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By shuffling biogeographical distributions, biological invasions can both disrupt long-standing associations between hosts and parasites and establish new ones. This creates natural experiments with which to study the ecology and evolution of host-parasite interactions. In estuaries of the Gulf of Mexico, the white-fingered mud crab (Rhithropanopeus harrisii) is infected by a native parasitic barnacle, Loxothylacus panopaei (Rhizocephala), which manipulates host physiology and behaviour. In the 1960s, L. panopaei was introduced to the Chesapeake Bay and has since expanded along the southeastern Atlantic coast, while host populations in the northeast have so far been spared. We use this system to test the host's transcriptomic response to parasitic infection and investigate how this response varies with the parasite's invasion history, comparing populations representing (i) long-term sympatry between host and parasite, (ii) new associations where the parasite has invaded during the last 60 years and (iii) naïve hosts without prior exposure. A comparison of parasitized and control crabs revealed a core response, with widespread downregulation of transcripts involved in immunity and moulting. The transcriptional response differed between hosts from the parasite's native range and where it is absent, consistent with previous observations of increased susceptibility in populations lacking exposure to the parasite. Crabs from the parasite's introduced range, where prevalence is highest, displayed the most dissimilar response, possibly reflecting immune priming. These results provide molecular evidence for parasitic manipulation of host phenotype and the role of gene regulation in mediating host-parasite interactions.

We used scanning electron microscopy (SEM) to describe cypris morphology in species of the barnacles Ibla and Paralepas, both of which are pivotal in understanding cirripede evolution. In Ibla, we also studied late naupliar stages with video and SEM. Special emphasis was put on the lattice organs, the antennules and the thorax and telson. In Paralepas we had settled specimens only and could therefore only investigate the carapace with the lattice organs. Cyprids of Ibla quadrivalvis and Paralepas dannevigi have five sets of lattice organs, grouped as two anterior and three posterior pairs. The organs are of the pore-field type and the terminal pore is situated anteriorly in the first pair, just as in the Rhizocephala and the Thoracica. In Ibla the armament of antennular sensilla resembles that found in the Thoracica but differs from the Rhizocephala. The absence of setules on the A and B setae sited terminally on the fourth antennular segment is a similarity with the Acrothoracica. The attachment disc is angled rather than facing distally and is encircled by a low cuticular velum. The thoracopods have two-segmented endopods and exopods as in the Thoracica, but the number, shape, and position of thoracopodal setae differ somewhat from other species of that superorder. Both Ibla and Paralepas cyprids have a deeply cleaved telson, but no independent abdominal part. In cypris morphology, Ibla and Paralepas show several synapomorphies with the clade comprising Rhizocephala and Thoracica and there are no specific apomorphies with either the Acrothoracica, the Rhizocephala or any particular subgroup within the Thoracica. This is in agreement with recent molecular evidence that Ibla (Ibliformes) is the sister taxon to all other Thoracica and the ibliforms therefore become the outgroup of choice for studying character evolution within the superorder. Paralepas, and other pedunculated barnacles without shell plates, are apparently not primitive but are secondarily evolved and nested within the Thoracica. J. Morphol., 2009. © 2008 Wiley-Liss, Inc.

vasa (vas)-related genes are members of the DEAD-box protein family and are expressed in the germ cells of many Metazoa. We cloned vasa-related genes (PpVLG, CpVLG) and other DEAD-box family related genes (PpDRH1, PpDRH2, CpDRH, AtDRHr) from the colonial parasitic rhizocephalan barnacle Polyascus polygenea, the non-colonial Clistosaccus paguri (Crustacea: Cirripedia: Rhizocephala), and the parasitic isopodan Athelgis takanoshimensis (Crustacea: Isopoda). The colonial Polyascus polygenea, a parasite of the coastal crabs Hemigrapsus sanguineus and Hemigrapsus longitarsis was used as a model object for further detailed investigations. Phylogenetic analysis suggested that PpVLG and CpVLG are closely related to vasa-like genes of other Arthropoda. The rest of the studied genes form their own separate branch on the phylogenetic tree and have a common ancestry with the p68 and PL10 subfamilies. We suppose this group may be a new subfamily of the DEAD-box RNA helicases that is specific for parasitic Crustacea. We found PpVLG and PpDRH1 expression products in stem cells from stolons and buds of internae, during asexual reproduction of colonial P. polygenea, and in germ cells from sexually reproducing externae, including male spermatogenic cells and female oogenic cells.

