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.