Viviparidae display two categories of life history traits-one constant and independent of habitat, another depending on ecological conditions. The relatively constant abundance is ensured by clustering in the same places at the same time, sex ratio (prevalence of females), size structure (presence of all size classes, with the largest snails forming the majority), and the high proportion of gravid females in the population. These traits undergo only seasonal variation in particular habitats. Adjustment to environmental changes involves the number of embryos per female and the size of reproducing females. Long-term life history studies were performed on Viviparus viviparus (L.) and V. contectus (Mill.) from various habitats (dam reservoir, river, oxbows connected to the river to varied extent) in Poland. The habitats differed in their surface area, depth and trophic status. The snails from the dam reservoir, ecotone zones in the river and stagnant oxbow lakes reproduced at a large body size, their fecundity being size-dependent. In very variable habitats, like oxbow lakes periodically joined to the river, viviparids reproduced at a younger age and smaller body size, while their fecundity did not increase with the body size. The reproduction was the most important factor determining the viviparid density in the studied habitats. This was mainly associated with the ovoviviparity which allowed for controlling the reproduction process. Reproduction is a flexible life history trait in the populations of V. viviparus and V. contectus.

Field and laboratory experiments were aimed at establishing the relationship between growth rate, age, mortality and fecundity of Viviparus viviparus (L.). Fecundity was found to depend on the female's size. The size (shell dimensions) did not affect the size of newborn snails; females of different size classes produced offspring of the same shell height (4.0 mm) and width (4.5 mm). In the first year of the experiment growth rate was higher in the field than in the laboratory. Sex could be recognised and developing embryos could be found in females in the middle of the second year of the experiment. Juvenile V. viviparus appeared in the laboratory when the females were 18 months old and had achieved size class III. Their shell increments were uniformly distributed, without visible dark winter rings or rings of summer growth inhibition. Winter and summer rings appeared in the second year in the field culture; the second winter ring appeared in the third year of field culture. In the field females at the end of their second year contained embryos; they produced offspring in the spring of the third year.

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Warniak is a shallow eutrophic pond-type lake, part of the Mazurian Great Lakes. Since the late 1960s it has been subject to long-term experiments: introducing fish stocks of varied size and composition, resulting in massive physical, chemical and biotic changes in the lake's ecosystem. The aim of this study was an assessment of changes in the lake's mollusc assemblages 17 years after the first investigations (1998). The comparison of malacofauna in 1998 and 2015 showed changes in its composition, a significant decline in species richness and abundance, reduction in vertical distribution, increase in spatial variation of composition, richness and abundance. The proportion of rare species (i.e. found in one or two samples) increased from 17.4% to 64.7%. The dominance pattern became simplified and the frequencies of most species decreased. In 2015 Anodonta cygnea (L.) was still the most widespread unionid and one of the three most frequent species. The changes in the malacofauna occurred despite the lack of significant changes in the lake trophy. The deterioration of conditions for molluscs may result mainly from the impoverishment of macrophytes and the decrease in their abundance.