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

Gastropoda Ten species, which are described below, belong to the subclass Prosobranchia. The Basommatophora are not considered in this paper. Although the composition of prosobranche fauna have changed since the regulation of the river, most of the species still exist in the region of Mosonmagyaróvár, where the Lajta flows into the Mosoni-Duna. The dominant families are Neritidae in the mainstream and Bithyniidae or Viviparidae in the large secondary branches. Family Neritidae Theodoxus danubialis (C. Pfeiffer, 1828) Localities: Mosoni-Duna near Dunaszeg 23. 10. 1990 (H); Lajta near the confluence with the Mosoni-Duna at Mosonmagyaróvár (Ungarisch Altenburg) 6. 9. 1991 (H); Mosonmagyaróvár 3. 4. 1990 (H); Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Miklóshalma) 9. 1991 (A/H); Nickelsdorf (Miklóshalma) 23. 6. 1991 (A/H); Zurndorf (Zurány) 23. 6. 1991 (A); Gattendorf (Lajtakáta) 30. 6. 1991 (A); Hollern, 1,5 km downstream of the village 26. 10. 1990 (A); Rohrau, subfossil from sandy sediments 1991 (A). T. danubialis (Figs 11-15) originates from the Pontosarmatian basin (Zilch & Jaeckel 1962). It is the most abundant species of Neritidae in the middle and lower Danube. Its distribution includes not only the tributaries of the Black Sea, but the adjacent river systems, as well, which were formerly connected with the Aegeopotamos river basin during Pliocene or Pleistocene, e. g. the Axios. or the Po river basin. Its fossil record is already documented for the Rhine river, where a faunal exchange took place during the Pleistocene situation of the watersheds (Kinzelbach 1985). Along the upper Danube, T. danubialis has a disjunct distribution (Leuchs & Tittizer 1989), whereas it generally appears in the middle reaches of the Danube and is completely absent from the Tisza river basin (Soós 1943, Pintér et al. 1979). This species is typical of the fauna of fast flowing reaches of the potamon and prefers a gravel or hard bottom. A unique feature that it settles in large and deep deadwater lakes, which are rich in oxygen (Kühwörther Wasser near Schönau, Nesemann). Its occurrence in the Kisalföld is restricted to the Danubian floodplain and small stretches near to the mouths of tributaries. The only river, which is inhabited by T. danubialis throughout the whole lower course, is the Lajta. This species is distributed in the mainstream as far up as Hollern. In the more slowly running branches, Bithynia tentaculata (Linnaeus, 1758) is more abundant than T. danubialis. Only in the Lajta-megyei-csatorna, which has a moderate water current, are both species present in high abundance. The rheophilic T. danubialis belongs to the most abundant makrozoobenthos and settles in all parts of the river bed. Its range extends with short interruptions throughout the Mosoni-Duna. Theodoxus tranversalis (C. Pfeiffer, 1828) Locality: Rohrau, subfossil from sandy sediments 1991 (A). This species also originates from the Pontosarmatian basin, but was originally distributed in the upper reach too (Zilch & Jaeckel 1962). Details of the habitat preferences of T. transversalis are not known. This species is more rheophilic than T. danubialis and is sensitive to water pollution (Falkner & Müller 1983). X. transversalis was thought to be almost extinct from the Danube. Today, relict populations exist only in the German section of the Danube (Peters 1989, Leuchs & Tittizer 1989). During intergacial periods T. transversalis appeared abundantly. The subfossil and fossil shells are frequently discovered in Danubian sediments as well as in some of its tributaries (Szádeczky-Kardoss 1938, Frank 1984).