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 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%.
A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile (nonmobile) and most are suspension feeders, but those in infraclass Rhizocephala are highly specialized parasites on crustaceans. They have four nektonic larval stages. Around 1,000 barnacle species are currently known. The name "Cirripedia" is Latin, meaning "curl-footed". The study of barnacles is called cirripedology.
Rhizocephala are derived barnacles that parasitise mostly decapod crustaceans, but can also infest Peracarida, mantis shrimps and thoracican barnacles, and are found 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. The name Rhizocephala derives from the Ancient Greek roots ῥίζα and κεφαλή, describing the adult female, which mostly consists of a network of thread-like extensions penetrating the body of the host.
Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults.
Thompsonia is a genus of barnacles which has evolved into an endoparasite of other crustaceans, including crabs and snapping shrimp. It spreads through the host's body as a network of threads, and produces many egg capsules which emerge through joints in the host's shell.
Sessilia is an unranked clade of barnacles, comprising the barnacles without stalks, or acorn barnacles. They form a monophyletic group and are probably derived from stalked barnacles. Sessilia is divided into two orders. The Verrucomorpha contain two families, Verrucidae and Neoverrucidae, and the remaining 14 families are in the order Balanomorpha.
Eulimidae is a family of very small parasitic sea snails, marine gastropod mollusks in the superfamily Vanikoroidea.
The Sacculinidae are a family of barnacles belonging to the bizarre parasitic and highly apomorphic infraclass Rhizocephala. The Sacculinidae is one of the two larger families of Rhizocephala, containing six genera:
The Peltogastridae are a family of barnacles belonging to the bizarre parasitic and highly apomorphic infraclass Rhizocephala. The Peltogastridae are by far the largest family of Rhizocephala. They comprise 14 genera, 3 of which were moved from the family Lernaeodiscidae.
The clade Multicrustacea constitutes the largest superclass of crustaceans, containing approximately four-fifths of all described crustacean species, including crabs, lobsters, shrimps, woodlice, prawns, krill, barnacles, crayfish, copepods, amphipods and others. The largest branch of multicrustacea is the class Malacostraca.
The Hexanauplia constitute a class of crustaceans, comprising two groups: the Copepoda and the Tantulocarida. The former subclass Thecostraca was elevated to class rank and removed from Hexanauplia in 2021.
Heterosaccus is a genus of barnacles in infraclass Rhizocephala. Like other taxa in this group, they parasitize crabs. Geoffroy Smith circumscribed the genus in 1906; he initially only included H. hians. Smith circumscribed a genus distinct from Sacculina due to a difference of the mesentery; in Heterosaccus, the mesentery does not stretch down to the mantle opening but rather only is present on the ring of attachment.
Polyascus is a genus of barnacles in infraclass Rhizocephala. It was circumscribed in 2003 by Henrik Glenner, Jørgen Lützen, and Tohru Takahashi. They included three species, all transferred from Sacculina. The generic name polyascus refers to the typical presence of multiple external sac-like female bodies, known as externae. In Polyascus species, these originate from asexual reproduction.
Sylon hippolytes is a castrating parasite that infects the shrimp Pandalina brevirostris.
Akentrogonida was formerly a suborder of barnacles belonging to the group Rhizocephala, now an infraclass. In research published by Chan et al. in 2021, the suborders Akentrogonida and Kentrogonida were removed from the infraclass Rhizocephala, leaving 13 families as children of Rhizocephala without intermediate orders or suborders.
Kentrogonida was formerly a suborder of barnacles belonging to the group Rhizocephala, now an infraclass. In research published by Chan et al. in 2021, the suborders Kentrogonida and Akentrogonida were removed from the infraclass Rhizocephala, leaving 13 families as children of Rhizocephala without intermediate orders or suborders.
Duplorbidae is a family of barnacles belonging to the infraclass Rhizocephala.
Polysaccus is a genus of barnacles belonging to the monotypic family Polysaccidae.
Polyascidae is a family of parasitic barnacles in the class Thecostraca. There are at least two genera and about eight described species in Polyascidae.
