2 COMMON EUROPEAN VIVIPARID SPECIES HYBRIDIZE

During a morphological/electrophoretic study on the European Vivparida e, a hybrid but fertile female specimen [Viviparus contectus (Millet, 1813) x V. viviparus (Linnaeus, 1758)] was found in the Niepolomice Fo rest, South Poland. The specimen showed intermediate characters in its shell, anatomy, and embryonic shell. Polyacrylamide gel electrophores is of that specimen confirmed its hybrid origin, which was marked in e ight enzyme systems: specific homozygotes in both species along with a typical heterozygote in the hybrid. Interspecific differences in allo zyme pattern, with different alleles in at least one locus in most enz yme systems studied, seem to indicate that V. contectus and V. vivipar us are very old species, and their isolating mechanisms preventing hyb ridization could have become not efficient enough after such a long ti me since the speciation event. This is all the more probable, now that they occur sympatrically very rarely.