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The male larvae of many parasitic barnacles are planktonic and are seasonally released. To achieve reproductive success, a male must be accepted by a receptive female that has successfully infected a host. To understand the seasonality of the breeding biology of parasitic barnacles, we developed an evolutionary game theoretical model for the seasonal pattern in the production of male larvae. Assumptions are that female parasitic barnacles become receptive following a given seasonal pattern. The parental females (mothers) choose the timing of producing their own male larvae to achieve maximum reproductive success. In the evolutionarily stable seasonal pattern, the production of male larvae often shows a sharp peak on a single day, indicating strongly synchronized production of male larvae, even when the supply of receptive females is distributed over the breeding season. When the total number of male larvae is large, the evolutionarily stable male production pattern may include multiple peaks, but it never shows a continuous distribution. This is very different from the game model previously developed for the emergence pattern of butterflies, where evolutionarily stable male emergence is always continuously distributed over a fraction of the mating season. As planktonic larvae, male parasitic barnacles have a naturally limited ability to find receptive females, and females may stay receptive for many days, whereas in butterflies, newly emerged females are mated within a day of their emergence.

Parasitism by rhizocephalans can cause morphological, physiological, and behavioral changes in their host crabs, thus potentially also affecting the structure and function of host populations in the benthic ecosystem. The dynamic variation in crab populations caused by rhizocephalans may affect the colonization of epibionts on the crabs, but there is little literature on host/parasite/epibiont relationships. This study investigated the patterns of colonization by the thecostracan barnacle Octolasmis on crabs parasitized by the rhizocephalan externae of Diplothylacus sinensis, in order to quantify the interaction of the epibionts and rhizocephalan. Two species of epibiont barnacles, Octolasmis bullata and O. wanwickii, occurred on the host crab species Portunus sanguinolentus: O. bullata was found in the branchial chambers of all rhizocephalan-parasitized crabs, with an intensity of 74.6 +/- 39.1 (mean +/- SD) barnacles per host; O. warwickii was found attached to the outer surfaces of 34.6% of parasitized crabs, with an intensity of 49.3 +/- 32.3 barnacles per host. Their patterns of colonization, including Octolasmis prevalence, intensity, and distribution, were not similar to those crabs unparasitized with the rhizocephalan externae in previous reports. Differences between the two groups of crabs were closely related to the parasitism by rhizocephalan externae of D. sinensis. Because the evidence in this study suggests that the parasite had arrested or reduced a series of host behaviors, such as molting, grooming, and burying, to protect the parasitization of rhizocephalan externae, although these behavioral patterns are natural antifouling mechanisms for the epibionts. As a result, Octolasmis could benefit from the behavioral changes of hosts in the rhizocephalan-crab association. Whereas, massive colonization of Octolasmis on rhizocephalan-parasitized crabs, especially long-term parasitized hosts, could pose a severe burden and lead to an increase in host mortality.

Rhizocephalans of the order Akentrogonida are parasitic barnacles that infest decapods, stomatopods, peracarids, and other cirripedes. Within this order, Thompsoniidae is found in decapods and stomatopods, and is a family comprised of species characterized by having multiple globular reproductive bodies (externae) erupting from the host’s extremities and abdominal surface, and connected internally by a root-like system. Two of the four genera currently accepted within Thompsoniidae, Thompsonia and Thylacoplethus, contain perhaps the most morphologically derived species within Akentrogonida. Herein we describe two new species of Thylacoplethus, T. porcellanus and T. umanguvatus, from the porcellanid and hermit crabs Petrolisthes scabriculus (Dana, 1852) and Calcinus morgani Rahayu and Forest, 1999, respectively. We also redescribe Thompsonia japonica Hafele, 1911 from a xanthid brachyuran crab based on morphological characters and DNA sequences of the nuclear 18S rDNA gene. The new species from the porcellanid crab comprises the first record of an akentrogonid rhizocephalan parasitizing a species of Porcellanidae. We reconstruct a molecular phylogeny of Akentrogonida using new and published DNA sequences, and we discuss our findings within an evolutionary context of host speciation. All species in the four thompsoniid genera are briefly reviewed, and a replacement name is proposed for the preoccupied Pottsia Hoeg and Lutzen, 1993.

