INTRASPECIFIC VARIATION IN RESISTANCE TO DESICCATION AND CLIMATIC GRADIENTS IN THE DISTRIBUTION OF THE LAND SNAIL XEROPICTA-VESTALIS

Land snails are subject to desiccating conditions in their terrestrial habitat. Our previous studies have revealed significant differences in resistance to desiccation among closely related species and among genera that share a similar life style, suggesting that the distribution pattern is correlated with the abiotic regime in its specific habitat. Our present study extends the scope of comparison to the intraspecific level, by examining the resistance to desiccation in five populations of Xeropicta vestalis, a Mediterranean-dwelling species that ranges from the 1000 mm to the 200 mm isohyet. The resistance to desiccation varied among populations in correlation with the specific habitat of each population and with the north-to-south and the west-to-east climatic gradients. Furthermore, in one case it exceeded the resistance of other, desert-dwelling species (Trochoidea simulata, Sphincterochila zonata). We suggest that, in spite of its physiological capacity to invade deserts, X. vestalis is prevented from doing so because it is an annual, semelparous species. The Negev Desert is an unpredictable environment, susceptible to year-to-year fluctuations to such an extent that one rainless year would wipe out all its populations.