Sharks are predators, usually thought of as at the top of the food chain in ocean ecosystems. As such, they can accumulate a lot of poisons or toxins, as we have seen recently in the news with scares of mercury in tuna. But it turns out that some of their parasites can be accumulating the toxins in extremely high concentrations and there is speculation that in doing so, they may be protecting their hosts. A recent study by scientists at the University of Aberdeen, Scotland, and the University of Tehran, Iran, found that tapeworms such as Anthobothrium, had concentrations of cadmium and lead, two heavy metals, that were as much as 455 times that of their hosts. The scientists suggest in their paper that parasites can serve as useful bioindicators - squishy little canaries in coal mines, if you will.
You can read about this in the original paper or on this site.
Image is of the type species of the genus Anthobothrium, A. cornucopia, from its original description in 1850.