GEOGRAPHIC-VARIATION IN REPRODUCTIVE TRAITS OF HELIX-ASPERSA MULLER STUDIED UNDER LABORATORY CONDITIONS

The reproductive characteristics of the land snail Helix aspersa were investigated under artificial conditions in ten populations exposed to contrasting selective pressures in their natural environments. Two of them were studied for two different years. Significant geographic var iation was detected not only in fecundity (clutch number, clutch size significantly related to shell size) but also in the timing of mating and egglaying. Thus, seasonal adjustments (breeding season and duratio n), related to the geographic location of populations, seemed to be pa rtially preserved under uniform laboratory conditions. In order to ass ess the extent of genetic or environmental determination of variation in these characters, three successive generations of snails from four ecologically distinct regions were reared under the same artificial co nditions. This experiment revealed that a large proportion of the init ially observed variation in natural populations from Lorient and Toulo use, France, and in snails from St. Denis, La Reunion, was environment ally induced. Animals born and reared in the laboratory exhibit simila r traits: they mate two or three times, lay a mean of 1.3 clutches cor responding to between 120 and 130 eggs per snail. On the other hand, s nails from Algeria retain their natural characteristics (larger shell size, larger clutches with larger eggs) under artificial conditions. I n the context of intraspecific life-history variation of Helix aspersa , observed combinations of traits might illustrate two tactics: (i) Sn ails from Algeria have a large size (H. a. maxima), which allows them to have a higher egg production in comparison with ''norms'' of the sp ecies (i.e. all other known populations), but not with respect to thei r shell volume (smaller than possible clutch volume). This production Gould compensate for a high mortality, which would affect all age cate gories in the field. (ii) Life-history patterns of populations from mo re or less recently colonized habitats, always dependant on human acti vities, would be considered as the second tactic of the species: stabl e populations of smaller adults with a smaller egg production and cons iderable plasticity in life-history traits.