EFFECT OF ZN, CU, PB, AND CD ON FITNESS IN SNAILS (HELIX-ASPERSA)

Juveniles and adults of the Brown garden snail (Helix aspersa Mull.) w ere fed on an artificial diet contaminated with Zn (ca. 40-12,000 mu g . g(-1) dry wt), Cu (ca. 9-1600 mu g . g(-1) dry wt), Pb (ca, 0.4-12, 700 mu g . g(-1) dry wt), Cd (ca. 0.16-145 mu g . g(-1) dry wt), and a ll four metals mixed together for 120 days. Significant negative expon ential 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 EC(20(consumption)) v alues for juveniles were (in mu g . g(-1)): Zn, 855; Cu, 248; Pb, 5290 ; Cd, 60; and for adults: Zn, 1240; Cu, 275; Pb, 3120; Cd, 147. In mix ed treatment EC(20(consumption)) values were substantially lower indic ating the additive effect of pollution with these four metals, EC(20(c onsumption)) for Zn in mixed treatment was 329 mu g . g(-1) for juveni les and 661 mu g . g(-1) for adults. The following EC(20) values were estimated for fecundity (in mu g . 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 sit es. Comparing Zn and Cd, for example, even though Zn is ca. 13-24 time s less toxic than Cd, it is usually present in plants and forest litte r in concentrations ca. 100 times greater than those of Cd, Thus, of t hese two metals, Zn appeared to be potentially the most important poll utant in ecologically relevant situations. No effect of any treatment on mortality was found during the 4-month experiment. The calculated s cenarios of population dynamics under the stress of chronic pollution with mixtures of the four metals revealed that the delayed reproductio n due to estivation of snails may be the main cause of population decl ine at high metal concentrations in food. However, at concentrations a t and below ca. 1000 mu g Zn . g(-1) in food, if the reproduction is n ot delayed the population may persist for a long time (0.25 control nu mber after 50 years). (C) 1996 Academic Press, Inc.