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

I. During the upper Pliocene, the Palaeo-Danube flowed into southeasterly direction, using the present-day catchment area of the lower Lajta. The upper Palaeo-Lajta only was a small tributary of the Vienna plain (Figs 2 A,B). II. In the lower Pleistocene (Günz/Mindel glacial), the Palaeo-Danube occupied the narrow valley of the Porta Hungarica (Fig. 2C). A watershed was formed at the place of the present-day gate of Bruck. Originally, the middle reach of the Lajta between Rohrau and Götzendorf flowed into westerly direction (PalaeoLeitha-Fischa-river on Fig. 2C). At last, it changed the direction of stream erosion during the middle Pleistocene. III. The course of the Lajta was created during the upper Pleistocene, since the Danube shifted its mainstream into a northerly direction. At the same place, a stream erosion from the Vienna into the Kisalföld plain generally started, because of tectonic depressions of the erosion base (Fig. 2D). On the other hand, the accumulation of alpine gravels shifted the tributaries into a downstream direction. Simultaneously, the course of the present-day Lajta changed, because of river capture. It originated from the Pleistocene Palaeo-Danube and Palaeo-LeithaFischa river, which mainly belong to the zoogeographical region of the upper Danube basin. Although the fauna of the upper Danube were radically influenced by the alpine glaciations, the former connections between the Palaeo-Lajta and PalaeoDanube are still reflected in the present-day faunal composition. Material and methods The Lajta was investigated during periods of low water level in the late summer or autumn. The author collected qualitatively with a handnet (mesh size of 1 mm). Samples were preserved in 70% Ethylalcohol. After examination, samples were transferred to the Zoological Department of the Hungarian Natural History Museum in Budapest. Lists of localities were added for all species, (A) indicates all localities in the Austrian reach and (H) in the Hungarian reach of the Lajta. Results The recent fauna of the lower Lajta Bivalvia Thirteen species of freshwater mussels were recorded in the lower reach of the Lajta. The Sphaeriidae family is well represented, including several members characteristical of potamon rivers. In contrast, the Unionidae family is nearly extinct due to water pollution and the channelization of the formerly meandering mainstream. Family Sphaeriidae Several forms of the Sphaerium corneum group have been identified, but their taxonomical status remains unclear. Three forms, often labeled or characterized as subspecies, are more likely closely related species. All taxa were found in rivers of the Kisalföld, where they appear sympatrically in the lower reach of the river Rábca at Hanságliget near Bősárkány. Only the typical form Sphaerium corneum (Linnaeus, 1758) is distributed widely in the western palaearctic region. It has no special autecological preferences. This species tolerates organic pollution and is very abundant in different types of