Hoeg, J.T. and Kolbasov G.A. 2002. Lattice organs in y-cyprids of the Facetotecta and their significance in the phylogeny of the Crustacea Thecostraca. — Acta Zoologica (Stockholm) 83 : 67‐79 Scanning and transmission electron microscopy (SEM and TEM) were used to study lattice organs in facetotectan y-cyprids from the White Sea and from Norwegian and Bahamian waters. The larvae represent at least four and possibly five different species of Facetotecta. Y-cyprids have five pairs of lattice organs in the head shield (carapace) organized into two anterior pairs and three posterior pairs. Both groups of lattice organs are arranged around a large central pore. The facetotectan lattice organs are elongate areas with a longitudinal keel, just as in the Ascothoracida and some Cirripedia Acrothoracica. The terminal pore of the organs is situated posteriorly in all five pairs. TEM confirms that the organs have the same general morphology as in the Cirripedia and Ascothoracida, namely, a cuticular chamber into which project ciliary segments from the chemosensory cells. Unlike Cirripedia the cuticular roof of the chamber lacks any pores. We conclude that five pairs of lattice organs represent an autapomorphy for the Thecostraca, which supports the monophyly of this taxon. In the ground pattern the terminal pore is posterior in all five pairs. The anterior position of the pore in lattice organ pair 2 is apomorphic for the Cirripedia, while within this taxon an anterior position also in pair 1 is apomorphic for a monophylum comprising the Thoracica and the Rhizocephala. Minute pores in the roof of the organs is another apomorphy of the Cirripedia, but its elaboration into pores visible with SEM may have been subject to some homoplasy. Since lattice organs are omnipresent in the settling instar of the Thecostraca they probably serve a critical role for the function of these cypris or cypris-like larvae.

Rhizocephalan barnacles are parasites of Crustacea. They lack even the rudiments of an alimentary canal, but infiltrate their hosts with a nutrient-absorbing system of rootlets. We review the ultrastructure of the rootlets using light microscopy, SEM, and TEM in nine species from five families, representing both suborders of the Rhizocephala: from the Kentrogonida Peltogaster paguri, P. curvatus, Peltogasterella sulcata, Cyphosaccus norvegicus (Peltogastridae); Lernaeodiscus porcellanae (Lernaeodiscidae); and Sacculina carcini (Sacculinidae); and from the Akentrogonida Clistosaccus paguri (Clistosaccidae); Chthamalophilus delagei, and Boschmaella japonica (Chthamalophilidae). With the exception of Chthamalophilus delagei, the root system of the investigated species shares numerous apomorphies at the ultrastructural level and displays at all levels specializations that maximize the surface area. The rootlets consist of a cuticle, an epidermis and a subjacent layer of axial cells that often, but not always surround, a central lumen. The rootlets are at all times enclosed in a less than 0.5 μm thick cuticle, which is never molted. The cuticle consists of an inner homogeneous layer with a slightly fibrous structure and an outer, less than 15-nm thick electron-dense layer, from which numerous microcuticular projections extend into the hemolymphatic space of the host. The microcuticular projections consist of the outer electron-dense layer and sometimes a core of the more translucent homogeneous layer. They vary among the species from being simple in Sacculina carcini to exhibiting complex branching patterns in Peltogasterella sulcata and Cyphosaccus norvegicus. Beneath the cuticle the epidermal plasma membrane is thrown into irregularly shaped projections. The epidermal cells are joined by long septate junctions and exhibit the characteristics of a transporting epithelium. Experiments with acid phosphatase revealed activity both in the epidermis and among the microcuticular projections. The projections may therefore form a domain that is important in absorption and extracellular digestion of nutrients from the host. The axial cells contain abundant endoplasmic reticulum and seem to convert absorbed carbohydrates into lipid, which is stored in large droplets. Subepidermal muscle cells cause sinuous movements of the rootlets, but it remains unknown how nutrients are transported along the rootlets towards the external reproductive body. In C. delagei the single, bladder-shaped rootlet lacks both the apical projections in the epidermis, the electron-dense cuticle layer, and the microcuticular projections. We review previous studies on the rhizocephalan root system and discuss functional and phylogenetic aspects of the morphology. J. Morphol. 249:9–42, 2001. © 2001 Wiley-Liss, Inc.