A thompsoniid rhizocephalan parasite, Diplothylacus sinensis (Keppen, 1877), was discovered in the semi-open Honghai Bay in the South China Sea, occurring in 26.4% of 1852 (934 males and 918 females) Portunus sanguinolentus (Herbst, 1783) (Decapoda: Portunidae) collected monthly between November 2012, and October 2013. Of the sampled crabs, 243 males and 246 females were infected with externae of D. sinensis or bore attachment scars on the body surface, giving an overall prevalence of infection of 26.0% and 26.8% in male and female crabs, respectively. The infection rate changed monthly with the highest prevalence in December (50.6%) and the lowest in June (1.3%). New externae filled with eggs or embryos were observed throughout the year, which indicates a year-round reproductive cycle, but virgin externae were more commonly present in late spring or early summer. Carapace width and wet weight of infected crabs of both sexes were significantly smaller than those of uninfected crabs. Compared to normal crabs, the pleon of infected females was significantly wider and the first pleopod of parasitized males was shorter. On addition, crabs infected by the rhizocephalan parasite were more susceptible to other epibionts, which included Octolasmis warwickii Gray, 1825 (10.1%), Chelonibia patula (Ranzani, 1818) (6.8%), Obelia spp. (7.2%) and Membranipora membranacea (6.5%) on the carapace surface, and Octolasmis bullata (Aurivillius, 1894) (64.7%) on the gills.

Of 2282 individuals in 23 hermit crab species collected from four coasts of the southern part of mainland Japan along the Pacific coast, 87 individuals in seven species were infested by six rhizocephalan species. Among the ten combinations between hosts and rhizocephalans, seven combinations were new records. Morphological identification of the rhizocephalan species from different host species and/or sites was consistent with molecular identification using DNA sequences (COI and 16S rRNA genes). Pagurus filholi and P. nigrivittatus were infested by both Peltogaster postica and P. lineata. The host preference of the two rhizocephalan species reciprocally shifted from one species to the other among the four collection sites. Rhizocephalans may choose the host inhabiting the preferred environment for the parasite within the range of their host species.

We studied spatial variation in the prevalence of the rhizocephalan barnacle Lernaeodiscus porcellanae on its porcelain crab host, Petrolisthes cabrilloi, at four southern California intertidal sites separated by only a few km. The prevalence of rhizocephalan externae varied significantly among sites in 2008-2009, with the southernmost site, White Point, consistently showing higher prevalence than the others. Externa prevalence was a good proxy of estimated true prevalence, i.e., the prevalence of rhizocephalans as a whole, not just those that had formed externae. We examined several hypotheses that might explain the observed spatial variation in prevalence. Host susceptibility to infection (indicated by the proxy of damage to host limbs, some of which are used to remove parasite infective stages), did not differ among sites. At all sites, prevalence was slightly higher in female crabs than in males, and the sex ratio at White Point was slightly female-biased while that at the other sites was mal...

A sacculinid rhizocephalan parasite was recently rediscovered in New Zealand after original recordings from collections in 1931. This new species of Heterosaccus occurred in 93.5% of 1118 Metacarcinus novaezelandiae (Decapoda: Cancridae) collected from Wellington Harbour between May 2006, and March 2007. Of the parasitized crabs, 22.1% had a rhizocephalan externa, 62.7% had a scar, and 15.2% had only the interna. Prevalence of Heterosaccus sp. was similar for male and female crabs and all were castrated. Many of the infected males displayed major morphological changes, which included a wider pleon and extra pairs of biramous pleopods compared to uninfected males (feminisation). Newly emergent externa and externa filled with eggs or embryos were found throughout the year, which indicates a year-round reproductive cycle. However, virgin externae were more common at the beginning of the southern hemisphere summer. A literature survey of Rhizocephala in the South Pacific revealed overall few describe...

