Relationships between pollutant-induced digestive toxicity and the assimilation and subcellular partitioning of elements by grass shrimp Palaemonetes pugio

Aquatic invertebrates inhabiting urbanized estuaries are typically exposed to pollutants through multiple pathways, including the diet. Biochemical and physical processes within invertebrate guts can be impacted by ingested pollutants, which may influence the assimilation of essential nutrients as well as pollutants. Pre-assimilatory digestive toxicity may result from pollutants circulating in gut fluid. Post-assimilatory toxicity could be due to incorporation of a pollutant into consumer tissues as a result of chronic exposure. This series of studies investigated the influence of chronic exposure to impacted field conditions or pre-exposure to dietary metal (Cd and Hg) in the laboratory on the assimilation of elements (organic carbon, Cd and Hg), subcellular partitioning of assimilated metal and digestive physiology (gut residence time [GRT], feces elimination rates [FER], gut pH and digestive protease activities) in the grass shrimp Palaemonetes pugio. Carbon and Cd assimilation and endpoints related to digestion were also assessed for naive shrimp following ingestion of a pulse of Cd-contaminated food. Based on these studies, it appears that grass shrimp may be able to maintain carbon assimilation in the laboratory under different forms of pollutant-induced dietary stress. For field-collected shrimp, this phenomenon may be attributable to digestive plasticity (e.g., increased GRT to compensate for reduced digestive enzyme activities). Increased assimilation of Cd and Hg was observed for shrimp collected along an impact gradient. Enhanced non-essential metal assimilation may have implications for accumulation and toxicity in impacted shrimp. In the case of Cd assimilation, a positive correlation with GRT and negative relationship with protease activities suggests that digestive plasticity may also influence assimilation of non-essential elements in the field. Variability in Cd assimilation by shrimp pre-exposed to dietary metal in the laboratory was not dose-dependent, which may be related to interactions between post-assimilatory impacts on gut physiology. Increased assimilation of Hg by Hg pre-exposed shrimp may have been related to a corresponding increase in gut pH (i.e., decreased concentrations of H+ ions in circulating gut fluid).