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

MISCELLANEA ZOOLOGICA HUNGARICA Tomus 7. 1992. p. 15-38 Species composition and zoogeography of the invertebrate fauna at the lower reaches of the Lajta River by H. Nesemann (Received March 10,1992) Abstract: The invertebrate fauna of the lower Lajta River is discussed, based on records of several important groups of freshwater invertebrate macrofauna. The faunal composition resembles that of the Danube and is unique for the lowland tributaries of the Kisalföld plain. Throughout the lower reaches of the Lajta, an extensive, longitudinal zonation was illustrated. The former situation of the watersheds and the upper Pleistocene origin of this river is still indicated in the present-day faunal composition. Key words: Porifera, Bryozoa, Hirudinea, Bivalvia, Gastropoda, Crustacea, distribution, Lajta Introduction Since 1987 the species of several important taxonomical groups have been intensively studied by the author. The results are presented for 47 species, comparing their occurrence to the general distribution in other lowland tributaries of the Danube. The Lajta is one of the larger tributaries of the middle reach of the Danube. Similarly to the Danube, the Lajta flows eastwards passing the Vienna and the Kisalföld plains. The middle and lower reaches flow through the lowlands, and the mainstream has a higher water current, compared to the neighbouring Morava (March), Rába (Raab) and Rábca (Rabnitz) rivers. The river originates in the eastern Alps. There are no important tributaries along the lower reach of this river. Here the Lajta enters the Pannonian plain, which is characterized by a warm and dry summer climate (Koncek 1965). The floodplain of the middle reach is wooded and has a large number of dead arms, which are connected with the mainstream during flood periods. In the Kisalföld plain, the Lajta is divided up into several river beds and flows into the Danube (Mosoni-Duna) near to the Szigetköz island (Fig. 1). This part of the Hungarian plain is the most downstream situated basin for the accumulation of alpine gravels (Fink 1966). The Lajta has a freshwater fauna, which is derived from two geographical regions. Palaeogeography of the Kisalföld The Danube is an example of a river valley of inhomogeneous origin (Orghidan 1966). Since the regression of the former Parathetys sea, the Danube has flowed through the Kisalföld, deluging the brackish lakes and depositing fluvial sediments (Fink 1966). This process continued during Pleistocene, helped by tectonic depressions in the Hungarian plains (Szádeczky-Kardoss 1938). As a result water flow in the Danube increased, and the current shifted in a northerly direc-