The strangest of all parasitic crustaceans is Cymothoa exigua, which attacks fish and takes up residence in their mouth.

An excerpt from the article 22 facts about crustacean

This crustacean feeds on the blood of its host. It attaches itself to the tongue and slowly replaces its function until it leads to its complete disappearance. At this point, the Cymothoa replaces the tongue completely and the fish begins to use the parasite as a substitute. It is the only known parasite that replaces the organ of its host.