Field observations on feeding of the land snail Helix aspersa Muller

摘要

Feeding of H. aspersa was observed at monthly intervals. Three natural populations in Galicia (NW-Spain) were studied. At two sites only a few plants constituted the bulk of the diet and in spring the snails' diet had the highest diversity (H'). Inthe third population, feeding and distribution of H. aspersa were observed in a small plot with permanent patches of Urtica dioica. Nearly one half of feeding snails fed upon U. dioica. Most of the other observations were on Mentha suaveolens, Ranunculusrepens and Gramineae. The diversity of the snails' diet showed seasonal variation with the maximum in the autumn months. Comparison between the availability of the different plant species and their contribution to the snails' diet showed that the snailsdid not eat at random; U. dioica was eaten much more than expected from its occurrence and grasses were strongly under-represented in the snails' diet. Temporal changes of availability were significantly correlated with the amounts eaten in the case ofUrtica, but not for the other food plants. The distribution of the snails in the plot was significantly correlated with that of Urtica. Chemical analyses of the food plants revealed Urtica as the species with the higher protein, ash and calcium contents.The strong preference of H. aspersa for U. dioica could be explained by the value of Urtica as food or by its suitability as habitat for the snails. The largest proportions of green material in the snails' diet occurred in the spring and juveniles ate more green material than adults in the three populations.