Ecogenetic comparison of the selfing terrestrial slugs Arion silvaticus and A-circumscriptus in Belgium (Mollusca : Pulmonata : Arionidae)

The distribution of the predominantly selfing terrestrial slugs Ai ion silv aticus and A. circumscriptus was compared in ten habitat types in Belgium. The two species occurred in respectively nine and five of these habitat typ es. A comparative allozyme analysis revealed that A. silvaticus was genetic ally more variable than A. circumscriptus, since 15 homozygous multilocus g enotypes (strains) were found in A. silvaticus, while A. circumscriptus onl y involved two strains. We tentatively argue that this result, combined wit h the wider habitat range of A. silvaticus fits into Van Valen's niche-widt h variation hypothesis and is the first demonstration of a positive relatio n between ecological and genetic variability (i.e. strain diversity) in aut ogamous pulmonates. Pastures and fir-woods are the most diverse habitat typ es in terms of strain composition. In many populations several of these str ains co-occurred. This seems at variance with many other (mainly) self-fert ilizing multistrain species, the local populations of which consist of sing le strains. There was a high niche-overlap (no ecological differentiation) between strains for which enough data were available.