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

Bryozoa Plumatellafungosa(?a\\as, 1768) Localities: Bruck, in secondary branches 16. 7. 1991 (A); Rohrau, in secondary branches 2. 6. 1990,14. 5.1991 (A). P. fungosa is a warm stenotherm species, which settles abundantly in the lower reaches of large rivers, deadwaters and secondary branches. It is spread throughout the holarctic region (Lacourt 1968). Phimatella emarginata Allman, 1844 Locality: Kleine Leitha (Kis-Lajta) at the Albrechtsmühle 30. 7.1991 (A). The cosmopolitan P emarginata is distributed widely on all continents of the world. It occurs in eutrophic rivers and deadwaters (Lacourt 1968). Porifera Spongilla lacustris (Linnaeus, 1758) Localities: Rét-árok (Leithakanal) 30. 7. 1991 (A/H); Kleine Leitha (Kis-Lajta) at the Albrechtsmühle 30. 7. 1991 (A). This species is distributed throughout the holarctic region. It prefers stagnant or slowly running waters. Ephydatia fluviatilis (Linnaeus, 1758) Locality: Bruck, in secondary branches 16. 7.1991 (A). This cosmopolitan species lives mainly in running waters, although it also occurs abundantly in brackish waters, e. g. the Nordostsee-Kanal. The subfossil Mollusca fauna of the lower Lajta Since the second half of the last century, the river bed and habitat have been changed by human activities. This process is still reflected in the longitudinal zonation and the distribution patterns of several species of freshwater fauna. The importance of human interventions can be demonstrated by differences in the present day faunal composition as compared to the subfossil record. Similarly to the neighbouring Rába and Rábca rivers, the Lajta originally had a meandering river bed. It was channelized during the 19th century when the main course was radically shortened to less than a third of its former length. On the other hand, channelization created a great number of deadwater reaches and stagnant water bodies. They were formed by cutting short stretches of the river. Aside from one early paper (Clessin 1888), there is no information on faunal distribution-before the channelization. In 1991, an alluvial river bed near the castle of Rohrau was dredged. This presented an opportunity to investigate the subfossil fauna representing the former state of the Lajta. The following species were collected in high abundances: Unio crassus cytherea, Anodonta anatina, Pisidium amnicum, Theodoxus danubialis, Theodoxus transversalis, Lithoglyphus naticoides, Microcolpia acicularis, Valvata piscinalis. All of these species, except for A anatina and V. piscinalis, disappeared from the middle reach of the river in the Vienna plain. Most of them, however, still survive in the Kisalföld reach. The subfossil faunal composition resembles that of the recent Lajta-megyei-csatorna and the Lajta at Mosonmagyaróvár. All of these species still exist in the Danube and the Mosoni-Duna. It appears that the main effect of the channelization was a downstream shifting of biological zonation.