The Life-Cycle and Productivity of the Land Snail Theba pisana (Mollusca: Helicidae)

Abstract

The land snail T. pisana was sampled over a 4-yr period at Tenby, South Wales [UK]. Newly-hatched snails appeared in late summer and autumn; mortality was high during the 1st winter; maturity was attained after 2 yr; hibernation occurred in both winters; and breeding occurred only once, with death soon afterwards. Samples from other localities covering the whole range of T. pisana suggest both annual and biennial life-cycles with breeding in summer and autumn in Britain and northern France, and later in autumn and in winter in the Mediterranean. An adequate combination of warmth and humidity is necessary for growth. Snails estivate when it is too hot or dry (in southern, Mediterranean localities), and hibernate when it is too cold (Britain and probably nothern France). The length of the life-cycle (1 or 2 yr) is probably determined by the duration of estivation or hibernation, and this may show considerable local variation. Thirty pairs of virgin snails were used to investigate lifetime production of eggs by T. pisana in the laboratory. T. pisana was confirmed as an obligate outcrosser. The single laboratory breeding season lasted from July 1980-March 1981; most snails died soon after this. Mean clutch production per pair was 5, with a mean of 77 eggs in each. Mean egg dry weight was 0.769 mg and was c [circa] 16% of wet weight. Mean total production per pair was 368 eggs, and 678 eggs if pairs producing less than 100 were omitted. This may well under-estimate production in the wild. Late-season clutches were smaller than early ones. Larger snails started to lay earlier and produced larger clutches. They may also have produced more clutches, and consequently more eggs in total.