Sacculina

Sacculina
Sacculina carcini on a male swimming crab
Scientific classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Thecostraca
Subclass: Cirripedia
Infraclass: Rhizocephala
Family: Sacculinidae
Genus: Sacculina
Thompson, 1836
Type species
Sacculina carcini
Thompson, 1836 

Sacculina is a genus of barnacles that is a parasitic castrator of crabs. They belong to a group called Rhizocephala. The adults bear no resemblance to the barnacles that cover ships, whales, and piers; they are recognised as barnacles because their larval forms are like other members of the barnacle class Cirripedia. The prevalence of this crustacean parasite in its crab host can be as high as 50%.