Editing Rhizocephala (section)

=== 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>