Gene flow between populations of two invertebrates in springs

Summary 1. Using allozymes, we analysed genetic structure of the freshwater gastropodBythinella dunkeriand the freshwater flatwormCrenobia alpina. The two species are habitat specialists, living almost exclusively in springs. The sampled area in Hesse (Germany) covers a spatial scale of 20 km and includes two river drainages. From the biology of the two species we expected little dispersal along rivers. However, the possibility exists that groundwater provide suitable pathways for dispersal. 2. InB. dunkeriheterozygosity decreased from west to east. For some alleles we found clines in this geographic direction. These clines generated a positive correlation between geographic distance and genetic differentiation. Furthermore patterns of genetic variation within populations suggested that populations may have been faced with bottlenecks and founder effects. If populations are not in population genetic equilibrium, such founder effects would also explain the rather high amount of genetic differentiation between populations (10%). 3. ForC. alpinathe mean number of alleles decreased with increasing isolation of populations. Genetic differentiation between populations contributed 19% to the total genetic variation. Genetic differentiation was not correlated to geographic distance, but compared withB. dunkerivariability of pairwise differentiation between pairs of populations was higher inC. alpina. 4. OverallB. dunkeriappears to be a fairly good disperser, which may use groundwater as dispersal pathway. Furthermore populations seem to be not in equilibrium. In contrastC. alpinaforms rather isolated populations with little dispersal between springs and groundwater seems to play no important role for dispersal.