Dr. Fűköh Levente szerk.: Malakológiai Tájékoztató 12. (Eger, 1993.)

SZABÓ, S.: The effect of becoming waterless and experiments of livingplace reconstraction on Monusca living in the soid laces of Upper Kiskunság

The problem of becomming waterless periodically of the sodic lakes and the longlasting waterless of the two examined territories propose the question of mollusc fauna's survival. The literary datas, the skins found on the surface and the fossil materials of drill-samples done in 50-150 cm depth on both territories prove that these parts were richer in species earlier compared to the beginning of the experiment. According to the literature and my experience the reason of individual's decay in large numbers is the increase of becoming concentrated with decrease of the water at same time. (Szabó, S. 1990 b). According to Richnovszky the species and individuals settle in - mostly by the birdmigration - year by year (Richnovszky, A. 1989). I examined this question - mentioned above - on ground and among laboratorial circumstances and experienced decay in large numbers in species of Lymnaea peregra and Lymnaea stagnalis for example in case of sudden waterlevel fall in the storage lake number I of Apaj after stopping the pumps. Contradiction to this in the 1-2 cm deep slowly ebbing water of "Fehérszék" and on its substratum the individuals of Lymnaea peregra show complete activity. The fact, that Í experienced during the drought in 1991 is inconsistent with the common, multitudious and complete decay. I digged down three plastic pools with opened bottom in 15 cm depth on the territory of Apaj in the open part of fish-pond number I and I didn't touch the bottom of the pond. Afterward I filled up the plastic ponds and supplied with water 2-3 times a week because of the evaporation. According to the expansion of territory individuals of Anisus spirorbis already appeared after a week. Also in the 10-20 cm deep water developed after the rainstormes in July 1991 snails were appeared on both territories and their abundancia , constancia and dominancia value and dispersionpicture were similar to the value of the pre-drying period. ^Based on my experience it seems the activity of species decreases because of the unfavourable oecological elements and the majority of individuals goes through difficulties in their oecological refugiums" during the unfavourable period. Escaping factors of the snails according to the results until now are: the drastic decrease of the waterlevel, the sudden increase of the water's electrical conducticity (total salinity), rise of the pH value over 9, and obtaining 25 mg/1 Na2C03 content of water. After the period of summer rainy weather there was no major bird-moving. I don't preclude the possibility of remote pick - up of species by birds - but not return in large numbers -, because it is my only explanation for appearance of Perforatella rubiginosa on territory of Apaj and for appearance of Armiger crista, Segmentina nitida in lake "Háromszögi". I examined the survival in mud by laboratorial experiments in aquariums and by 50150 cm deep drilling - 10 drillings on one territory. The mortality of species is extreme, it is between 20-80%. A lot of individuals decay particulary from mature specimen of Lymnaeas, and it is quite less in case of Planorbarius corneus.The most individual of Anisus spirorbis and Planorbis planorbis go through the unfavourable period. Nearly every species forms thin skin or chalky inclusion on its mouth during the time of becoming waterless: Planorbarius corneus, Planorbis planorbis, Anisus spirorbis, Segmentina nitida. Most of the individuals of Anisus spirorbis and Planorbis planorbis stay on the surface and other species hide in still humid mud but not deeper than 10-15 cm, and the Planorbarius corneus and Lymnaea peregra slip into the polygonal rent of parched bed.