Natural history of Theba pisana in Israel (Pulmonata: Helicidae)

Authors


  • Professor A. Cain, Dr S. Jones and Mr R. Cowie, kindly criticized an earlier draft of this paper. I am grateful to Mrs Rachel Pimstein for her devoted technical assistance.

Abstract

Theba pisana in southern Israel is a semelparous species with an annual life cycle. Young snails hatch in early winter, grow rapidly in spring and by summer they reach, first adult shell-size, then adult shell-weight. Shell-size is negatively related to density, suggesting that, as in some other landsnails, there may be a density-dependent inhibition of growth. Summer forces the snails into 3–4 months of aestivation.

After the first winter rains the snails, awakened perhaps by the sudden drop in barometric pressure, emerge from aestivation. Only now are most of the spermatozoa and mature oocytes formed (maturation of gametes actually starts some two weeks before the first rains), and the snails copulate. No evidence of precocious breeding was found. The snails sometimes dig a few “sham” holes, perhaps to discourage egg-eating predators, before finally depositing their eggs (mean clutch size: 80 eggs) in the last hole.

In mid-winter, the snails die. In a habitat where the sheltering conditions of the vegetation offer the snails more protection, the dying off process of the population is stretched over a few more months. This could, perhaps, be the first step towards the evolution of a bi-annual cycle, such as is typical of T. pisana in some other areas of its distribution.

Ancillary