The distribution of the bush-dwelling snail Theba pisana (Helicidae) in Israel is restricted to the coastal plain. It is known as an annual semelparous species, but recent studies revealed that part of the population has a biennial life cycle.

The present study examined the water economy and resistance to desiccation throughout the life cycle under natural conditions in the field, and by exposing samples of various size (age) groups to three weeks of laboratory desiccation during various seasons.

We found that resistance to desiccation in Theba pisana is age-dependent as young snails lost more water than adults during desiccation. In general, high rates of water loss during desiccation were noted in growing snails during winter and spring, and low rates were recorded during the summer aestivation in all age groups. A small part of the natural population ceased growing and entered an early aestivation, thereby avoiding the otherwise high rates of water loss.

We conclude that resistance to desiccation is entrained in the annual life cycle of T. pisana and that survival under natural desiccating conditions depends on the maturation, and on the ontogenetic development of the water preserving mechanisms. The age-dependent water economy of the snail dictates the timing of aestivation and the establishment of a new set-point which is reflected in the clustering behaviour, secretion of a calcareous epiphragm, and a selective compartmentalization of body fluids.