We describe techniques that enable the preservation of tissues from the rhizocephalan barnacle, Loxothylacus texanus, inside the body cavities of blue crab hosts, Callinectes sapidus, in a manner that minimizes the degradative activities of hepatopancreatic enzymes. These procedures allow the extraction and amplification of both parasite and host 18S rDNA within the same sample and enable one to distinguish between parasitized and unparasitized crab tissue in as little as two weeks after infection, well before any external manifestations of the parasites. Two PCR-based approaches were taken to identify the presence of L. texanus. In the first approach, a set of primers specific for L. texanus was used to specifically amplify 18S sequence in a background of C. sapidus DNA or the DNA of other barnacle species. In the second approach, a set of general primers was used to amplify 18S sequence from C. sapidus and a variety of barnacle species. The products of this PCR were then digested with an enzyme that recognizes a restriction site present only in the L. texanus PCR product to yield a unique pattern of fragments. With these techniques, we could detect as few as five parasitic cypris larvae in water samples, as well as L. texanus in the tissue of a small crab collected from the field and in the four anterior periopods of a crab bearing the external stage of the parasite. In experiments with potential hosts of varying sizes and molt stages, we confirmed that the parasite was significantly more effective in infecting crabs less than 30 mm carapace width than larger individuals. These techniques will facilitate future investigations of ecological and physiological interactions between these important crustacean parasites and their hosts and will help to determine the economic impact of this parasite on blue crab fisheries.

Parasitization by sacculinids (Cirripedia: Rhizocephala: Sacculinidae) induces severe modifications in morphology, behavior, and reproduction of their host crabs. To understand the mechanisms involved, it is important to have comprehensive information on their association. However, such information has been poorly available, mainly because of scarcity of distinguishing characters of sacculinids hamper identification of them. In the course of the investigation of the associations using the molecular marker (COI gene) and morphological observation to identify sacculinids, three species of Sacculina, S. confragosa, S. imberbis, and S. yatsui, were found to parasitize a single host crab, Pachygrapsus crassipes, sympatrically in a restricted area. This is the first finding for three-on-one association in a single locality.

Sacculina sinensis (Cirripedia: Rhizocephala) is a parasitic barnacle first reported in Hong Kong by Boschma (1933) to infect the intertidal crab, Leptodius exaratus. Since then, no studies have been conducted to investigate its distribution and morphology, which are important in providing background information for further taxonomic and ecological studies. In the present study, S. sinensis has shown to have a limited distribution in Hong Kong, being confined to only four out of the nine sheltered boulder shores sampled and with a mean infestation rate of 11.3 percent. The localized distribution of S. sinensis in Hong Kong is probably due to its short larval development times (four naupliar stages and one cypris stage, completed in about six days) and hence limited larval dispersal distance. The externa of S. sinensis is ovoid in shape with two distinct shoulders, and the outer cuticle surface is spinous. From histological studies, the male receptacles are located posteriorly in the visceral mass...

The Rhizocephala is a group of extremely reduced parasitic crustaceans, that exclusively parasitize other Crustacea. In the family Sacculinidae, the external sac-like part (externa) of the adult parasite contains the reproductive apparatus and is attached beneath the abdomen of the host crab. Hosts with more than one externa may occur and are in most cases believed to have arisen from multiple cyprid larvae. However, in three species of the genus Sacculina, multiple externae have been shown to originate by asexual reproduction from a single parasitic cypris larva. We present a phylogenetic analysis of ten species of Sacculina and outgroups based on partial sequences from the cytochrome oxidase 1 (CO1) and the entire 18s rDNA gene. A separate parsimony analysis from the 18s rDNA and CO1 genes resulted in two trees with almost identical topologies. Both genes strongly support a monophyletic, asexually reproducing clade and fail to support a monophyletic Sacculina genus. As a consequence we have est...

