Editing Rhizocephala (section)

== Description ==

=== Adults ===

[[File:Haeckel Sacculina.jpg|thumb|''[[Sacculina carcini]]'' (adult female) parasiting a crab (''[[Carcinus maenas]]''). [[Haeckel]]'s drawing shows the internal network of filaments from the parasite.]]

Adult rhizocephalans lack [[appendage]]s, [[segmentation (biology)|segmentation]], and all internal organs except [[gonad]]s, a few muscles, and the remains of the [[nervous system]]. Females have a [[Arthropod cuticle|cuticle]], which is never shed.<ref>{{cite journal |last1=Bresciani |first1=José |last2=Høeg |first2=Jens T. |title=Comparative ultrastructure of the root system in Rhizocephalan barnacles (Crustacea: Cirripedia: Rhizocephala) |journal=Journal of Morphology |date=July 2001 |volume=249 |issue=1 |pages=9–42 |doi=10.1002/jmor.1039 |pmid=11410937 |s2cid=27832764 }}</ref> 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.{{cn|date=October 2024}}

=== Nauplius and cypris larvae ===

[[Nauplius (larva)|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 ([[cyprid]]s) after several [[ecdysis|moults]]. Like the nauplii, the cyprids are lecithotrophic (non-feeding).<ref>{{cite book |last1=Martin |first1=Joel W. |last2=Olesen |first2=Jørgen |last3=Høeg |first3=Jens T. |last4=Høeg |first4=Jens |title=Atlas of Crustacean Larvae |date=2014 |publisher=JHU Press |isbn=978-1-4214-1197-2 |page=99 |quote=The Acrothoracica and Rhizocephala have exclusively lecithotrophic nauplii. The cypridoid larva is always lecithotrophic }}</ref> 

=== Parasitism ===

The female cypris larva in Kentrogonida settles on a host and [[Metamorphosis (biology)|metamorphoses]] into a specialized juvenile form, a kentrogon. This has no visible segmentation and no appendages except the [[antennule]]s. These are used to attach the larva to the host; their only purpose is to inject a cell mass, the [[vermigon]], into the host's [[hemolymph]] through a retractive hollow stylet on the larva's head. The kentrogon stage seems to have been secondarily lost in all of the Akentrogonida, where the cypris injects the vermigon through one of its antennules.<ref>{{cite book |last1=Glenner |first1=Henrik |last2=Høeg |first2=Jens T. |chapter=A Scenario for the Evolution of the Rhizocephala |title=Modern Approaches to the Study of Crustacea |year=2002 |doi=10.1007/978-1-4615-0761-1_42 |isbn=978-1-4613-5228-0 |pages=301–310 |url=https://www.researchgate.net/profile/Jens-Hoeg/publication/289891819_A_Scenario_for_the_Evolution_of_the_Rhizocephala/links/5931677eaca272fc55f5c223/A-Scenario-for-the-Evolution-of-the-Rhizocephala.pdf}}</ref> The vermigon grows into a network of root-like threads through the host's tissue, centering on the [[digestive system]] and especially the [[hepatopancreas]], and absorb nutrients from the hemolymph. The 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.<ref name="Glenner"/><ref>{{cite journal |last1=Elumalai |first1=V. |last2=Viswanathan |first2=C. |last3=Pravinkumar |first3=M. |last4=Raffi |first4=S. M. |title=Infestation of parasitic barnacle Sacculina spp. in commercial marine crabs |journal=Journal of Parasitic Diseases|date=2014 |volume=38 |issue=3 |pages=337–339 |doi=10.1007/s12639-013-0247-z |pmid=25035598 |pmc=4087306 }}</ref><ref>{{cite journal |last1=Noever |first1=Christoph |last2=Keiler |first2=Jonas |last3=Glenner |first3=Henrik |title=First 3D reconstruction of the rhizocephalan root system using MicroCT |journal=Journal of Sea Research |date=July 2016 |volume=113 |pages=58–64 |doi=10.1016/j.seares.2015.08.002 |bibcode=2016JSR...113...58N |doi-access=free |hdl=1956/12721 |hdl-access=free }}</ref>

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 [[Homology (biology)|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.<ref>{{cite journal |last1=Hoeg |first1=J. T. |title=Male Cypris Metamorphosis and a New Male Larval Form, The Trichogon, in the Parasitic Barnacle Sacculina carcini (Crustacea: Cirripedia: Rhizocephala) |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=24 September 1987 |volume=317 |issue=1183 |pages=47–63 |doi=10.1098/rstb.1987.0047 |bibcode=1987RSPTB.317...47H |doi-access= }}</ref><ref>{{cite book |last1=Gould |first1=Stephen Jay |title=Leonardo's Mountain Of Clams |date=2010 |publisher=Random House |isbn=978-1-4090-0038-9 |page=315 |quote=In Sacculina and close relatives, the virgin externa contains no opening }}</ref><ref>{{cite book |first1=Waltraud |last1=Klepal |chapter=A Review of the comparative anatomy of the males in cirripedes |pages=250–304 |chapter-url={{Google books|OKFt7CCF-jkC|page=250|plainurl=yes}} |title=Oceanography And Marine Biology |date=1999 |publisher=CRC Press |isbn=978-0-203-40068-5 |quote=The 'trichogon' was discovered, a new post-cypris male larval form which may be considered as an extremely reduced dwarf male }}</ref>

