Studies have shown that the trophic transfer of certain metals in aquatic systems may be controlled by the internal distribution of metal within prey and that this distribution may be influenced by detoxification mechanisms (i.e. metals bound to metal-binding proteins [metallothioneins, MT] may be more available to predators than metals associated with insoluble cellular constituents). The purpose of this investigation was to examine the interactive effects of Cd and Zn exposure on the accumulation and subcellular distribution of these metals in the brine shrimp Artemia franciscana. Particular attention was given to the partitioning of metals to a subcellular compartment containing heat-stable proteins (HSP) (e.g. MT), heat-denatured proteins (HDP) (e.g. 'enzymes') and organelles, here considered as trophically available metal (TAM). Adult A. franciscana were exposed for 3 d to Cd (control, 1, 89 or 445 µM and Zn-control) or Zn (control, 1, 89 or 445 µM and Cd-control) through solution using radioisotopes ( 109 Cd and 65 Zn) as tracers of stable metals. Following exposure, various operationally defined subcellular fractions were obtained. Increased binding of Cd to HSP resulted in increased partitioning of Cd to the TAM compartment (i.e. TAM-Cd% increased from 57 to 80% over the range of Cd exposures). This increase in TAM was greater than proportional at the 1 µM Cd exposure concentration and could result in a 'bioenhancement' of Cd trophic transfer to predators. Exposure to elevated concentrations of Cd (89 and 445 µM) also resulted in a ~50% suppression in Zn accumulation by A. franciscana. Upon increasing Zn exposure, a shift in the subcellular partitioning of Zn among TAM fractions (from HSP to HDP and organelles) maintained a TAM-Zn% of ~65%. As a verification of the TAM concept, a direct relationship was observed between the partitioning of Cd and Zn to the TAM compartment of A. franciscana exposed to the Cd-control/Zn-control treatment and absorption of these metals by the grass shrimp Palaemonetes pugio.

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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.

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