We used nucleotide sequence data from the ribosomal RNA (rRNA) gene cluster to determine the affinities of rhizocephalans reported to be Sacculina carcini which parasitize different portunid hosts: the green crab Carcinus maenas from Sweden; C. maenas from England; Liocarcinus marmoreus from Ireland; and Liocarcinus holsatus from Wales. There were no differences in nucleotide sequence data from the 3′ region of the small subunit (SSU) (160 bp) and internal transcribed spacer 1 (ITS1) (274 bp) rRNA of these parasites. The same region was sequenced from Sacculina granifera from Queensland, Australia, parasitic on Portunus pelagicus; Sacculina oblonga from Kyushu, Japan, parasitic on Cyclograpsus intermedius; and an undescribed rhizocephalan from Victoria, Australia, parasitic on Nectocarcinus integrifrons. The consensus nucleotide sequence of S. carcini from the ITS1 rRNA differed from S. granifera by 52% (146/282 sites), from S. oblonga by 56% (162/289 sites) and from the rhizocephalan from N. int...

A new species of rhizocephalan. Succulina oblonga, is described and illustrated on the basis of material collected from Amakusa Island, Kyushu, Japan. It parasitizes the littoral crab Cyclograpsus intermodius. It could not be determined whether earlier records of sacculinid species from Japanese C. intermedius belong to the same species.

At Nha Trang, Vietnam, two species of colonial rhizocephalans of the family Thompsoniidae parasitize three species of Stomatopoda. Thylacoplethus squillae Hoeg and Lutzen is already known from Japan. Pottsia serenei, new species, may be differentiated from Pottsia plesionikae by its smaller size, relatively longer stalk, much larger eggs, and the presence of compound eyes in the cypris larvae. A third colonial species, Diplothylacus sinensis, is recorded from Portunus sanguinolentusat three localities in Vietnam

Sacculina polygenea is a parasite of the intertidal crab Hemigrapsus sanguineus in Kyushu, Japan. Populations from 2 localities differing widely in prevalence (1.1-1.7% and 41.6-79.6%) were compared. When the parasite externae perish, they are replaced by new ones in almost every crab observed irrespective of locality. The abdomen of 29 parasitized crabs from both localities contained numerous internal tumors (primordial parasites). Their number varied between 10 and 92 per crab in the low-prevalence locality and 20 and 71 in the high-prevalence locality. We conclude that the tumors (and externae) of each crab arise by asexual reproduction through budding from the root system of the parasite. The parasites generally become external in crabs 1.5-2 years old. We estimate that the externae die and are replaced 1-3 times during the life of a crab, the host molting and growing between replacements. Concomitantly, the average number and size of the externae increase at each instar. Sacculina polygenea offers the first proven case of asexual reproduction in the family Sacculinidae.

Heterosaccus lunatus is a rhizocephalan barnacle which parasitizes the crab Charybdis callianassa in Moreton Bay, Queensland. Its free-swimming larvae are lecithotrophic and there are 4 naupliar stages before metamorphosis to the cypris stage. Sexes can be distinguished during this larval phase by size, the male larvae being larger at all stages. Experiments determined the effect of differing salinities on larval development of H. lunatus. A development index (D.I.) was calculated for each salinity tested and larval development was arbitrarily deemed successful if the D.I. was greater than 50%, i.e., > 50% of the nauplii became cyprids. Successful larval development occurred over the salinity range 24-40‰.

