(First link is now correct)
The BBC is reporting parasitic louse found in a red snapper in Britain. Cymothoa Exigua drains blood from the fish's tongue and then takes up residence inside the mouth. It seems to even be a functional replacement for the tongue.
A nice picture is here.
The beeb quotes a scientist as being very excited by the find, but I spent 30 seconds on google and found references going back to the late 80s...An entry from early this year is at Stranger Fruit
Other lovely hitchhikers are described here.