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

Pisidium casertanum (Poli, 1791) Localities: Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9. 1991 (A/H); Zurndorf (Zurány) 23. 6.1991 (A). P. casertanum is present in abundance in all types of permanent waters. In the Lajta, it was collected from a few localities of the Kisalföld. Pisidium obtusale (Lamarck, 1818) Localities: Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9. 1991 (A/H); Rohrau, in a secondary branch 14. 5.1991 (A). The mussel lives mainly in swampy ponds and is widely spread throughout Central Europe (Zilch & Jaeckel 1962). It was rarely found in secondary branches of the Lajta. Family Unionidae Anodonta anatina (Linnaeus, 1758) Localities: Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 1989 and 1991 (A/H); Rét-árok (Leithakanal) 30. 7. 1991 (A/H); Rohrau, in a secondary branch 2. 6. 1990, 9. 1991 (A). This species is the most abundant taxon of the genus Anodonta. It is distributed throughout the western Palaearctic region except the Mediterranean tributaries, where a sibling species occurs (Kinzenbach 1989). It was often confused withA cygnea (Linnaeus, 1758), which prefers the more stagnant waters of the lowlands. For a long time, the two species were not distinguished correctly or considered under the collective name Anodonta cygnea (Haas 1969). Unlike A. cygnea, the euryoecius^4. anatina settles in different types of running and stagnant waters of the lowlands. This species also tolerates moderate level of water pollution and lives entrenched in sand, mud or gravel bottoms. It is very abundant in the floodplains of all the rivers of the Kisalföld and also lives in lowland streams, e. g. Cúhai Bakony-ér and Russbach. In the Lajta, A. anatina was rarely found in large secondary branches near the castle of Rohrau, which are recently used as a fishery. Moreover this species was recorded frequently from the large branches in the Kisalföld. It is the only member of the family Unionidae, which still lives in the river Lajta. Frank (1984) alleged that A cygnea occurs in the Lajta-megyei-csatorna. The published data are based on a confusion with A anatina. Unio crassus cytherea (Küster, 1846) Localities: Rohrau, subfossil from sandy sediments of a secondary branch 1991 (A); Nickelsdorf (Miklóshalma) subrecent shells 1987 (A/H); Mosonmagyaróvár (Ungarisch Altenburg), subrecent shells along the gravelly banks 9.1991 (H). The distribution of Unio crassus includes central and eastern Europe, Asia Minor, and Syria. The species is absent from the western Mediterranean region, the Iberian Peninsula, and the British Isles. It prefers running waters and the littoral zone of large lakes, but is sentitive to water pollution (Haas 1969, Modell 1941, Reischütz & Sackl 1991). Unio crassus shows an extensive geographical specialization. A large number of subspecies can be distinguished. They originate from the Pliocene and are very important for the documentation of former connections between different river basins. Pleistocene changes of the watersheds can be demonstrated with the persistence of distribution patterns of Unio crassus subspecies. Especially the former connection of the upper Danubian river basin with adjacent water systems (Rhine, Rhone) had been investigated using U crassus since the beginning of this century (Kobelt 1908, Haas & Schwarz 1913, Zwiesele 1914).