Rhizocephala are derived barnacles that parasitise mostly decapodcrustaceans, but can also infest Peracarida, mantis shrimps and thoracican barnacles, and are found from the deep ocean to freshwater.[1][2] Together with their sister groups Thoracica and Acrothoracica, they make up the subclass Cirripedia.[3] 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 (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.[4]
As adults they lack appendages, segmentation, and all internal organs except gonads, a few muscles, and the remains of the nervous system. Females also have a cuticle, which is never shed.[5] Other than the minute larval stages, there is nothing identifying them as crustaceans or even arthropods in general. The only distinguishable portion of a rhizocephalan body is the externa; the reproductive portion of adult females.
Nauplii released from adult females swim in water for several days without taking food (the larva has no mouth and no intestine) and transform into cypris larvae (cyprids) after several moults. Like the nauplii, the cyprids are lecithotrophic (non-feeding).[6] The female cypris in Kentrogonida settles on a host and metamorphoses into a specialized juvenile form called a kentrogon, which has no visible segmentation and has no appendages except the antennules that are used to attach itself to the host, and whose only purpose is to inject a cell mass named the vermigon into the host's hemolymph through a retractive hollow stylet on its head. The kentrogon stage seems to have been lost in all of the Akentrogonida, where the cypris injects the vermigon through one of its antennules.[7] The vermigon grows into root-like threads through the host's tissue, centering on the digestive system and especially the hepatopancreas, and absorb nutrients from the hemolymph. This network of threads is called the interna. The female then grows a sac-like externa, which consists of a mantle, a mantle cavity, an ovary and a pair of passageways known as cell receptacles, extruding from the abdomen of the host.[8][9][10]
In the order Kentrogonida, the virgin externa contains no openings at first. But it soon molts to a second stage that contains an orifice known as the mantle departure, and which leads into the two receptacle passageways — once assumed to be the testes in hermaphroditic parasites before the realization that they were actually two separate sexes — and starts releasing pheromones to attract male cyprids. From inside the body of the male cypris that succeeds in entering the departure, a unique and very short lived male stage called the trichogen emerges through the antennule opening. It is the homologue of the female kentrogon, but is reduced to an amoeboid unsegmented cuticle-covered mass of cells consisting of three to four cell-types: the dorsolateral, the ventral epidermis, the inclusion cells, and the postganglion. The externa have room for two males, one for each of the receptacles, which increase the heterozygosity of the offspring. Once inside, the trichogen will shed its cuticle before reaching the end of the passageway.[11][12][13]
In the order Akentrogonida, which form a monophyletic group nested within the paraphyletic Kentrogonida,[14] the male does not develop into a trichogon, and the cypris injects its cell mass through its antennule and directly into the body of the immature externa. The offspring also hatch directly into fully developed cyprids instead of nauplius larvae (except for a few species of kentrogonid rhizocephalans, which hatch into cyprids like the akentrogonids, the kentrogonids have kept their nauplius stage). In species like Clistosaccus paguri, the male injects its cluster of cells which migrates through the connective tissue of the mantle and into the receptacle. But in forms like Sylon hippolytes the receptacle is absent, and the males cells implant in the ovary instead.[15] While only a single male can settle in each receptacle, which is the rule in Kentrogonida, the number of implanted males in Akentrogonida can range from just one to more than ten.[16]
The small cluster of cells injected by the male cypris will, once it reaches its destination inside the female, differentiate into a loosely connected mass of sperm-producing germ cells. Being nothing more than sperm-forming cells, these adult male rhizocephalans represent the simplest form of male in the entire animal kingdom. Mature female externa releases eggs into its mantle cavity where eggs are fertilised by sperm from the hyper-parasitic male(s). Due to the larval sexual dimorphism in the Kentrogonida, the females produce two different egg sizes; small female eggs and larger male eggs.[17][18] It appears the sex determination in Akentrogonida is environmental.[19]
In Peltogasterella gracilis, the externa produces several batches of larvae before it drops off the host, taking the male(s) inside with it. After the original externa disappear, the host moults and the interna grows buds that each develops into a new virgin externa. The females commonly has two cypris cell receptacles. With more than one externa, and new ones replacing the old ones, each female Peltogasterella can receive sperm from numerous males during its lifetime.[citation needed]
The externa is where the host's egg sac would be, and the host's behaviour is chemically altered: it is castrated and does not moult until aged externa(e) drop(s) off. The host treats the externa as if it were its own egg sac.[8] This behaviour even extends to male hosts, which would never have carried eggs, but care for the externa in the same way as females.[8][failed verification]
Classification
Following an updated classification of barnacles by Chan et al. (2021), the subgroups Akentrogonida and Kentrogonida were not retained, leaving 13 families as children of the infraclass Rhizocephala.[3][20]
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%.
