"Parasites suck toxins from sharks"

[chucky]

http://www.flmnh.ufl.edu/fish/sharks/InNews/toxins2007.html

"Together, such studies leave researchers thinking that these intestinal creatures are protecting their hosts." and I think if.. the host is protecting them or making their life comfortable by eating right.

[jycemrsh]

It is the mutual understanding that keeps them going. The host and the "parasites" work for their own self but it is the nature that has designed it in such a way that they are interdependent on each other. Both of the possibility that you mentioned are true. Both the host and the parasite help each other in their existence.