Rhizocephala is a group of crustaceans that exclusively parasitizes other crustaceans. It is taxonomically placed within the class Cirripedia, the barnacles, with which it shares a unique larval type, the cyprid. The main objective of the cyprid is to find and irreversibly attach to a suitable substratum and initiate metamorphosis. In the presumed sister group to Rhizocephala, the true barnacles or Thoracica, metamorphosis leads to a juvenile filter-feeding version of the adult organism. In Rhizocephala the female cyprid settles on the integument of a crustacean and undergoes metamorphosis into a kentrogon that possesses a hollow cuticular-tube structure, the stylet, which penetrates the integument of the host and acts as a guide tube for the prospective internal parasite. The first, hitherto unknown endoparasitic stage of a rhizocephalan, the vermigon, was recently discovered (Glenner and Hoeg [1995] Nature 377:147–150) and its migration through the hemolymph of the host, as well as its internal development, was described in Glenner et al. ([2000] Mar Biol 136:249–257). The present article provides detailed information on kentrogon and vermigon formation, the injection process, and the succeeding developmental stages up to the stage of the earliest primordium reported from the literature. The anlage of the ovary is traced back to the free-swimming cypris stage and it is implied that the mesoderm and ectoderm of the endoparasite are already differentiated in the cyprid. J. Morphol. 249:43–75, 2001. © 2001 Wiley-Liss, Inc.

Knowledge about the Rhizocephala (parasitic barnacles) has increased exponentially over the past two decades. This introduction broadly reviews this progress, touching on rhizocephalan morphology, life-cycles, larval biology, and the effects on the crustacean hosts. Members representing both orders, the Kentrogonida and Akentrogonida, are discussed. The recent discovery regarding the vermigon stage further highlights the intricacy of the kentrogonid life-cycle. Because rhizocephalans are found in most world regions, occurring on their respective hosts from the deep ocean to freshwater, their importance is now being recognized.

SEM investigation of laboratory-reared cyprids of the parasitic barnacle Heterosaccus lunatus has revealed morphological differences between the male and female larvae, particularly the attachment organ of their antennules. The detailed features of the attachment organ (segment III) and segment IV for each sex are described and compared with those of other rhizocephalan cyprids, particularly Sacculina carcini, a reportedly closely related species. The absence of the large posterior sac from segment III in the H. lunatus male highlights the need for careful larval investigations on other sacculinid species. Settlement of female H. lunatus cyprids occurred on the gill lamellae of the host crab with the subsequent kentrogon stage taking the sacculinid form. The inclusion of H. lunatus in the Sacculinidae was therefore confirmed, based on the morphology of the cypris antennules and kentrogon.

The morphology and biology of Polysaccus japonicus Hoeg & Lutzen, 1993 is described. Nearly all infected hosts are adults, the females of which become castrated. Each bears 4–50 (female) externae on the underside of the abdomen. The externae are interconnected via a root system within the host's perineural blood sinus. The externae pass through three moults. Cypris larvae inject spermatogonia into the immature externa's mantle cavity, which is where spermiogenesis occurs later. Multiplication of the male cells is probably supported by secretion from paired mantle cavity glands, presumably homologous to the receptacles of kentrogonid rhizocephalans. Late during the 2nd intermoult the visceral mass ruptures to release the ova into the mantle cavity where they become fertilized. The third moult results in opening of the mantle aperture which initiates a period of vigorous peristalsis of the muscular mantle. Following emission of the cypris larvae the externae perish leaving distinct scars. The root system always contains buds of various sizes and stage of development. When the scarred host moults, the most advanced buds emerge to give rise to a new generation of externae. This is thought to be repeated for the rest of the host's life. The antennule of the cyprid has a relatively very long and slender 3rd segment, a unique construction among Akentrogonida. Because of this and the presence of special mantle cavity glands, a new family, Polysaccidae, is erected, which in some characters is intermediate between the Kentrogonida and the Akentrogonida.