Sequence data from PCR-amplified 18S rDNA were used to infer phylogenetic relationships among the thecostracan crustacean taxa Ascothoracida (Ulophysema oeresundense), Acrothoracica (Berndtia purpurea, Trypetesa lampas), Rhizocephala (Loxothylacus texanus), and Thoracica (Calantica villosa, Lepas anatifera, Octolasmis lowei, Chthamalus fragilis, Chelonibia patula, Tetraclita stalactifera, Balanus eburneus). Sequence-divergence estimates ranged from 1.8% for 2 acrothoracican taxa to 24.3% for an ascothoracidan and a balanomorph thoracican. Parsimony, invariants, and neighbor-joining analyses all showed the Ascothoracida and Acrothoracica to be sister taxa and very early thecostracan lineages widely separated from the Rhizocephala, which in turn is the sister group to the Thoracica. A comparison of the secondary structure of the V3 variable domain in the 18S rRNA molecule for these taxa supports this result somewhat: the acrothoracican V3 structure is phenetically more similar to that of the ascothoracidan and branchiopods than to the other cirripedes. There is some disagreement between a morphology-based phylogeny and the current one based on molecular characters. It may result from molecular evidence suggesting an early origin and divergence of the Acrothoracica from the main cirripede lineage, as well as a substitution rate in the lineage leading to the Rhizocephala + Thoracica that is very much higher (10-12 times) than those in the other thecostracan lineages examined. Within the Thoracica, the sessile balanomorphs appear to be derived from a lineage related to scalpelloid lepadomorphs, but variation in the 18S molecule among balanomorphs is insufficient to permit inference of relationships within this group. We conclude that the Thecostraca, represented here by the subclasses Ascothoracida and Cirripedia (the latter including the Acrothoracica, Rhizocephala, and Thoracica), is a monophyletic taxon.

The distribution of the internal root system, the morphology of several external stages, and the cypris larva are described for the rhizocephalan Thompsonia reinhardi, new species. The morphology and various events during the life cycle are compared to other species of Thompsonia.

A revision of the suborder Akentrogonida Hafele (Cirripedia: Rhizocephala) is proposed, based upon characters concerning ontogeny, larval morphology, and metamorphosis. The original diagnosis of the suborder is amended to include several genera, previously considered to be incertae sedis. The family Sylonidae Boschma is reduced to a synonym of Clistosaccidae Boschma. The diagnosis of the family Chthamalophilidae Bocquet-Vedrine is amended to include the genera Boschmaella Bocquet-Vedrine and Bocquetia Pawlik. The new families erected are: Duplorbidae, new family, for the genera Duplorbis Smith, Cryptogaster Bocquet-Vedrine and Bourdon, and Arcturosaccus Rybakov and Hoeg; Mycetomorphidae, new family, for the genus Mycetomorpha Potts, and Thompsoniidae, new family, for the genus Thompsonia Hafele. The genus Pirusaccus Lutzen is still considered as incertae sedis, though its possible affinity to Thompsoniidae is emphasized. A list of all species of the Rhizocephala Akentrogonida is given with notes on hosts and geographical distribution. A key allows identification of all the families of the Rhizocephala.

Externae (reproductive structures) of Loxothylacus texanus were found on 10.3% of 1,521 blue crabs (Callinectes sapidus) examined in Galveston Bay, May through July 1989. Infections were found in areas of the bay where annual salinities average between 15 and 30 ppt. No externae were found on 577 crabs caught in intertidal areas. Of 944 crabs collected in subtidal habitats, 156 (16.5%) bore the parasite's extemae. In a separate analysis of 284 externae collected from 237 host crabs, the number of externae per crab ranged from 1―4 (mean = 1.2). The volume of externae (in cm3) ranged from 0.5-5.8 (mean = 3.3). The carapace width (in mm) of host crabs ranged from 49-100 mm (mean = 68.9). The developmental sequence of externae as indicated by external coloration is: white, tan, yellow, brown, salmon, and purple.

Since 1993, Heterosaccus dollfusi has been a very common parasite on the crab Charybdis longicollis along the Mediterranean coast of Israel. It accompanied from the Red Sea this Lessepsian migrant crab. The parasite is more common on male than female crabs, where it becomes external on hosts of many sizes shortly after a molt and undergoes characteristic growth stages. The parasite usually causes the complete loss of pleopods in both sexes and a feminine broadening of the abdomen in male crabs. Parasite survival is the same in both sexes of the host. More than one parasite per host is frequent, each probably originating from an individual cypris larva rather than from asexual budding. Hosts with single parasites outlive those with multiple infections.

