Effect of Zn, Cu, Pb, and Cd on Fitness in Snails (Helix aspersa)

Juveniles and adults of the Brown garden snail (Helix aspersaMüll.) were fed on an artificial diet contaminated with Zn (ca. 40–12,000 μg·g−1dry wt), Cu (ca. 9–1600 μg·g−1dry wt), Pb (ca. 0.4–12,700 μg·g−1dry wt), Cd (ca. 0.16–145 μg·g−1dry wt), and all four metals mixed together for 120 days. Significant negative exponential regressions of food consumption and fecundity on concentrations were found for all treatments. Growth rate was affected significantly only by Zn and mixed treatments. The calculated EC20(consumption)values for juveniles were (in μg·g1): Zn, 855; Cu, 248; Pb, 5290; Cd, 60; and for adults: Zn, 1240; Cu, 275; Pb, 3120; Cd, 147. In mixed treatment EC20(consumption)values were substantially lower indicating the additive effect of pollution with these four metals. EC20(consumption)for Zn in mixed treatment was 329 μg·g−1for juveniles and 661 μg·g−1for adults. The following EC20values were estimated for fecundity (in μg·g−1): Zn, 1740; Cu, 533; Pb, 6140; Cd, 120; Zn in mixed treatment, 2210. The relative toxicities of the four metals were compared with their ratios in contaminated field sites. Comparing Zn and Cd, for example, even though Zn is ca. 13–24 times less toxic than Cd, it is usually present in plants and forest litter in concentrations ca. 100 times greater than those of Cd. Thus, of these two metals, Zn appeared to be potentially the most important pollutant in ecologically relevant situations. No effect of any treatment on mortality was found during the 4-month experiment. The calculated scenarios of population dynamics under the stress of chronic pollution with mixtures of the four metals revealed that the delayed reproduction due to estivation of snails may be the main cause of population decline at high metal concentrations in food. However, at concentrations at and below ca. 1000 μg Zn·g−1in food, if the reproduction is not delayed the population may persist for a long time (0.25 control number after 50 years).