Loxothy/acus ihlei, a rhizocephalan parasite of the mud crab, Scylla sermla (Fors kill) , is reported for the first time in Australian waters. The parasite is described and found to be variable in a number of characters. Most infected S. serra/a. collected from commercial catches in the Northern Territory, Australia, were significantly smaller than uninfected crabs, and their abdominal flap s were relatively larger than those of mature females. Annual prevalence of infec tion was low (2·1 %), with the monthly variation ranging from negligible early in the year to around 7% in November. Fisheries for the mud crab, Scylla serrata (Forskal), have substantial commercial importance throughout the Indo-West Pacific region, with annual catches of around 23000 t in 1992 (FAO 1994). Loxothylacus ihlei was first described by Boschma (1949) as a parasite of S. serrata in Indonesian waters. He gave no detai Is on its effects on S. serrata, and despite the host's extensive distribution and substantial commercial importance, the parasite has received no further study. In this paper, we report L. ihlei as a parasite of S. serrata in Australian waters for the first time. Its influence on the morphometry of the host and its prevalence in commercial catches "from the Northern Territory is reported. Materials and methods Infected S. serrata were cO.llected as part of a monitoring programme on the commercial mud crab fishery in the Northern Territory, Australia. Commercial catches were sampled over a 2-3-day period at monthly intervals between June 1990 and December 1993. During these sampling periods. fisherman were re­ quested to keep their entire catch. including undersized and early post-moult crabs. Sex, carapace width (CW: width including posterolateral spines), total weight (TW: whole live weight, excluding crabs with missing chelae), moult stage and physical damage were recorded from each crab. The sixth abdominal segment width (SASW: widest part of the sixth abdominal segment. excluding setae) was measured for all females. a sub-sample of males. and most parasitized crabs, since parasitism causes broadening of the abdominal flap (Weng 1987; Hochberg. Bert. Steele & Brown 1992). The pres­ ence of externae (Fig.) A). the scar from loss of the externae or distinctive morphological changes caused by parasitism were used as an indication of infected crabs, whilst all other crabs were considered as uninfected. Dissection for intemae was not undertaken.

A morphological study was carried out on the fournaupliar stages of Sacculina carciniusing mainly scanning electron microscopy. Frontal horns were present and throughout development the typical nauplius limbs remained simple and gnathobases were lacking. Such features are characteristic of other lecithotrophic barnacle nauplii. The presence of a vestigial ventral thoracic process was evident on the stage III nauplius and was even more prominent on the stage IV nauplius. These observations confirm that the rhizocephalan nauplius is close to the thoracican nauplius form and lend strong support for the retention of the Rhizocephala within the Cirripedia.

Putative sensory structures, called lattice organs, were studied with scanning electron microscopy from ascothoracid or cypris larvae representing a wide range of families of the Ascothoracida and Cirripedia. These organs, situated dorsally on the carapace were, with few exceptions, always found in two anterior and three posterior pairs. The lattice organ morphology displayed by the Ascothoracida, a seta–like structure with a terminal pore, is believed to be the most plesiomorphic condition. Within the Cirripedia lattice organ morphology varied from types resembling the Ascothoracida in the Acrothoracica and the lepadomorph Capitulum mitella, to an elongate pore field with a larger terminal pore in most Thoracica and Rhizocephala. Akentrogonid Rhizocephala seem to display the most apomorphic condition. While lattice organ morphology was generally constant at the family level, cases were seen where closely related species such as Chthamalus stellatusand Chthamalus montaguishowed minor, but clear cut differences. Lattice organs in 2 + 3 pairs are argued to represent a synapomorphy for the Ascothoracida and the Cirripedia. The results confirm that the cyprid morphology at the ultrastructural level will prove to be of high value in estimating phylogeny within the Cirripedia.

