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)

an Basin, the range of distribution includes mainly the Danube. F acicularis occurs in the Danube and is common in thermal springs of this river basin too, wherein it lives together with the Pliocene relict Theodoxus prevostianus (C. Pfeiffer, 1828). These populations are completely separated from those in large rivers. In general E acicularis inhabits slowly flowing reaches of the Danube and large tributaries (Rába). It is present in high abundances in secondary branches downstream Vienna, but its occurrence in the main stream starts at first downstream Gönyü (BOTHÁR 1966). This species is less common than F. esperi in the whole Hungarian reach. Fagotia esperi (A Ferussac, 1823) (Fig. 14) Localities: Danube at Dunaföldvár (H), km 1560.8, 31. Jan. 1991, leg. WIMMER & NESEMANN; Danube at Érd-Ofalu (H), km 1625.4, 30. Jan. 1991, leg. WIMMER & NESEMANN; Danube at Visegrád (H), km 1693.6, 31. May 1989, leg. PUKY & NESEMANN; Szentendre Duna at Szentendre (H), 31. May 1989, leg. PUKY & NESEMANN; Szentendre Duna between Tahitótfalu and Leányfalu (H), 31. May 1989, leg. PUKY & NESEMANN; Mosoni Duna at Dunaszeg (H), 23. Oct. 1990. The second member of the Thiaridae is the most frequent prosobranchiate snail together with Lithogfyphus naticoides (C. Pfeiffer, 1828) (BOTHÁR 1966). F. esperi is a characteristic member of the Danubian Potamon biocoenosis. Its distribution is extended upstream towards the Mosoni Duna, but no records are known from the Austrian reaches or the large tributaries. The Holocene evidence is known from the lower reach of the River Lajta. The species, rarely found in earlier times, becomes a common member of the Danubian fauna during the last 25 years. CONCLUSIONS Zoogeographical barriers and pathways We can distinguish between several groups of the Potamon biocoenosis, which had been able to inhabit the upper courses of the rivers, originally starting from the lower or middle reach of the river Danube since the end of pleistocen glaciation. According to SOÓS (1934) the invetigated rivers are divided in districts or zones as shown on Fig. 19. The most important barriers or pathways are the narrow valleys, where the Danube breaks through the mountains. The reaches are often characterised by riffles, rocky slopes with a strong velocity of flow and high gradient and the absence of deadwaters and floodplains (see Fig. 18). The narrow valleys are unfordable barriers for many non-rheophile aquatic organisms. The whole course of the river Danube is divided into isolated basins and plains (SOÓS 1934). For zoogeographical history the most important plains (Fig. 18) along the upper and middle reach exist in Bavaria between Regensburg and Vilshofen (Gauböden), in Austria between Vienna and Hainburg (Wiener Becken, Kisalföld) and in Hungary (Kisalföld, Alföld). A large number of species seems to be restricted to the central Hungarian Great Plain (Alföld, zone 1 on Fig. 19). Their distribution is bordered at the middle reaches of the rivers Tisza and Danube (downstream the Dunakanyar). Some examples presented in this paper are Dina apathyi, Theodoxus fluviatilis, perhaps Placobdella costata, but also the amphipods Obesogammarus obesus and Orchestia cavimana. The "Wiener Pforte", upstream towards zone 5 is a barrier for