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Certain parasites can manipulate their hosts to enhance their own fitness and transmission success. Rhizocephalan barnacles are a notable example, inducing significant changes in their crustacean host's morphology, physiology, and behaviour. Until recently, it was believed that the spider crab Scyra aff. ferox is parasitized by a single species Sacculina pilosella in the Sea of Japan. However, previous molecular studies have revealed a complex of unrelated species, Sacculina pugettiae (fam. Sacculinidae) and Parasacculina pilosella (fam. Polyascidae). These rhizocephalans can even parasitize one host specimen simultaneously. Here, we investigated the interaction of S. pugettiae and P. pilosella with the nervous system of their common host. Species were identified based on receptacle morphology, with further validation through 18S gene phylogenetic analysis. The parasites interacted differently with the host nervous system: S. pugettiae had goblet‐shaped organs in the ganglion periphery, while P. pilosella lacked these structures, instead possessing numerous neuropil rootlets. In case of simultaneous infection, both goblet‐shaped organs and neuropil rootlets were present. Histochemistry revealed the presence of muscular rosettes in the rootlets of both species, suggesting a similar organization of the muscular system despite phylogenetic differences. Scanning electron microscopy confirmed the presence of host cell projections enveloping the rootlets. This study provides insights into the morphological features of rhizocephalan‐decapod interaction and highlights differences in their interaction with host nervous tissue between families. Our results also confirmed the loss of the goblet‐shaped organs in Polyascidae. [ABSTRACT FROM AUTHOR]

The results of this study showed that out of 287 individuals (135 males and 152 females) of the spider crab Menaethius monoceros collected from the Red Sea and the Gulfs of Suez and Aqaba, 15 individuals (6 males and 9 females) were infected with Parasacculina pilosella (Van Kampen & Boschma, 1925) (Polyascidae: Rhizocephala: Cirripedia: Crustacea). A total of 20 externae were recorded, including 11 singles, three doubles, and only one triple on the infected crab abdomens. The overall infection rate was 5.23% for the combined sexes at all sites, declined to 4.44% in males, but increased to 5.92% in females, reaching a higher rate of 16.07% in the populations of Marsa Alam and Shalateen sectors during summer. The examined externae have variable colors, sizes, and shapes based on their sexual maturity, which has been classified into four stages: immature or virgin, early mature, maturing, and late (spent externae). The largest number of externae was eight extruded on the 3rd abdominal segment, while it decreased to six, two, and one on the 4th, 2nd and 5th segments, respectively. The size of externae varied from 1.3 × 1.0 × 1.0 to 6.3 × 4.5 × 2.5 mm (Breadth × Height × Thickness). These results showed changes in the external secondary sexual characters of the infected males, represented by the remarkable broadness of the abdomen fringed with marginal setae and invagination of the sternum, but were limited in infected females.

The results of this study showed that out of 287 individuals (135 males and 152 females) of the spider crab Menaethius monoceros collected from the Red Sea and the Gulfs of Suez and Aqaba, 15 individuals (6 males and 9 females) were infected with Parasacculina pilosella (Van Kampen & Boschma, 1925) (Polyascidae: Rhizocephala: Cirripedia: Crustacea). A total of 20 externae were recorded, including 11 singles, three doubles, and only one triple on the infected crab abdomens. The overall infection rate was 5.23% for the combined sexes at all sites, declined to 4.44% in males, but increased to 5.92% in females, reaching a higher rate of 16.07% in the populations of Marsa Alam and Shalateen sectors during summer. The examined externae have variable colors, sizes, and shapes based on their sexual maturity, which has been classified into four stages: immature or virgin, early mature, maturing, and late (spent externae). The largest number of externae was eight extruded on the 3rd abdominal segment, while it decreased to six, two, and one on the 4th, 2nd and 5th segments, respectively. The size of externae varied from 1.3 × 1.0 × 1.0 to 6.3 × 4.5 × 2.5 mm (Breadth × Height × Thickness). These results showed changes in the external secondary sexual characters of the infected males, represented by the remarkable broadness of the abdomen fringed with marginal setae and invagination of the sternum, but were limited in infected females. [ABSTRACT FROM AUTHOR]

Introduction: This study undertakes a detailed examination of the host range and distribution patterns of Rhizocephala, a group of specialized parasitic barnacles, within Korean marine environments. It aims to expand the understanding of their biodiversity and ecological impact. Methods: Employing both morphological observation and mitochondrial DNA sequencing, the study identifies the variety of rhizocephalan species infecting decapod hosts in Korean waters, aiming to document their diversity and distribution. Results: The investigation identifies 26 rhizocephalan species parasitizing 28 decapod hosts, including the discovery of three new parasite-host relationships not previously reported. It was observed that while a few rhizocephalan species infect multiple hosts, most have a narrow host range. Additionally, the study maps out regional differences in Rhizocephala diversity across three Korean marine ecoregions, noting the highest diversity in the warmer East China Sea ecoregion and lower diversity in the colder Yellow Sea and East Sea ecoregions. Discussion: The findings underscore the necessity for continued taxonomic research on Rhizocephala and their host species to better understand and manage these parasites, which hold significant ecological and economic importance. The study suggests that geographical variations in host-parasite relationships are influenced by regional climatic conditions, particularly winter water temperatures. [ABSTRACT FROM AUTHOR]

