Rhizocephala are derived barnacles that are parasitic castrators. Their hosts are mostly decapod crustaceans, but include Peracarida, mantis shrimps and thoracican barnacles. Their habitats range from the deep ocean to freshwater. Together with their sister groups Thoracica and Acrothoracica, they make up the subclass Cirripedia. Their body plan is uniquely reduced in an extreme adaptation to their parasitic lifestyle, and makes their relationship to other barnacles unrecognisable in the adult form. They also exhibit the most extreme sexual dimorphism of all animals. The females inject themselves into a host and take over their bodies, while the males inject themselves into a settled female and cease being independent organisms through the degeneration of all tissues except the ones responsible for spermatogenesis. The name Rhizocephala derives from the Ancient Greek roots ῥίζα (rhiza, "root") and κεφαλή (kephalē, "head"), describing the adult female, which mostly consists of a network of thread-like extensions penetrating the body of the host.

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