Lead Assimilation and it's Variability in the Snail Helix aspersa

Hel aspersa is known to accumulate Pb in its tissues during environmental exposure, and lose the metal when exposure ends. The effect of previous exposure to Pb on subsequent metal assimilation was investigated, together with the contribution of a range of factors to the variability in measured metal concentrations between individual snails. Snails from a number of sites, with a wide range of Pb pollution, were investigated. Assimilation of Pb from the diet was lower in He/&x aspersa from a grossly contaminated site (CP) than in animals from a ‘clean' control site (RD). High tissue Ca concentrations in the CP population were thought to be linked with an enhanced excretion efficiency for Pb, with rapid loss of Ca occuring after ingestion of Pb. Comparison of snails from several sites provided further evidence for differentiation by the CP snails in respect of Pb assimilation. Rates of uptake and the amount of variability were lower in the CP population. The main site of Pb storage, probably bound to Ca/Mg pyrophosphate granules, is in the hepatopancreas. Changes in Ca levels in the soft tissues, and possibly the shell, followed Pb exposure. The role of Ca in regulating assimilation of Pb was therefore investigated. There was no evidence of any direct correlation between dietary Ca concentrations and Pb assimilation. Genetic adaptation to high levels of environmental Pb by the CP snails was investigated in an experiment with laboratory-bred F, animals. No firm conclusions could be reached, due to high levels of variability in the data. Comparisons of data from both juveniles and adults showed similar patterns of Pb assimilation, indicating the possibility of a genetic adaptation by this population. Only CP juveniles showed any increase in fresh weight when fed on a Pb-dosed diet. The potential role of the microbial gut flora in metal assimilation was investigated in a series of experiments in which snails were fed antibiotic-dosed diets. There was no evidence that the gut flora had been altered by these treatments. Variability of metal concentrations within populations was a serious problem in all experiments. Sources of variability in the experimental work and methods of reducing their influence in future studies are examined. The implications to biological monitoring programmes and ecotoxicological studies are discussed. Modification of their Ca metabolism, together with increased rates of excretion, are thought to be the main mechanisms employed by the CP snails to reduce Pb concentrations in the tissues.