Evaluating Helix aspersa as a sentinel for mapping metal pollution

The common garden snail has been proposed as a sentinel species for mapping environmental concentrations of toxic metals, since it accumulates high levels in its soft tissues. The capacity of Helix aspersa to quantify site differences is assessed here from a survey of 23 populations around UK and Wales with a wide range of contamination. Five adult snails were collected from each site during 1 week in 1990. Each replicate snail was partitioned into two soft tissue components (the hepatopancreas and the ‘rest’) and their Pb, Cd, Cu and Zn levels compared with those in the soil (as an integrative long-term measure of exposure) and the leaves of one food plant, Taraxacum (as a measure of recent dietary exposure). Median concentrations in one or both tissue fractions correlated with both soil and vegetation levels for each metal except Cu. Whole soft tissue Zn, Pb and Cd concentrations were also correlated with either soil or Taraxacum or both. However, Cd and Zn levels were not independent of tissue dry weight and there were also interference effects between Pb, Cd and Zn in the ‘rest’ and between Cd and Zn in all fractions. Lead in the hepatopancreas correlated with both soil and dietary sources, and was independent of dry weight. There was some correlation of Cd with Pb in this tissue. Outliers largely determined the relationship between hepatopancreatic Pb and both soil Pb and vegetation Pb. If sites with very high levels in each source were excluded from the analysis, the significant correlation of hepatopancreatic Pb with vegetation and soil concentrations disappear. This study emphasizes that sentinels have to do more than simply rank sites by their tissue concentrations: a prime criterion in their selection should be their capacity to quantify the magnitude of site differences.