Snails as bioindicators
Snails are one of the many biological tools used by researchers to characterise soil pollution, to identify or predict damage to ecosystems and to monitor the evolution of pollution in time and space. This gastropod is notable for its ability to concentrate pollutant substances (such as metallic trace elements) in its tissues, making it possible to assay the toxic element.
Use of snails as bio-indicators of heavy metal pollution
The use of snails as biological indicators is particularly appropriate for metals, which they accumulate in their organs. Scientists from Italy and USA evaluated the transfer of cadmium in situ in a soil-plant-snail system. Under realistic conditions of environmental contamination, the garden snail (Helix aspersa) revealed the bioavailability of cadmium in the soil. The results obtained constitute important information for the rational use of terrestrial snails as a tool for environmental biomonitoring.
The scientists analysed the accumulation and elimination kinetics of cadmium in H. aspersa over a laboratory experiment (six months). For three months, juvenile snails were exposed to both natural and artificial cadmium (Cd) contaminated soils (exposure phase). Five concentrations (50 to 800 micrograms/g), were selected to estimate the concentrations causing 50% inhibition of growth. The results showed that their internal Cd concentrations reached a steady state after 14 days of exposure, whatever the soil type and concentration of exposure.
Then they were transferred to a clean environment for three months (depuration phase). During the depuration phase, elimination enabled a decrease of internal concentrations, but they did not return to base line. Significant amounts of Cd (6 to 35 mg. kg-1) were thus stored in the long-term in snails and became potentially transferable and toxic in foodwebs.
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. With each metal the effects increased with increasing concentration. Snails exposed to heavy metals continued to be reproductively active, but the egg production was highly variable.
The results obtained with cadmium compared to those of other authors working with earthworms and soil arthropods show that snails H. aspersa give responses to concentrations much more rapidly and with more sensitivity (comparable to those of earthworms).
Snails have also been used to determine metal levels, such as lead in urban environments.
Concentrations of cadmium, lead, copper and zinc were measured in individuals of Arianta arbustorum from different urban sampling sites. In comparison to snails from a reference site, the animals collected in the city showed higher concentrations of cadmium, lead, and copper, indicating elevated levels of metal pollution. The most pronounced difference in tissue concentrations between control animals and contaminated snails was observed for lead.
Arianta arbustorum is a suitable species for biomonitoring, because it is widespread, resident and easy to collect; it has a high capacity for metal accumulation and shows different concentrations depending on metal contamination of the sampling area.
An interspecific comparison of metal concentrations in terrestrial gastropods was conducted to define background levels and classes of burden. Three pollution levels are distinguished on the basis of the snails' metal burden: no pollution (class 1: reference sites), moderate (class 2: traffic and other human activities in urban areas), high pollution (class 3: mining and heavy industry)
Land snails are a good indicator of environmental health as they are impacted by overgrazing, fire and invasive grasses. In other words, a decline in their populations could signal the a whole land area has been damaged. 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.
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