Since mudsnails are able to avoid contaminated sediment and that the contaminants in sediment are not uniformly distributed, the mudsnail Peringia ulvae was exposed to cadmium (Cd) spiked sediment and assessed for avoidance response in a heterogeneous contamination scenario. Four Cd concentrations were prepared and disposed in patches on dishes, which were divided in 25 fields (six fields for each sediment concentration); 24 organisms were deployed in the central field, with no sediment. Observations were made at 2, 4 and 6 h (corresponding to immediate response), 8, 10 and 12 h (very short term), and 24 h (short term). A trend to avoid contaminated patches was observed in the immediate and very short term. After 24 h exposure, the organisms exposed to the highest level of contamination seemed to have lost the ability to move and avoid contaminated patches. In a contamination scenario in which non- and contaminated sediment patches are heterogeneously distributed, local mudsnail populations can simply rearrange their locality without needing to move to a different habitat. Such less contaminated patches can become donor areas in a future recolonization scenario.
