Subcellular distribution of cadmium in two aquatic invertebrates: change over time and relationship to Cd assimilation and loss by a predatory insect.

We set out to determine if the efficiency of cadmium (Cd) assimilation and loss by a freshwater predator (the alderfly Sialis velata) differs when it is exposed, for various lengths of time, to Cd in either an insect (Chironomus riparius) or a worm (Tubifex tubifex). Prey were exposed to Cd in sediments for up to 28 days and then fractionated to measure Cd distributions in their cells. Cadmium subcellular distributions varied little over time for a given preytype but differed substantially between the two prey species; for example, the cytosol comprised a larger proportion of Cd in the insect (76%) than in the worm (34%). The predator assimilated proportionally more Cd from the insect (72%) than from the worm (46%) and these assimilation efficiencies were similar to the proportion of prey Cd that would theoretically be available to it (cytosolic Cd + organelle Cd). However, measurements of Cd in the predator's feces confirmed that to obtain an exact 1:1 relationship between predator assimilation efficiency and prey subcellular distribution we had to assume that approximately 50% of the Cd associated with the organelle fraction of T. tubifex was unavailable for digestion by the predator. Losses of Cd from the predator also varied depending on the type of prey that were the source of its Cd.