L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Nesemann, H., Pöckl, M.; Wittmann, K. J.: Distribution of epigean Malacostraca in the middle and upper Danube (Hungary, Austria, Germany)

main course of the middle Danube, because it never has been found in larger tributaries or in Lake Balaton. O. obesus prefers large river flows, is most characteristic for the potamon biocoenosis, and is chiefly found in the gravel. Stygobromus ambuláns (Fr. Müller 1846) Material: see Nesemann (1993), further records: swamps in Carinthia, Burgenland (Strem), 1995. This gammarid species is an original element of the Pontic fauna. Today, its distributional range includes all drainage systems of the Black Sea. In different large running water systems, S. ambuláns has advanced very far northwestward, reaching Central Europe (Straskraba 1962). It was at first described for the tributaries of the Elbe River. The Western European populations are isolated from those in the Pannonian lowlands. Along the Danube River, the western distribution limit of the Pontic population is formed by the basins of Lake Fertő (Andrikovics et al. 1982), the upper Rába, the lower Lajta and the Morava basin (Nesemann, unpublished). The recently recorded isolated population in the basin of the Lake Ammersee in Bavaria is the westernmost locality in the Danube basin (Hess & Heckes, in prep.). Disjunct populations live in the area of River Po and in the inneralpine basin of Klagenfurt. S. ambuláns is found in different kinds of water bodies, from the lowlands to the lower mountains. The species inhabits puddles, temporary pools, swamps, springs, wells, swampy banks of streams and rivers, and muddy shores of lakes (Dudich 1927). In the present study, S. ambuláns was collected in many kinds of waters (Fig. 5) which all seem to have one attribute in common: the deficiency of oxygen. Under these conditions no other gammarid species would be able to survive. In large rivers, S. ambuláns is a typical and locally very abundant component of the fauna, being chiefly restricted to the reed-zones (Ráckevei Duna; Hortobágy in Tisza-system) and swampy meadows of the backwaters (Lajta, Strem). In the Danube, S. ambuláns is found in adequate habitats of side-branches, where water movement is low. It was mainly found in dense packs of decaying leaf litter of decidous trees, in decaying reed-stems and other plant remains. Niphargus (Phaenogatwnarus) hrabei Karaman, 1932 Material: see Nesemann (1993), additional records: Danube, in a temporary deadwater reach near Klosterneuburg (rkm 1939), 17. 05. 1995, leg. Nesemann, H.; Danube deadwater reach near Schönau (rkm 1909), 10. 05. 1987, leg. Wittmann, K. J. This amphipod is common in the temporarily flooded deadwater regions of the riverin forests and floodplains along the Danube. It inhabits similar habitats as S. ambuláns. It seems to be largely restricted on the Danubian valley, whereas S. ambuláns is common in the floodplains of the Danubian tributaries. It was recently found in the Bavarian section of the Danube (Hess & Heckes, in prep.).