Background. Rhizocephalan interaction with their decapod hosts is a superb example of host manipulation. These parasites are able to alter the host's physiology and behavior. Host-parasite interaction is performed, presumably, via special modified rootlets invading the ventral ganglions. Methods. In this study, we focus on the morphology and ultrastructure of these special rootlets in Polyascus polygeneus (Lützen & Takahashi, 1997), family Polyascidae, invading the neuropil of the host's nervous tissue. The ventral ganglionic mass of the infected crabs were fixed, and the observed sites of the host-parasite interplay were studied using transmission electron microscopy, immunolabeling and confocal microscopy. Results. The goblet-shaped organs present in the basal families of parasitic barnacles were presumably lost in a common ancestor of Polyascidae and crown "Akentrogonida", but the observed invasive rootlets appear to perform similar functions, including the synthesis of various substances which are transferred to the host's nervous tissue. Invasive rootlets significantly differ from trophic ones in cell layer composition and cuticle thickness. Numerous multilamellar bodies are present in the rootlets indicating the intrinsic cell rearrangement. The invasive rootlets of P. polygeneus are enlaced by the thin projections of glial cells. Thus, glial cells can be both the first hosts' respondents to the nervous tissue damage and the mediator of the rhizocephalan interaction with the nervous cells. One of the potential molecules engaged in the relationships of P. polygeneus and its host is serotonin, a neurotransmitter which is found exclusively in the invasive rootlets but not in trophic ones. Serotonin participates in different biological pathways in metazoans including the regulation of aggression in crustaceans, which is reduced in infected crabs. We conclude that rootlets associated with the host's nervous tissue are crucial for the regulation of host-parasite interplay and for evolution of the Rhizocephala. [ABSTRACT FROM AUTHOR]

Background Rhizocephalan interaction with their decapod hosts is a superb example of host manipulation. These parasites are able to alter the host’s physiology and behavior. Host-parasite interaction is performed, presumably, via special modified rootlets invading the ventral ganglions. Methods In this study, we focus on the morphology and ultrastructure of these special rootlets in Polyascus polygeneus (Lützen & Takahashi, 1997), family Polyascidae, invading the neuropil of the host’s nervous tissue. The ventral ganglionic mass of the infected crabs were fixed, and the observed sites of the host-parasite interplay were studied using transmission electron microscopy, immunolabeling and confocal microscopy. Results The goblet-shaped organs present in the basal families of parasitic barnacles were presumably lost in a common ancestor of Polyascidae and crown “Akentrogonida”, but the observed invasive rootlets appear to perform similar functions, including the synthesis of various substances which are transferred to the host’s nervous tissue. Invasive rootlets significantly differ from trophic ones in cell layer composition and cuticle thickness. Numerous multilamellar bodies are present in the rootlets indicating the intrinsic cell rearrangement. The invasive rootlets of P. polygeneus are enlaced by the thin projections of glial cells. Thus, glial cells can be both the first hosts’ respondents to the nervous tissue damage and the mediator of the rhizocephalan interaction with the nervous cells. One of the potential molecules engaged in the relationships of P. polygeneus and its host is serotonin, a neurotransmitter which is found exclusively in the invasive rootlets but not in trophic ones. Serotonin participates in different biological pathways in metazoans including the regulation of aggression in crustaceans, which is reduced in infected crabs. We conclude that rootlets associated with the host’s nervous tissue are crucial for the regulation of host-parasite interplay and for evolution of the Rhizocephala.

Parasitic barnacles (Thecostraca: Rhizocephala) are among the most striking parasitic manipulators in the animal kingdom. "Kentrogonid" rhizocephalans significantly alter the physiology and behaviour of brachyuran crabs, while "akentrogonid" influence on hosts remains poorly studied. The specialised rootlets invading the nervous system that are presumably crucial for the host-parasite interplay were described in basal rhizocephalan families that display a kentrogonid life cycle. In this study, we present a histological description of such rootlets in the "akentrogonid" Diplothylacus sinensis (Keppen, 1877) (family Thompsoniidae) in the ventral nervous ganglia of the crab Portunus pelagicus (Linnaeus, 1758). The morphology of these rootlets varies greatly in cuticle structure, gross organisation of the rootlet tissues and presence/absence of large vacuoles. It is assumed that rootlet modification appears during its development and successive growth from the penetration site into the ganglia's neuropil. The absence of the goblet-shaped organs in D. sinensis supports the hypothesis that these structures were secondarily lost in the crown lineages of the Rhizocephala. • Diplothylacus sinensis has an abundant net of rootlets invading hosts nervous system • Rootlets of D. sinensis are not modified into goblet-shaped organs • Rootlets of D. sinensis have diverse types of tissue organisation • Crown groups of rhizocephalans presumably have lost goblet-shaped organs [ABSTRACT FROM AUTHOR]