The Rhizocephala are considered to be monophyletic due to several synapomorphies in the ontogeny of the cndoparasitic phase. The various types of metamorphosis described in the Rhizocephala are discussed and compared to metamorphosis in the Cirripedia Thoracica and Acrothoracica. In males and females of the suborder Kentrogonida. the cyprid settles and metamorphoses into a new instar, in males the trichogen and in females the infective kentrogon. The kentrogon goes through yet another. incomplete moult associated with the development of the stylet. Within the three kentrogonidan families. the Iernaeodiscid-peltogastrid type of kentrogon differs from the sacculinid type in the mode of attachment to the host. in the complexity of internal anatomy. in the position and penetration of the stylet, and in whether or not the cyprid carapace must be shed prior to penetration of the stylet. In the Akentrogonida metamorphosis never results in a new instar. Where observed (Clistosaccidae and Thompsoniidae). both male and female cyprids settle and penetrate into their substrate (female parasite or new host) with one of the antennules. Using the antennule as a syringe. male cyprids inject spermatogonia while female cyprids injects embryonic cells developing into an endoparasite. By comparison with metamorphosis in the Cirripedia Thoracica and Acrothoracica it is concluded that the presence of a metamorphic moult leading to a post-cyprid instar is plesiomorphic and that the trichogon and kentrogon are homologous with the first metamorphosed juvenile in these outgroups. The abbreviated ontogeny in the Akentrogonida without metamorphic moult and post-cyprid larval instars is considered apomorphic. This contradicts the long-held supposition that the Akentrogonida are the most‘primitive’Rhizocephala and dovetails with new information that this suborder contains many advanced traits. Within the Kentrogonida. the lernacodiseid-peltogastrid type of kentrogon is considered more plesiomorphic than the sacculinid type, which resembles the clistosaccidthompsoniid type in having the antennules involved in the penetration process. The homologization of the kentrogon with a juvenile barnacle indicates that presence of a kentrogon is plesiomorphic within the Rhizocephala and that the Kentrogonida is paraphyletic.

Akentrogonid rhizocephalans morphologically resembling the genus Thompsonia are revised as a result of examination of new material. The species concerned are all obligatorily colonial and have ovoid or cylindrically shaped externae with a terminal stalk and a much reduced anatomy. A numerical cladistic analysis of all Rhizocephala Akentrogonida using the Hennig 86 program leads to a redefinition of the Thompsoniidae HOeg and Rybakov, 1992. Autapomorphies for the Thompsoniidae are primarily the morphology of the attachment to the host and the total absence of a mesentery. The cladistic analysis refutes that the Thompsoniidae should have a plesiomorphic morphology and branch off very low on the rhizocephalan phylogeny. The family now comprises four genera: Pottsia gen. n. (monotypic), Diplothylacus gen. n. with two species, Thompsonia Kossmann with five species. A revived and redefined Thylacoplethus Coutiere includes eight species. The genera are distinguished by the location of the spermatogenic tissue, the site where the eggs are fertilized, the presence or absence of a mantle pore and the way it is formed, the number or absence of oviducts, and the number of cuticular annuli on the stalk. All 16 species, of which six are new to science, are described when necessary, and, if possible, illustrated. A phylogeny for the redefined family is proposed. Thylacoplethus is morphologically closest to the hypothetically ancestral thompsoniid and is likely paraphyletic. The new genus Polysaccus with two species, one of them new to science, and the monotypic genus Pirusaccus Lutzen resemble thompsoniids in externa morphology and in being obligatorily colonial.

