Reproduction as a variable life history trait in freshwater snail Viviparus viviparus (Linnaeus, 1758) (Gastropoda: Architaenioglossa: Viviparidae)

Studies on Viviparus viviparus populations were carried out in freshwater bodies: in a dam reservoir, in outlet river stretches and in oxbow lakes variously connected to the river. Sex structure, fertility (understood as the number of fertile females in the population) and reproduction were analysed. The sex ratio in V. viviparus changed during the vegetation period. The largest percentage of females was found in summer, with slightly less in spring and the least in autumn. Fertile female percentages were dominant in all habitats and were very high in spring and summer with up to 90% in through-flow oxbow lakes. The most numerous were viviparids of the II, III, and IV size classes. The youngest snails with shell height and width less than 8 mm were least numerous. Smaller snails of the II size class dominated in spring while the number of III and IV size class viviparids increased in summer and autumn. Most embryos were noted in females in spring and summer during their intense reproduction while the number of embryos was almost twice as low in autumn. The highest values of reproductive effort (the IEI index) were noted in all habitats in summer which differed markedly from those in spring and autumn. The lowest fertility of V. viviparus was found in an oxbow lake cut off from the river with a mean value of 4 to 12 embryos/female and in a dam reservoir which had a mean number of 4 to 13 embryos (female). Snails from through-flow oxbow lakes were more fertile at 8 to 25 embryos (female) and also from outflow river sections at 7 to 25 embryos (female). The mean number of embryos increased with the females shell size. The greatest fertility in through-flow oxbow lakes was found in females of the II and III size classes. In spring, the mean number of embryos per female of the II size class (16 embryos) and that of the III size class (17 embryos) from through-flow oxbow lakes was greater than or comparable with that found in the largest females in other habitats. However, in females of the IV size class the greatest number of embryos per female was found in viviparids from river outlets.