Phenotypic plasticity in reproductive traits: importance in the life history of Helix aspersa (Mollusca : Helicidae) in a recently colonized habitat

Reproductive traits of the land snail Helix aspersa Muller were investigate d under artificial conditions from two samples, one collected from a popula tion exposed to unpredictable human pressures in its natural environment, i .e. a recently created polder area with intensive agriculture, and the othe r from a snail farm in which animals were reared under constant conditions defined as 'optimal' for growth and reproduction. Results were compared wit h data collected from natural populations of the same region (Brittany) and from habitats spanning the environmental heterogeneity of the range of the species. A large part of the variation among populations could be explaine d by different phenotypic covariances between shell size, clutch size and e gg size, but not by the number of clutches per snail. Thus, the higher egg production of snails from the polders was related to (i) a strong correlati on between clutch size and shell size, shell size being in the upper limit of the overall range for the region concerned; (ii) an uncommonly low egg w eight in comparison with the 'norm' of Helix aspersa, this trait seeming to be involved in a trade-off with clutch size. Second clutches were smaller than the first ones, but their eggs were significantly heavier. This differ ence may be linked to a size-dependent mortality of juveniles during winter which arises in all populations in which hibernation occurs as an adaptati on to low temperatures. In addition to the selective regime usually involve d for populations of helicid snails from Western Europe, several unpredicta ble mortality factors occurred in the polders area: herbicide and pesticide treatments (lethal for young snails), human predation (lethal for adults) and burning (lethal for all snails). Life-history patterns of Helix aspersa are discussed in relation to its ability to successfully colonize a large range of habitats modified by humans, to such an extent that it can become a pest. (C) 2000 The Linnean Society of London.