Desiccation and rehydration in land snails -: A test for distinct set points in Theba pisana

Land snails are subjected to annual cycles of activity and aestivation in relation to seasonal changes in temperature, humidity, and water availability and, therefore, should be adapted with a range of behavioral and physiological mechanisms that will ensure their survival under their specific microhabitat conditions. This study tested the physiological responses to combinations of desiccation and rehydration in a Mediterranean land snail, Theba pisana. Normothermic desiccation for three weeks resulted in about 10% loss of the initial whole snail mass. Extension of desiccation to five weeks resulted in a total loss of 14.2%. This mass loss resulted in depletion of the extra-pallial fluid compartment, while body fluid was closely maintained. Chloride and osmotic concentrations of the extra-pallial fluid increased significantly during desiccation, while urea concentration decreased significantly. When allowed to rehydrate for various time periods (48, 72, 96, and 120 h), the snails recovered all mass loss, and all osmotic constituents returned to normal levels. When these snails were dessicated again, they differed in rates of mass loss in correlation with the length of recovery period. These findings suggest that water is preferentially moved to, and urea transported to the soft body tissue, facilitating buildup of the osmotic gradient between the two compartments and the close regulation of the soft body hydration state. Upon rehydration, the high urea concentration in the soft body tissue facilitates water uptake. I conclude that land snails use two distinct set points for body hydration: one is the level of osmotic and urea concentration in the body fluid after desiccation, and the other is the hydration state after rehydration. Such a mechanism may function during natural activity/aestivation cycles. Thus, during long-term aestivation. a new set point of water economy is established, in association with metabolic depression. An extended period of contact with moisture is needed to change this set point. Urea accumulation and osmotic changes in the soft body tissue may play a major role in such a mechanism.