Arcturosaccuskussakini, new genus, new species, is a rhizocephalan parasite of the ultraabyssal isopod crustacean Antarcturus zenkewitchi Kussakin, collected off the east coast of Iturup Island, Kuril Islands, Russia. The morphology of the Rhizocephala Akentrogonida is reviewed and compared to this new species. Arcturosaccus has multiple layers of cuticle in the mesenteric canal, and a few spermatogenic islets floating free in the mantle cavity. By these characters, the new species seems most closely related to the genera Duplorbis and Cryptogaster, but differs from them by (1) the presence of the paired ovary, (2) the position of the mesentery, and (3) the mantle aperture, and by the shape of the externa. Naupliar instars are absent in the life cycle of Arcturosaccus kussakini and the larvae hatch as cyprids.

Cyprid settlement and host invasion was studied in the rhizocephalan Clistosaccus paguri (Clistosaccidae) using transmission electron microscopy. Male and female larvae appear to be morphologically identical. The infestive kentrogon instar known from the Peltogastridae, Lernaeodiscidae, and Sacculinidae is absent in the Clistosaccidae. Instead, the cyprid itself uses an antennule to penetrate through the host integument. This is the same mechanism used by functional male cyprids to implant spermatogonia into juvenile female parasites, and it is the first demonstration of "akentrogonid" host invasion in the the Rhizocephala. The primordial parasite is not preformed in the cyprid, but develops from injected, undifferentiated cells almost directly below the site of settlement. The full clistosaccid life cycle without kentrogons is described in detail, and compared to that found in the "classical" kentrogonid rhizocephalans. It is concluded that the traditional classification of the Rhizocephala into the suborders Kentrogonida and the Akentrogonida should be reconsidered, since the Clistosaccidae was previously included in the former.

Larval and adult morphology is studied in rhizocephalan parasites of the genus Ptychascus (Sacculinidae) infesting semiterrestrial crabs of the families Ocypodidae and Grapsidae. A parasite population from fiddler crabs (Uca) in Costa Rica represents a new species. Its cyprids have a sensory armament on the antennules that is distinct from that found in cyprids of Ptychascus glaber Boschma, the only previously described member of the genus. It is argued that larval characters will prove useful in rhizocephalan taxonomy. The main adaptations to life on semiterrestrial hosts in the genus Ptychascus are the lack of nauplii and the male cyprid's ability to settle and metamorphose almost immediately after release from the parasite. This abbreviated development probably ensures that the larvae can settle before being carried away from the habitat of the host crabs. Cyprid morphology is similar to that found in other sacculinid rhizocephalans. Furthermore, a TEM study shows that male cyprids metamorphose into a spine-covered trichogon as in other members of the Sacculinidae. It therefore seems that most features of the typical sacculinid life cycle are maintained.

The elaborate life history of the rhizocephalan parasite Lernaeodiscus includes a number of counterdefensive measures specifically adapted to circumvent defense systems of the host crab Petrolisthes. Lernaeodiscus is dioecious. The externa produces large male and small female cyprids. The female cyprids invade the host by means of a kentrogon stage. The sole site of invasion is the gills. The female parasite later erupts on the ventral side of the abdomen as a virgin externa. This has to be hyperparasitized by a male cyprid in order to reach sexual maturity. The life cycle is completed in about 5 months. Autogrooming is the crab's primary defense against infestation and ordinarily only crabs slightly deficient in this regard become infested. Once inside, the parasite gains control of the host crab and induces it to accept the parasite as "self"—in effect, as its own reproductive system. This nullifies all remaining defenses the crab might have against the parasite. In addition, infested male crabs become behaviorally and morphologically female, so that they provide maternal care for the external reproductive body of the parasite, as do infested female crabs, rather than attempting to remove the parasite from their body. While the parasite has diverted all of the reproductive resources and capabilities of the crab to its own use, the crab otherwise carries out its life as a normal member of the intertidal community.