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

running water. Typically this mussels is found in gravel, sand or stone habitat in the rivers and streams of the plains (Zilch & Jaeckel 1962). Sphaerium scaldianum (Normand, 1844) has a thick shell and prefers lowland-rivers with a moderate current velocity. It is common in sandy sediments. It is distributed throughout the continental inland waters of east- and central Europe. The third and most interesting taxon is Sphaerium nucleus (Studer, 1820), which is thin-shelled. The external side of the shell is interrupted by greyish, irregular growth lines. S. nucleus differs from the other taxa by its habitat preference and ecology. It prefers the temporarily flooded ponds of deadwater reaches and old river beds, which are poor in oxygen and filled up with organic matter. It is also abundant in the submerged vegetation zone along the banks of slowly running rivers, e. g. the Mosoni-Duna. In the Lajta, Bivalvia of the genus Sphaerium are less common than in other large Danubian tributaries of the Kisalföld. Only a few specimens were screened from muddy bottom. They belong to the next two of the characterised species. Sphaerium nucleus (Studer, 1820) Locality: Rohrau, in a secondary branch 14. 5.1991 (A) S. nucleus was collected from a secondary branch near Rohrau, where it lives sympatrically with Musculium lacustre (O. F. Müller, 1774), Pisidium subtruncatum Malm, 1855, and Pisidium obtusale (Lamarck, 1818). Sphaerium corneum (Linnaeus, 1758) Localities: Zurndorf (Zurány) 23. 6.1991 (A), Mosoni-Duna near Mosonmagyaróvár (Ungarisch Altenburg) 13. 9.1991 (H). S. corneum (Fig. 8) was rarely found in the fast flowing mainstream. It was washed from small accumulations of sand found between stones on the bank. Musculium lacustre (O. F. Müller, 1774) Localities: Mosoni-Duna at Mosonmagyaróvár (Ungarisch Altenburg) 13. 9. 1991 (H); Rohrau, in a secondary branch 14. 5.1991 (A). M. lacustre (Fig. 7) is a characteristic member of the fauna of small, temporary flood ponds, which occur in the floodplain of lowland rivers (Zilch & Jaeckel 1962, Falkner 1989, Nyilas & Sümegi 1991). Individuals were collected in high abundance from swampy areas of secondary branches. Pisidium amnicum (O. F. Müller, 1774) Localities: Mosoni-Duna at Mosonmagyaróvár (Ungarisch Altenburg) 13. 9. 1991 (H); Lajta at the confluence into the Mosoni-Duna 6. 9. 1991 (H); Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9.1991 (A/H). P amnicum (Fig. 6) is mainly found in the potamon of rivers and streams of the lowlands. Its distribution includes the Palaearctic region (Zilch & Jaeckel 1962). This rheophilic species prefers the gravelly or sandy bottom of calcareous running waters, which are rich in oxygen. Many populations have become extinct because of water pollution. P amnicum is no longer found in the upper and middle reaches of the Lajta. The fossil record shows that, originally, P. amnicum settled as far upstream as Rohrau. Living populations exist only in the Kisalföld. Compared to the present, P. amnicum was more common during Pleistocene (SzádeczkyKardoss 1938). This species was as common in the fast flowing Danubian mainstream as Theodoxus tranversalis (C. Pfeiffer, 1828). It is a leading fossil in fluvial gravels from the pleistocene era. Some rivers with living populations are listed below. Large specimens were collected in high numbers from the MosoniDuna and from the main course downstream of Budapest at Dunaföldvár. A few