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

tion. The river entered and widened several narrow valleys, e. g. the Porta Hungarica and the Duna kanyar, which were, up till the end of Pliocene, used only by its tributaries, the Morava, Hron and Ipoly. Additionally the basin of the Lake Fertő was originally formed by a secondary branch of the Danube (Szádeczky-Kardoss 1938). During Pleistocene glaciations, the freshwater fauna of the upper Danube was strongly influenced by environmental changes and, consequently, a special faunal composition formed. Several species of the Danubian fauna were able to survive periods of glaciation. They all tolerate low water temperatures, high water current and are well adapted to the conditions of the rhithron (Balon 1968). In contrast to the upper Danube, the rivers of the middle and lower Danube basin had served as a refuge for the fauna of the potamon. The most recent faunal borders were established since the Würm glaciation. From both geomorphological and zoogeographical point of view, the Lajta is the most important connecting link between the upper and middle Danube basin (Szádeczky-Kardoss 1937). Regarding the changes of the flowing direction, the geomorphological history of the lower Lajta is easy to understand. The middle reach follows the northern slope of the Leitha mountains in a north-easterly direction. At Hollern, the river bends off to the southeast. This point, where the Vienna and the Kisalföld plains meet, is the so-called gate of Bruck or the Porta Austriaca. Here was a former watershed. Originally, the Palaeo-Danube entered the Kisalföld, using the lower reach of the present-day Lajta. After the Paleo-Danube shifted its main stream to north, using the Porta Hungarica to break through to the Carpathian mountains, the Palaeo-Lajta extended its lower reach into the former Danubian river bed. Three different situations of the watershed can be distinguished (Fig. 2). Fig. 2. Changes of watersheds and the direction of the rivers between the Vienna and Kisalföld plain. A - Pliocene, B - Upper Pliocene and lower Pleistocene, C - Middle Pleistocene, D - Upper Pleistocene and present-day situation