L. Forró - É. Murai szerk.: Miscellanea Zoologica Hungarica 6. 1991 (Budapest, 1991)

Nesemann, H.: Zoogeography and composition of leech fauna of Danubian lowland rivers in the Kisalföld compared with some molluscs (Hirudinea, Gastropoda)

Glossiphonia paludosa, Batracobdella algira, Fagotia acicularis, Viviparus acerosus and Unio tumidus zelebori (see also Fig. 12). The distribution patterns recently found for the crustaceans Limnomysis benedeni, Dikerogammarus villosus and Chaetogammarus ischnus are very similar (NESEMANN, unpublished). The "Hainburger Pforte" or "Porta Hungarica" (point of intersection, zone 2, 3 and 4) separates many lowland rivers from the upper Danubian river basin. The following species do not occur in the Austrian Danube even today: Erpobdella testacea, E. nigricollis, Trocheta cylindrica, Valvata (Borysthenia ) naticina and Fagotia esperi. Its range of extension is bordered at the Kisalföld area (Szigetköz), while the three leech species had been able to enter the plain of the lower river March (Morava, zone 3) into northwestern direction. The influence of tributaries of the Danubian fauna Another important fact for the biological zonation is the confluence of large tributaries, which are able to influence the regimen of the River Danube. The Hungarian distribution of several leeches and snails is recently bordered upstream at the confluence of the lowland river March. Isolated populations (Erpobdella nigricollis, Viviparus acerosus, Valvata (Borysthenia) naticina) of the Bavarian Danube, more than 300 km away from the Hungarian Great Plain, are restricted to the lowland-river-like reach upstream from Passau. They are considered postglacial relicts of its former spreading during warm periods (Atlanticum). The Austrian Danube is strongly influenced by the river Inn and other tributaries of the alpine mountains. The main stream between Passau and Vienna is more or less homogeniously populated with the typical biocoenosis of the upper Danube, including only the frequent leech species Dina punctata, Glossiphonia complanata and Erpobdella octoculata. The radical change of the biocoenosis downstream Vienna is not only caused by the abiotic characteristics. It is also understandable as a summarised result of the confluence of many typical lowland rivers between Bratislava and Szob (zone 2 on Fig. 19). Biological characteristics and river zonation Some interesting similarities are found between the faunal composition of the main stream of the Danube, the anabranches and dead-water reaches. The communities of several deadwaters near Vienna resemble the Hungarian main stream with high abundances of Glossiphonia paludosa, Dina lineata, Fagotia acicularis, Theodoxus danubialis, Viviparus acerosus, Lithogfyphus naticoides, Sphaerium rivicola ssp. and Unio tumidus zelebori. The listed species are common in the middle reaches. The uppermost localities are recorded from lakes and deadwaters of the alluvial floodplain, while its occurrence in the main stream is bordered at the Moson reach. Therefore the importance of "lakes" and secondary branches is easily understandable, which simulate environmental characteristics of high-order-streams (zone 1) (OTTO & BRAUKMANN 1983) along the upper and middle reaches (zone 4) of the river. Large anabranches with the possibility of faunal exchange contain the highest diversity of leech species (MIKUSKA & GEC 1970). Some interesting examples along the investigated reach of the Danube are to be shown: the Kühwörther Wasser between Mühleiten and Schönau, the Mosoni Duna and the Ráckevei Duna. In general most of the discussed leeches and molluscs, except Dina punctata, are typical members of the Danubian Potamon biocoenosis. They are restricted to the rivers of the Hungarian reaches or occur only in relict populations along the