L. Forró szerk.: Miscellanea Zoologica Hungarica 7. 1992 (Budapest, 1992)

Nesemann, H.: Species composition and zoogeography of the invertebrate fauna at the lower reaches of the Lajta River

The Danube and its tributaries belong to the oldest river basins in central Europe. According to their polygenetic origin, they are inhabited by several subspecies of Unio crassus. Some authors have alleged that U. crassus cytherea is the only subspecies in all Pontocaspian tributaries (Modell 1924). Actually, it is the only faunal member of the upper Danube, which was strongly formed by the Pleistocene glaciations. Its eastern distribution is bordered by the Kisalföld. Here U. crassus cytherea meets with another subspecies, which have originated in the lowland tributaries of the Danube. Such prominent faunal border have been documented for a number of freshwater organisms, e. g. the semiaquatic leeches. This border marks the general meeting point of freshwater fauna from the upper Danube with those from the middle Danube. Subrecent and subfossil shells of U. crassus cytherea (Figs 9-10) were collected from several localities in the lower course of the Lajta. The mussel is extinct in areas due to water pollution, but small populations still exist in streams around Leithaprodersdorf. This proves that the fauna of the Lajta originated from the upper Danube rather than from the lowland rivers of the Kisalföld. Figs 9-10. Unio crassus cytherea, Lajta at Rohrau, subfossil