Quantifying the uptake and release of cadmium and copper by the opossum shrimp Mysis relicta preying upon the cladoceran Daphnia magna using stable isotope tracers

Abstract

Daphnia magna were uniformly labeled with the stable isotope tracers 113Cd and 65Cu and fed to the invertebrate predator Mysis relicta to determine the levels of release, assimilation, accumulation, egestion, and excretion of Cd and Cu when consumed in food alone. Samples were analyzed using ICP-MS and data analyzed using a mass balance model. The sloppy feeding behaviour of mysids resulted in the release of metals from the prey Daphnia in particulate and soluble forms. Mysids have been observed to consume selectively the soft-body portion of Daphnia, resulting in the discarding of particulates, mainly exoskeleton, while eating. The trade-off of a higher handling time is a highly assimilable diet. Assimilation efficiencies of the two metals (72% for Cd and 21% for Cu) were considerably higher than previously published values for M. relicta, likely due to a previous overestimation of metal egestion. Excretion appears to be an important mechanism by which mysids rid their body of assimilated metals, resulting in the further transfer of metals into the readily available dissolved phase. After a 94-h depuration period, 47% of ingested Cd and 53% of ingested Cu remained associated with the mysids in either the gut, body, or exoskeleton.