Barnacles are a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and are hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. Around 1,000 barnacle species are currently known.
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.
The Acrothoracica are an infraclass of barnacles.
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.
Facetotecta is a poorly known subclass of thecostracan crustaceans. The adult forms have never been recognised, and the group is known only from its larvae, the "y-nauplius" and "y-cyprid" larvae. They are mostly found in the north Atlantic Ocean, neritic waters around Japan, and the Mediterranean Basin, where they also survive in brackish water.
Pisa armata is a species of crab from the eastern Atlantic Ocean.
Pollicipes polymerus, commonly known as the gooseneck barnacle or leaf barnacle, is a species of stalked barnacle. It is found, often in great numbers, on rocky shores on the Pacific coasts of North America.
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.
Loxothylacus panopaei is a species of barnacle in the family Sacculinidae. It is native to the Gulf of Mexico and the Caribbean Sea. It is a parasitic castrator of small mud crabs in the family Panopeidae, mostly in the Gulf of Mexico, the Caribbean Sea and the eastern Atlantic Ocean.
Sacculina carcini, the crab hacker barnacle, is a species of parasitic barnacle in the family Sacculinidae, in particular a parasitic castrator, of crabs. The crab that most often is used as a host is the green crab, the natural range of which is the coasts of Western Europe and North Africa. It can be found attached to the crab's abdomen and affects consumption rates by humans.
Clistosaccidae is a family of parasitic barnacles belonging to the bizarre and highly apomorphic infraclass Rhizocephala, which is part of the barnacle subclass Cirripedia.
Clistosaccus is a genus of barnacles which are parasitic on hermit crabs. It is a monotypic genus, and the single species is Clistosaccus paguri, which is found in the northern Atlantic Ocean and the northern Pacific Ocean.
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.
↑ Bresciani, José; Høeg, Jens T. (July 2001). "Comparative ultrastructure of the root system in Rhizocephalan barnacles (Crustacea: Cirripedia: Rhizocephala)". Journal of Morphology. 249 (1): 9–42. doi:10.1002/jmor.1039. PMID11410937. S2CID27832764.
↑ Martin, Joel W.; Olesen, Jørgen; Høeg, Jens T.; Høeg, Jens (2014). Atlas of Crustacean Larvae. JHU Press. p.99. ISBN978-1-4214-1197-2. The Acrothoracica and Rhizocephala have exclusively lecithotrophic nauplii. The cypridoid larva is always lecithotrophic
↑ Hoeg, J. T. (24 September 1987). "Male Cypris Metamorphosis and a New Male Larval Form, The Trichogon, in the Parasitic Barnacle Sacculina carcini (Crustacea: Cirripedia: Rhizocephala)". Philosophical Transactions of the Royal Society B: Biological Sciences. 317 (1183): 47–63. Bibcode:1987RSPTB.317...47H. doi:10.1098/rstb.1987.0047.
↑ Gould, Stephen Jay (2010). Leonardo's Mountain Of Clams. Random House. p.315. ISBN978-1-4090-0038-9. In Sacculina and close relatives, the virgin externa contains no opening
↑ Glenner, Henrik; Høeg, Jens T.; Stenderup, Jesper; Rybakov, Alexey V. (May 2010). "The monophyletic origin of a remarkable sexual system in akentrogonid rhizocephalan parasites: A molecular and larval structural study". Experimental Parasitology. 125 (1): 3–12. doi:10.1016/j.exppara.2009.09.019. PMID19786021.
↑ Korn, O. M.; Shukalyuk, A. I.; Trofimova, A. V.; Isaeva, V. V. (September 2004). "Reproductive Stage of the Life Cycle in the Rhizocephalan Barnacle Polyascus polygenea (Crustacea: Cirripedia)". Russian Journal of Marine Biology. 30 (5): 328–340. doi:10.1023/B:RUMB.0000046552.07712.02. S2CID7703577.
↑ Leonard, Janet L. (2018). "Phylogenetic Character Distribution". Transitions Between Sexual Systems: Understanding the Mechanisms Of, and Pathways Between, Dioecy, Hermaphroditism and Other Sexual Systems. Springer. pp.224–5. ISBN978-3-319-94139-4.
↑ T. Høeg, Jens; Noever, Christoph; A. Rees, David; A. Crandall, Keith; Glenner, Henrik (2020), "A new molecular phylogeny-based taxonomy of parasitic barnacles (Crustacea: Cirripedia: Rhizocephala)", Zoological Journal of the Linnean Society, 190 (2): 632–653, doi:10.1093/zoolinnean/zlz140, hdl:1956/23159
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