A cymothoid isopod lives inside the mouth of a snapper fish. The isopod servers blood vessels in the fish's tongue, causing the tongue to atrophy and degenerate. The isopod then hooks its pereopods, or legs, to the base of the fish's tongue, essentially replacing the tongue. The isopod stays there for the rest of its life, feeding on blood, mucus, and stray pieces of food from the fish.
P for Parasitism, M for mutualism, or C commensalism