Incorporation of the Rhizocephala in the Cirripedia, reflecting the traditional view that these parasites evolved from a setose feeding barnacle, has recently been challenged in favour of rhizocephalans being the sister group to all other Thecostraca or a scenario where they evolved from a free-living, ‘precirripede’ ancestor. Adult morphology is useless in discussing the monophyly of the Cirripedia, since rhizocephalan adults are too reduced to furnish any phylogenetic evidence. But numerous, detailed similarities in nauplii and cyprids of the Thoracica, Acrothoracica and Rhizocephala as well as the ultrastructure of their sperm are synapomorphic relative to other Thecostraca and indicate that these three orders form a monophylum. There is evidence that the stylet in the rhizocephalan kentrogon is homologous to an element in the ancestral mouth field. If so, the Rhizocephala probably evolved before setose feeding was adopted, and constitute the sister group to the Acrothoracica and Thoracica. This conclusion is based on frail evidence so the term Cirripedia should be retained to comprise the Rhizocephala, Thoracica, and Acrothoracica. These three orders all possess remarkably similar cyprids, adapted to accomplish irreversible settlement by cement secretion and initiate metamorphosis, so their last common ancestor was most probably a permanently sessile organism.

Interna development within the intertidal tropical crab, Leptodius exaratus, requires up to 4.5 months; the roots become distributed from the CNS along the major nerves and, when the crab moults, results in emergence of numerous externae located symmetrically on the abdomen and bases of the thoracic appendages. As many externae drop off, their number is continuously reduced; the survivors reach sexual maturity and ovulate after 2.5 months and complete embryonic development after 2 more months. At the next moult a new and more numerous generation of externae appears, and this is repeated until the crab dies. Presence of externae significantly prolongs the host's normal intermoult period and minimizes its growth. The parasite has little effect on the host's sexual characters although females are normally sterilized. The cuticle of the externa is moulted once, leaving a distinct annulus around the stalk. Spermatogonia are presumably injected by male cyprids into the female externa and develop into sperm within one or more vesicular spermatogenic bodies in its mantle. The eggs are laid through a single oviduct into the mantle cavity where they become fertilized and develop into cyprids that are liberated at the disintegration of the externa.

Boschmaella japonica infests the barnacles Chthamalus challengeri and Balanus amphitrite amphitrite. Its morphology is very similar to the previously known members of the family Chthamalophilidae (Rhizocephala, Akentrogonida). Like Boschmaella balani and Bocquelia rosea it has an internal root system within the host. Transmission electron microscopy revealed that the ‘incubatory cavity’ in previous descriptions of the Chthamalophilidae is lined by a thin cuticle, and it is therefore homologous to the mantle cavity found in other Rhizocephala. The development of the male structure resembled events in Chthamalophilus delagei. It is evaginated from the mantle into the mantle cavity as a free-floating, epithelium-lined capsule, ‘the spermatogenic island’, which later fragments into numerous secondary spermatogenic islands, where spermatogenesis takes place. Based on new morphological evidence and a comparison with other rhizocephalans, it is suggested that all species of the Chthamalophilidae are gonochorists, in which male cyprids implant spermatogonia directly into the mantle of female externae. The larvae of Boschmaella japonica hatch as cyprids which, like those of other chthamalophilids, lack a thorax and thoracic cirri. This larval feature is thought to be an important factor limiting dispersal of these species. The incidence of multiple-infested hosts is very high in Boschmaella japonica.

A new rhizocephalan parasite which infests the barnacles Chthamalus challengeri and Balanus amphitrite amphitrite is described. It is known only from the type locality, Aburatsubo, Kanagawa Prefecture, Japan. Its position in the family Chthamalophilidae is assured by the cyprids lacking a thorax, by the position of the mesenteric canal opposite to the stalk, and by its host being a balanomorph barnacle. Among the three previously described species of the Chthamalophilidae, the new species most closely resembles Boschmaella balani, but it differs in details of the internal root system and in the externa being found both on the host body and on the inner mantle. About two-thirds of the infested hosts carry more than one externa.

