Carbon assimilation and digestive toxicity in naïve grass shrimp (Palaemonetes pugio) exposed to dietary cadmium.

Naïve grass shrimp Palaemonetes pugio were pulse-fed cadmium-contaminated meals containing carbon-14, fluorescent or near-infrared markers and analyzed for carbon assimilation efficiency, gut residence time, feces elimination rate, extracellular digestive protease activity or gut pH. Carbon assimilation efficiency (~83%), minimum gut residence time (~435 min) and proventriculus pH (~5.29 to ~6.01) were not impacted significantly by cadmium ingestion. A dose-dependent decrease in feces elimination rate (from ~14.4 to ~6.4 mm h(-1)) was observed for shrimp for 2 h following minimum gut residence time. Protease activities increased ~2.4-fold over the range of dietary cadmium exposures, however, this variation was not dose-dependent. Differential impacts of cadmium exposure on carbon and cadmium assimilation reported previously are consistent with work involving shrimp subjected to chronic field exposure. The influence of ingested cadmium on feces elimination rate may be related to pre-assimilatory impacts on packaging, intestinal transport or release of feces. Protease activities may have been influenced by pre-assimilatory interactions between available cadmium ions in gut fluid and enzyme-secreting cells of the hepatopancreatic epithelium or direct impacts on active enzymes.