Tissue-Level Biomarkers in Sentinel Slugs as Cost-Effective Tools to Assess Metal Pollution in Soils

In previous laboratory experiments, slugs were shown to be sensitive to metal pollution. Therefore, they might be invaluable instruments for biological assessment of soil pollution. The present investigation was carried out to validate previous laboratory results in a field study. Slugs were collected from an abandoned copper mine (Parys mountain top, PMT), from a site 7 km away from the mine (Parys mountain bottom, PMB), and from a clean site (Snowdonia Cwm Idwal, SCI) in Wales in early July 1994. Whole soft body and digestive gland Cd, Cu, and Zn concentrations were measured by means of atomic absorption spectrophotometry (AAS). The digestive gland was the main tissue for metal accumulation, with significant differences in tissue metal levels between samples from different sites. PMB presented the highest Cd and Zn levels and the highest Cu levels were found at PMT. In addition, metals were demonstrated in situ by autometallography as black silver deposits (BSD) on histological sections of digestive gland tissue. The extent of BSD within lysosomes of digestive cells was closely related to metal levels determined by AAS. Histochemistry revealed that Ca metabolism and structural and reserve connective tissues might be altered in slugs living in metal-polluted soils. Finally, tissue-level biomarkers of biological effect [mean epithelial thickness (MET), mean diverticular radius (MDR), mean luminal radius (MLR), MET/MDR and MLR/MET] were quantified by image analysis of digestive gland histological sections stained with hematoxylin-eosin. MET and MDR values of slugs collected from SCI were high, while slugs from PMB presented low MLR/MET associated with environmental stress induced by metal exposure. We conclude that exposure and effect biomarkers recorded in sentinel slugs could be sensitive, quick, and cheap indices of metal pollution in soils. A Slug Watch monitoring program could be developed similar to the Mussel Watch program, which is currently applied to assess environmental quality in coastal and estuarine areas.