Surface adsorption of metals onto the earthworm Lumbricus rubellus and the isopod Porcellio scaber is negligible compared to absorption in the body

In terrestrial organisms, bioaccumulation is usually based on a summation of the amount of metal adsorbed to the body wall and absorbed into the body. The relative proportions of metal adsorption and absorption are usually not quantified. In this study, the distinction between adsorbed and absorbed metals was investigated in two different terrestrial species exposed to metals for 2 weeks. The earthworm Lumbricus rubellus was chosen as representative for organisms mainly taking up metals via the dermal route, and the isopod Porcellio scaber as an organism taking up metals mainly via the alimentary tract. Cross-sections of whole animals were made using a cryostat and accumulated metals were localized by means of a phosphor screen (autoradiography). Radiolabels were used to determine the distribution of metals over the different organs and to distinguish between adsorption and absorption. Cd in the earthworm was mainly found in tissues of the chloragogenous region, whereas Zn was also found in various other organs and in the connective tissue. In the isopod, both Cd and Zn were mainly located in the hepatopancreas. Adsorbed amounts of Cd and Zn were negligible compared to internalized Cd and Zn concentrations for both organisms. Consequently, when focusing on effects of metal uptake for the organism itself, there is no need to correct for adsorption. This suggests that adsorption to the epidermis is not a rate limiting step in metal uptake by soil invertebrates.