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

specimens, washed from gravel, were found in the middle reach of the river Zala near Kustány. Pisidium supinum A. Schmidt, 1851 Localities: Mosoni-Duna at Mosonmagyaróvár (Ungarisch Altenburg) 13. 9.1991 (H); Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9.1991 (A/H); Kleine Leitha (Kis-Lajta) at the Albrechtsmühle 30. 7.1991 (A). This rheophilic species (Fig. 5) inhabits the sandy bottom of lowland rivers. It is a characteristic member of the potamon fauna and is strongly restricted to the Danube itself and lower courses of large tributaries (Pintér, Richnovszky & Szigethy 1979). P. supinum is also found in Lake Balaton and in the Morava river basin. In the Lajta, this species occurs only in the Kisalföld and does not occur in the middle reach as far upstream as Gattendorf. Pisidium henslowanum (Sheppard, 1823) Localities: Mosoni-Duna at Mosonmagyaróvár (Ungarisch Altenburg) 13. 9.1991 (H); Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9. 1991 (A/H); Rét-árok (Leithakanal) near Deutsch-Jahrndorf 30. 7.1991 (A). The distribution pattern of P. henslowanum (Fig. 3) is similar to that of P. supinum. The species does not reach the narrow valley between Hollern and Deutsch-Hasslau. It was frequently collected in the secondary branches of the Lajta in the Kisalföld, where faunal exchange with the Danube is possible. P. henslowanum is a member of the potamon biocoenosis, but it is not as rheophilic as R amnicum and P. supinum. Therefore it also appears in the Rét-árok, where it prefers the muddy river-bed and a low water current. Pisidium nitidum Jenyns, 1832 Localities: Mosoni-Duna at Mosonmagyaróvár (Ungarisch Altenburg) 13. 9.1991 (H); Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9. 1991 (A/H); Rét-árok (Leithakanal) near Deutsch-Jahrndorf 30. 7.1991 (A). P. nitidum (Fig. 4) lives in lowland-rivers and lakes with sandy or muddy sediments (Falkner 1989). This stagnophilic mussel was found in mud samples from the bank. In the Lajta, the species appears in the lower reach and is recently restricted to the Kisalföld. Pisidium subtruneatum Malm, 1855 Localities: Mosoni-Duna at Mosonmagyaróvár (Ungarisch Altenburg) 13. 9. 1991 (H); Lajta at the confluence into the Mosoni-Duna near Mosonmagyaróvár (Ungarisch Altenburg) 6. 9. 1991 (H); Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9. 1991 (A/H); Rét-árok (Leithakanal) near Deutsch-Jahrndorf 30. 7. 1991 (A); Zumdorf (Zurany) 23. 6. 1991 (A); Rohrau, in a secondary branch 14. 5.1991 (A); Bruck, in a secondary branch 16. 7.1991 (A). P. subtruneatum is distributed all over Europe. It appers in all types of running waters of the rhithron and potamon. The species prefers muddy bottom (Zilch & Jaeckel 1962, Falkner 1989). It is by far the most abundant of the Pisidium species in the Lajta. It was screened in high abundance from several slowlyrunning, secondary branches and was collected most frequently from the sandy river bed of the main stream. It is widely distributed throughout the whole floodplain. Therefore it is often the only member of this genus found in the middle reach around Bruck. Pisidium personatum Malm, 1855 Localities: Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9. 1991 (A/H); Rét-árok (Leithakanal) 30. 7.1991 (A/H); Zurndorf (Zurány) 23. 6.1991 (A). This species is generalist in its habitat preference. It also colonizes springs (Zilch & Jaeckel 1962). P. personatum was infrequently found in the lower Lajta.