Fish lice

Fish lice are the family Argulidae, or 'carp lice'. They are a group of parasitic crustaceans in the class Maxillopoda.
Although they are thought to be primitive forms, they have no fossil record. Argulidae is the only family in the order Arguloida (occasionally "Arguloidea"), although a second family, Dipteropeltidae, has been proposed.