''Hemioniscus balani'', a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

isopod Isopoda is an Order (biology), order of crustaceans. Members of this group are called isopods and include both Aquatic animal, aquatic species and Terrestrial animal, terrestrial species such as woodlice. All have rigid, segmented exoskeletons ...

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

, is a widespread parasitic castrator of

barnacle Barnacles are arthropods of the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea. They are related to crabs and lobsters, with similar Nauplius (larva), nauplius larvae. Barnacles are exclusively marine invertebra ...

s in the northern

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

. Its

range Range may refer to: Geography * Range (geographic), a chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra) ** Mountain range, a group of mountains bordered by lowlands * Range, a term used to i ...

extends from

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...

to the Atlantic coast of

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

, and as far west as

Massachusetts Massachusetts ( ; ), officially the Commonwealth of Massachusetts, is a U.S. state, state in the New England region of the Northeastern United States. It borders the Atlantic Ocean and the Gulf of Maine to its east, Connecticut and Rhode ...

. It is also commonly found on the Pacific coast of North America; it is not known if the Pacific and Atlantic populations are the same species, or if the Pacific population exists following human-assisted introduction.

Life history

''Hemioniscus balani'' parasitically castrates adults of several species of barnacles, including '' Semibalanus balanoides'', '' Chthamalus fissus'', '' Chthamalus dalli'', and '' Balanus glandula''. Rarely does more than one ''H. balani'' parasitize the same host, but under conditions of heavy infestation, a single barnacle may contain as many as seven ''H. balani'' individuals. Both the isopod parasite and its barnacle host are hermaphroditic crustaceans with a free-swimming larva and a sessile adult. ''H. balani'' is a protandrous hermaphrodite that develops into a female only after attaching to its adult host. While in its male swimming form, it resembles a typical isopod with antennae, jointed limbs, and a segmented exoskeleton. When fully mature, the adult female form of ''H. balani'' is reduced to a bloated, star-shaped egg sac, up to 8 mm in length inhabiting the mantle cavity of its barnacle host. The barnacle host species are simultaneous hermaphrodites, with both male and female gonads in each adult. ''H. balani'' feeds on its host's ovarian fluid, so the barnacle loses female reproductive ability, but can still produce sperm.

References

{{Taxonbar, from=Q3862741 Cymothoida Isopods of the Atlantic Ocean Crustaceans described in 1866 Parasites of crustaceans Parasitic crustaceans