Sacculina granifera was found in 12% of commercial sandcrabs, Portunus pelagicus, in Moreton Bay irrespective of sex. Although male and female crabs were randomly distributed about Moreton Bay, egg bearing females were most common at the seaward station. This and the distribution of epizoic barnacles on gills and carapace suggest the sexes school separately. The increased prevalence of barnacles on the carapace of infected crabs indicates Sacculina inhibits moulting. The distribution of interna infections in small crabs towards south and west and externa infections in large crabs in the easterly, seaward stations suggests Sacculina preferentially attacks young crabs as they move inshore and then induces crabs to behave like ‘berried’ (i.e. egg bearing) females by moving seaward as they grow. Morphologically female crabs are little changed by Sacculina, but the males show considerable modification which is reflected most accurately in the shortening of the chelar propodus to proportions similar to normal females. Infected crabs are sterile and internally the hepatopancreas becomes green rather than tan. Parasitized crabs were seen to groom their externae as ‘berried’ females groom their egg masses.

Cypris morphology in the Rhizocephala differs both between the sexes of individual species and between the species. The large aesthetasc on the third antennular segment is unique to male cyprids of the Lernaeodiscidae, Peltogasteridae and Sacculinidae (suborder Kentrogonida) and Mycetomorpha vancouverensis (suborder Akentrogonida). A fourth segmental aesthetasc, common to both sexes, is much longer in males than in females. A spinous process on the attachment disc is another male-specific character, but is absent altogether in the Lernaeodiscidae. It is argued that these aesthetascs help locate the settlement targets. Except in Mycetomorpha vancourverensis, aesthetascs have been secondarily lost in cyprids of the suborder Akentrogonida, and there are no other morphological means for separating the male and female settling stage. In cyprids of Sylon hippolytes (Sylonidae) and of Mycetomorpha vancouverensis and Thompsonia sp. (Akentrogonidae) the long and slender antennules are probably used in penetration without formation of a kentrogon, as already reported for Clistosaccus paguri (Clistosaccidae). Cyprids of Sylon hippolytes and Clistosaccus paguri are almost morphologically identical.

The anatomy of the female cypris is very similar to that of cyprids of the Thoracica. There is no development of a preformed kentrogon within the unsettled cypris. Kentrogon formation is completed within 10–20 min after settlement. Elimination of the spent cypris is necessary for successful development. The kentrogon includes a variety of tissues, such as muscles and part of the cement glands. The injection stylet develops in connection with a moult, and it penetrates into the crab gill through the ventral body wall of the kentrogon c. 50 h after settlement. Only a single cell, also recognizable in the unsettled cypris, is injected into the host, while the rest of the kentrogon degenerates after invasion. The speed of kentrogen development and much of the kentrogon anatomy can be interpreted as a defense against host crab grooming. The single invasion cell may either be considered as a stem cell or an egg.

The behaviour of crabs Portunus pelagicus infected with the rhizocephalan barnacle Sacculina granifera is examined and compared with that of normal uninfected crabs. With regard to alarm reactions, feeding and locomotion, the infected crabs behave as normal crabs. Their stance, defaecation and burying behaviour differ in that they must accommodate to the physical presence of a large body, the parasite, suspended between cephalothorax and abdomen. General body care is increased as a result of the increased epizoic fauna on the exoskeleton which develops when moulting ceases. Infected crabs with an externa resemble ovigerous females in terms of their moderate daylight activity and particularly the grooming care and attention given to the sac—a surrogate egg mass. Parasitized crabs excavate a depression and using the third walking legs (and often second and first as well) attend the externa as an ovigerous female attends her egg mass. The grooming of the externa is of equal periodicity to egg mass grooming behaviour, but is less prolonged at maximum intensity. Sac grooming behaviour begins 18 days after emergence of an externa and declines progressively if the externa is removed and the interna degenerates. In interactions with other crabs parasitized individuals initiate fewer and dominate fewer encounters than normal crabs of similar size and sex; and parasitized males, at least, are less active when with other crabs than when alone. It is concluded the parasite secretes a hormonal mimic which induces ovigerous behaviour which maximizes the survival of the parasite population.