wide ranges of pH, salinity, oxygen content and trophicity. Development of semi-submerged macrophytes (<i>Stratiotes</i>, <i>Hydrocharis</i>, <i>Salvinia</i>) is the main factor influencing occurrence of the species and its abundance.