Fig 2: Peltogaster (Peltogastridae) whole externa and root system attached to a part of the main abdominal root trunk From Bresciani & Hoeg 2001.
The life cycle (Fig 3) of the parasite involves a mature female externa which is fertilized by one or two cryptic dwarf males and which subsequently releases a series of broods of free swimming nauplii. The nauplii develop lecithotrophically, metamorphose into cypris larvae after about 5–6 days and become competent to settle after another 3–4 days in the plankton. Female cypris larvae must locate a suitable host while the male cypris larvae must find a parasitized host containing a virgin female. Some of the sensory structures involved in this behavior are the lattice organs and the aesthetasc setae found on antennules of the cyprids that have been proposed as olfactory organs. Females possess one and the males two. It had been demonstrated that larvae can use waterborne host metabolites to find a suitable host. 
Female cyprids settle on the exoskeleton of a host crab at the base of a plumose seta and metamorphose into a special stage known as the kentrogon. The kentrogon penetrates the exoskeleton of the crab with a hollow stylet and injects the primordial parasite into the hemocoelic fluid. After an internal phase of a few months to 3 years, the parasite produces an external virginal reproductive body (externa) situated under the abdomen of the host. The externa attracts male cyprids, which implant as dwarf males and remain with the female externa for the duration of the latter’s lifetime and fertilize all its broods. Externa failing to receive at least one male cannot mature and eventually perish, leaving a scar on the host exoskeleton.
Fig 4: First photo shows the cyprid larva, antennae are marked with arrows, from Pasternak et al 2005. Second photo shows the kentrogon stage injecting cells in to the host body, from Hoeg 1987.
Sacculina carcini is a rhizocephalan parasite that attacks the crab C. maenas. S. carcini can have on their hosts severe and lasting effects on these include growth, morphology, physiology, and behavior of the host crab. The parasite can arrest the moult cycle of its host crab (which therefore suffers increased fouling). As a consequence of rootlet growth this parasite castrates both male and female hosts and phenotypically feminizes the males. Both sexes suffer alterations in behavior such as response to the externa and Sacculina eggs as their own brood. For example a common behavior of fecunded female crabs is to climb to some high rock and groom its abdomen to release the fertilized eggs from the brood pouch. Moving her claw the mother crab stir the water generating a flow. In the same way male parasitized by Sacculina will display a similiar behavior when the parasite offpring is ready to hatch, he will groom the Externa and release the next generation of his foster sons. “Sacculinized” host became an integrated developmental system that include the castrated crab, the breeding externa and the dwarf males. These creatures had been the focus of interesting cromosome and molecular studies that we can discuss in a next episode.
Fig 5: Rhizocephalan Extena on infected host species showing rootlets distribution in different parasite-host systems: A: Peltogaster paguri (Peltogastridae) on Pagurus bernhardus. B: Sacculina carcini (Sacculinidae) on Carcinus maenas. C: Sylon hippolytes (Clistosaccidae) on Spirontocaris lilljeborgi, from Bresciani & Hoeg 2001.
Cristian Villagra
References:
Bresciani J & Høeg JT (2001) Comparative Ultrastructure of the Root System in Rhizocephalan Barnacles (Crustacea: Cirripedia: Rhizocephala). Journal of Morphology, 249:9–42.
Herberts C (1982) Host-Parasite Relation between the Shore Crab Carcinus maenas and Sacculina carcini (Rhizocephala): Identification and Characterization of a Specific Fraction Correlated with Parasitism. Journal of Invertebrate Pathology, 39, 60-65.
Hoeg JT (1987) The relation between cypris ultrastructure and metamorphosis in male and female Sacculina carcini (Crustacea, Cirripedia). Zoomorphology, 107:299-311.
Le Mouchel-Vielh E, Rigolot C, Gibert J-M & Deutsch JS (1998) Molecules and the Body Plan: The Hox Genesof Cirripedes (Crustacea). Molecular Phylogenetics and Evolution, 9,382–389.
Øksnebjerg B (1999) The Rhizocephala (Crustacea: Cirripedia) of the Mediterranean and black seas: taxonomy, biogeography and ecology. Israel Journal of Zoology, 46: 1-102.
Pasternak Z, Garm A & Høeg JT (2005) The morphology of the chemosensory aesthetasc-like setae used during settlement of cypris larvae in the parasitic barnacle Sacculina carcini (Cirripedia: Rhizocephala). Marine Biology, 146: 1005–1013.
Threshera RE, Wernerb M, Høegc JT, Svaned I, Glennerc H, Murphy NE, Wittwer C (2000) Developing the options for managing marine pests: specificity trials on the parasitic castrator, Sacculina carcini, against the European crab, Carcinus maenas, and relatedspecies. Journal of Experimental Marine Biology and Ecology, 254: 37–51
Strathmann RR (1993) Hypotheses on the Origins of Marine Larvae. Annual Review of Ecology and Systematics, 24: 89-117. Walker G(1985) The cypris larvaea of sacculina carcini Thompson (Crustacea: Cirripedia: Rhizocephala) J. Eq. Mar. hoI. Ecof, 93:131-145.
