An isopod may sound like something
you could listen to music on, but in reality they are nasty parasites that
attack their hosts in an unusual and disgusting way.
At a length of about 2 centimeters long, the isopod will attach itself to
the tongue of a living fish and begin sucking blood from it. As the isopod
grows, more and more blood is extracted until the tongue actually
atrophies and dies, falling off.
Now that room has been made in the mouth, the isopod moves in and actually
replaces the fish's tongue with its body. The fish continues life normally
as the disgusting parasite continues to gorge itself inside of the
unfortunate fish's mouth.