Hemioniscus balani, a species of isopod crustacean, is a widespread parasitic castrator of barnacles in the northern Atlantic Ocean.

[2] Both the isopod parasite and its barnacle host are hermaphroditic crustaceans with a free-swimming larva and a sessile adult.

While in its male swimming form, it resembles a typical isopod with antennae, jointed limbs, and a segmented exoskeleton.

[4] When fully mature, the adult female form of H. balani is reduced to a bloated, star-shaped egg sac,[2] up to 8 mm in length inhabiting the mantle cavity of its barnacle host.

H. balani feeds on its host's ovarian fluid, so the barnacle loses female reproductive ability, but can still produce sperm.