[Effects of heavy metals on snail development. Use of snails as bio-indicators of heavy metal pollution for the preservation of human health].

The use of snails as biological indicators is particularly appropriate for metals, which they accumulate in their organs. The aim of the present experiment was to carry out a rigorous experimentation in the laboratory and in the wild in order to develop a methodology for the use of snails at a known stage of growth that would give precise information on the toxicity of heavy metals for different concentrations and durations of exposure. We have developed a test of toxicity based on the effects of a noxious and carcinogenic element, cadmium, on the land-snail Helix aspersa aspersa (H.a.a) of one month of age. Five concentrations (50 to 800 micrograms/g), were selected to estimate the concentrations causing 50% inhibition of growth (EC 50) at 14 days: 190 micrograms/g and at 28 days: 180 micrograms/g. A soil matrix contaminated with metals (soil including 800 micrograms/g Cr, 20 micrograms/g Cd, 800 micrograms/g Pb and 2000 micrograms/g Zn) was incorporated into the food at 50 and 75%, it too inhibited the growth of juvenile snails compared to incorporation of control soil. An accurate and rapid (2 to 4 weeks) method is therefore available for the evaluation of the toxicity of pollutants by ingestion. The first trials of this method in the wild consisted of placing batches of 2-month-old snails, identical to those used in the first lab tests, in locations that were either polluted or not. Differences in growth were observed depending on the locations; analysis of the levels of metal in the organs of the snails should enable us to check if there is a correlation between these levels and the growth rates. The results obtained with cadmium compared to those of other authors working with earthworms and soil arthropods show that snails give responses to concentrations comparable to those of earthworms and much more rapidly and with more sensitivity than those of collembolla for example. The ease of handling snails and the perfect control of their breeding are essential factors in carrying out reliable bioassays in toxicology and in ecotoxicology.