In the order Akentrogonida, which form a monophyletic group nested within the paraphyletic Kentrogonida,<ref>{{cite journal |last1=Glenner |first1=Henrik |last2=Høeg |first2=Jens T. |last3=Stenderup |first3=Jesper |last4=Rybakov |first4=Alexey V. |title=The monophyletic origin of a remarkable sexual system in akentrogonid rhizocephalan parasites: A molecular and larval structural study |journal=Experimental Parasitology |date=May 2010 |volume=125 |issue=1 |pages=3–12 |doi=10.1016/j.exppara.2009.09.019 |pmid=19786021 }}</ref> 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.<ref>{{cite book |last1=Castro |first1=Peter |last2=Davie |first2=Peter |last3=Guinot |first3=Danièle |last4=Schram |first4=Frederick |last5=Klein |first5=Carel von Vaupel |chapter=Fig. 71-12.24. Rhizocephalan life cycle: A-J, life cycle of the akentrogonid rhizocephalan |chapter-url={{Google books|g6a9CwAAQBAJ|page=714|plainurl=yes}} |title=Treatise on Zoology - Anatomy, Taxonomy, Biology. The Crustacea, Volume 9 Part C (2 vols): Brachyura |date=2015 |publisher=BRILL |isbn=978-90-04-19083-2 |page=714 }}</ref> 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.<ref>{{cite book |last1=Thiel |first1=Martin |last2=Wellborn |first2=Gary |title=The Natural History of the Crustacea: Life Histories |date=2018 |publisher=Oxford University Press |isbn=978-0-19-062028-8 |page=385 }}</ref>

=== Reproduction ===

The small cluster of cells injected by the male cypris differentiates into a loosely connected mass of sperm-producing germ cells, once it reaches its destination inside the female. Being nothing more than [[Spermatogenesis|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 [[Hyperparasite|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.<ref>{{cite journal |last1=Nagler |first1=Christina |last2=Hörnig |first2=Marie K. |last3=Haug |first3=Joachim T. |last4=Noever |first4=Christoph |last5=Høeg |first5=Jens T. |last6=Glenner |first6=Henrik |title=The bigger, the better? Volume measurements of parasites and hosts: Parasitic barnacles (Cirripedia, Rhizocephala) and their decapod hosts |journal=PLOS ONE |date=5 July 2017 |volume=12 |issue=7 |pages=e0179958 |doi=10.1371/journal.pone.0179958 |pmid=28678878 |pmc=5497970 |bibcode=2017PLoSO..1279958N |doi-access=free }}</ref><ref>{{cite journal |last1=Korn |first1=O. M. |last2=Shukalyuk |first2=A. I. |last3=Trofimova |first3=A. V. |last4=Isaeva |first4=V. V. |title=Reproductive Stage of the Life Cycle in the Rhizocephalan Barnacle Polyascus polygenea (Crustacea: Cirripedia) |journal=Russian Journal of Marine Biology |date=September 2004 |volume=30 |issue=5 |pages=328–340 |doi=10.1023/B:RUMB.0000046552.07712.02 |s2cid=7703577 }}</ref> It appears the sex determination in Akentrogonida is environmental.<ref>{{cite book |last1=Leonard |first1=Janet L. |chapter=Phylogenetic Character Distribution |chapter-url={{Google books|0rWZDwAAQBAJ|page=224|plainurl=yes}} |title=Transitions Between Sexual Systems: Understanding the Mechanisms Of, and Pathways Between, Dioecy, Hermaphroditism and Other Sexual Systems |date=2018 |publisher=Springer |isbn=978-3-319-94139-4 |pages=224–5 }}</ref>

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 [[Budding#Animal reproduction|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 her lifetime.{{Citation needed|date=July 2017}}

The externa is where the host's egg sac would be, and the host's behaviour is chemically altered: it is [[castration|castrated]] and does not moult until the aged externa drops off. The host treats the externa as if it were its own egg sac.<ref name="Glenner"/> This behaviour even extends to male hosts, which would never have carried eggs, but care for the externa in the same way as females.<ref name="Glenner">{{cite book |last1=Glenner |first1=Henrik |last2=Høeg |first2=Jens T. |year=2002 |chapter=A scenario for the evolution of the Rhizocephala |editor=Elva Escobar-Briones & Fernando Alvarez |title=Modern Approaches to the Study of Crustacea |pages=301–310 |publisher=Springer |isbn=978-0-306-47366-1 |chapter-url={{Google books|OKFt7CCF-jkC |page=301 |plainurl=yes}} }}</ref>