Rhizocephala

Rhizocephala are a highly degenerate order of parasitic crustaceans that live on crabs and hermit crabs. These parasites are hermaphroditic, though complementary males can occur. When young, they go through stages similar to a developing barnacle.1

Once attached to a host, they become limbless, unsegmented sacs with rootlike processes that penetrate the host to absorb its juices. The presence of the parasite not only castrates the host but can also feminize a male host in terms of morphology and behavior. The parasite develops a reproductive body that emerges from the host, and the host then cares for the parasite's larvae as if they were its own.12