THE NUMBER OF DIFFERENT KINDS OF PARASITES IS LEGION, as is the amazing range of their life styles. Discoveries in the 1980s brought to light a most unusual form of parasitism. Unusual because it involves the first-known case in animals of a parasite replacing a host structure and thereafter taking over the function of the vanished structure. It actually becomes a substitute for the very organ it destroyed. The parasite in question is a marine isopod, a creature very similar to the woodlouse or sow bug. Its host is various types of fish, such as snappers, and the sparid fish with the heart-warming scientific name of Boop boops. The structure it destroys and replaces is the fish's tongue!

Upon entering the fish's mouth, the isopod attaches itself to the base of the fish's tongue. Exactly how it destroys the tongue is not known for sure. It does not simply eat it. It seems that it gorges on blood at the base of the fish's tongue, thereby decreasing the flow of blood to the rest of the tongue. Soon the tongue degenerates and vanishes, leaving only a stub. The isopod is attached to the base of the fish's mouth by seven pairs of hook-like appendages.

Interestingly, fish found with isopods instead of tongues appear to be in perfectly healthy condition. Fishologists believe that the isopod actually takes over the function of the tongue. In a normal fish, a patch of teeth (called vomerine) in the roof of the moth help chew food before swallowing. The food is held in place by the tongue while this is taking place. Scientists have noted that the isopod is remarkably similar in shape to the normal tongue. And many specimens of parasite show slight abrasion on their back where the vomerine teeth should make contact. These facts have caused scientists to conclude that the parasitic isopod is just as good as the real thing — the fish's own tongue!

Who could ever have thought of such a thing?