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

Discussion Longitudinal zonation of the Lajta-Mosoni-Duna system The fauna of the Lajta is unique in composition as compared with other lowland tributaries of the Kisalföld. Some evidences are listed below. - The sympatrical occurrence of Neritidae (Theodoxus) and Thiaridae (Microcoria, Esperiana) is an attribute of Danubian fauna and was never recorded from other tributaries. Most of the Danubian prosobranche snails are restricted to short stretches near to the mouth. - The pannonian subspecies Unio tumidus zelebori (Zelebor, 1851) is absent from the Lajta as well as from the upper Danubian river basin. This mussel could not enter the upper Danube since the end of Pleistocene. In contrast to the Lajta and upper Danube, it is one of the most abundant species in adjacent water systems, e. g. Rába, Rábca, Zala and Morava. The distribution pattern of Viviparus acerosus in the Kisalföld resembles that of Unio tumidus. - The rheophilic leech Dina punctata allopatrically occurs instead of Trocheta cylindrica Örley, 1886. The latter species has a western distribution border in the Kisalföld, whereas Dina punctata has the eastern border of its area, which is formed just like that of the freshwater mussel Unio crassus cytherea. - Two dominant members of the Lajta fauna belong to the faunal composition of gravel in fast flowing rivers. D. punctata and the prosobranche snail, Theodoxus danubialis, are as abundant in this river as in the Kisalföld reach of the Danube. There is no other river where they occur in high density. In the main course of the Lajta, the number of species increases proportionally as one moves downstream (Fig. 24). Freshwater mussels and leeches are abundant in the area of the Kisalföld. Prosobranche snails mainly occur in the same region. The distribution area of some members of the fauna of lowland rivers borders on the lower reach: Pisidium amnicum, Pisidium supinum, Pisidium henslowanum, Pisidium nitidum, Esperiana esperi and Erpobdella nigricollis. They are restricted recently to the Kisalföld. Another group of species enters the Vienna plain and includes Synurella ambuláns, Glossiphonia concolor and Dina lineata. The Lajta forms the western boundary of their distribution area. Ecological zonation of the floodplain of the Lajta The freshwater fauna of the investigated region can generally be divided up into four ecological groups, which are characteristic of different types of water bodies in the floodplain (Table 1). The first group contains seven species, which lives only in the main stream. These so-called Lajta species prefer gravel or hard bottom and are rheophilic. A great number of the remaining taxa belong to the stagnophilic fauna, which only occur in deadwater reaches and secondary branches. The third group, the euryoecius species, tolerate various habitats and are able to settle in different water bodies. The last group includes several members of the fauna of the middle reach of the Danube, which are strongly restricted to the Kisalföld.