Background: Rhizocephalan barnacles stand out in the diverse world of metazoan parasites. The body of a rhizocephalan female is modified beyond revealing any recognizable morphological features, consisting of the interna, a system of rootlets, and the externa, a sac-like reproductive body. Moreover, rhizocephalans have an outstanding ability to control their hosts, literally turning them into “zombies”. Despite all these amazing traits, there are no genomic or transcriptomic data about any Rhizocephala. Methods: We collected transcriptomes from four body parts of an adult female rhizocephalan Peltogaster reticulata: the externa, and the main, growing, and thoracic parts of the interna. We used all prepared data for the de novo assembly of the reference transcriptome. Next, a set of encoded proteins was determined, the expression levels of protein-coding genes in different parts of the parasite’s body were calculated and lists of enriched bioprocesses were identified. We also in silico identified and analyzed sets of potential excretory / secretory proteins. Finally, we applied phylostratigraphy and evolutionary transcriptomics approaches to our data. Results: The assembled reference transcriptome included transcripts of 12,620 protein-coding genes and was the first for any rhizocephalan. Based on the results obtained, the spatial heterogeneity of protein-coding gene expression in different regions of the adult female body of P. reticulata was established. The results of both transcriptomic analysis and histological studies indicated the presence of germ-like cells in the lumen of the interna. The potential molecular basis of the interaction between the nervous system of the host and the parasite's interna was also determined. Given the prolonged expression of development-associated genes, we suggest that rhizocephalans “got stuck in their metamorphosis”, even at the reproductive stage. Conclusions: The results of the first comparative transcriptomic analysis for Rhizocephala not only clarified but also expanded the existing ideas about the biology of these extraordinary parasites.

Sex allocation theory predicts the adaptive allocation of resources to male versus female reproduction in simultaneous hermaphrodites in response to individual characteristics or environmental factors. Because parasites uptake resources from their hosts, their presence could affect the sex allocation of the hosts. We investigated the effects of infestation status and infestation intensity by the rhizocephalan barnacle Boschmaella japonica on reproduction, including sex allocation, of the host intertidal barnacle Chthamalus challengeri. Feeding activity was also examined as a factor related to resource intake. Both male and female reproductive investment decreased with increasing parasite infestation, and the sex allocation of large infested hosts was more male-biased than that of large uninfested hosts. Moreover, in contrast to the model prediction that male investment does not change under resource limitation, male investment decreased in infested hosts whose resources were taken by parasites. This reduction in male investment could be explained by changes in mating group size, since infested hosts have shorter penises and consequently are able to access fewer mating partners., (© The Author(s) 2025. Published by Oxford University Press on behalf of the European Society of Evolutionary Biology.)

Background: Rhizocephalan barnacles stand out in the diverse world of metazoan parasites. The body of a rhizocephalan female is modified beyond revealing any recognizable morphological features, consisting of the interna, a system of rootlets, and the externa, a sac-like reproductive body. Moreover, rhizocephalans have an outstanding ability to control their hosts, literally turning them into “zombies”. Despite all these amazing traits, there are no genomic or transcriptomic data about any Rhizocephala. Methods: We collected transcriptomes from four body parts of an adult female rhizocephalan Peltogaster reticulata: the externa, and the main, growing, and thoracic parts of the interna. We used all prepared data for the de novo assembly of the reference transcriptome. Next, a set of encoded proteins was determined, the expression levels of protein-coding genes in different parts of the parasite’s body were calculated and lists of enriched bioprocesses were identified. We also in silico identified and analyzed sets of potential excretory / secretory proteins. Finally, we applied phylostratigraphy and evolutionary transcriptomics approaches to our data. Results: The assembled reference transcriptome included transcripts of 12,620 protein-coding genes and was the first for any rhizocephalan. Based on the results obtained, the spatial heterogeneity of protein-coding gene expression in different regions of the adult female body of P. reticulata was established. The results of both transcriptomic analysis and histological studies indicated the presence of germ-like cells in the lumen of the interna. The potential molecular basis of the interaction between the nervous system of the host and the parasite's interna was also determined. Given the prolonged expression of development-associated genes, we suggest that rhizocephalans “got stuck in their metamorphosis”, even at the reproductive stage. Conclusions: The results of the first comparative transcriptomic analysis for Rhizocephala not only clarified but also expanded the existing ideas about the biology of these extraordinary parasites.

The endoparasitic crustacean Sacculina carcini (Cirripedia: Rhizocephala) has a much simpler morphology than conventional filter-feeding barnacles, reflecting its parasitic lifestyle. To investigate the molecular basis of its refined developmental program, we produced a draft genome sequence for comparison with the genomes of nonparasitic barnacles and characterized the transcriptomes of internal and external tissues. The comparison of clusters of orthologous genes revealed the depletion of multiple gene families but also several unanticipated expansions compared to non-parasitic crustaceans. Transcriptomic analyses comparing interna and externa tissues revealed an unexpected variation of gene expression between rootlets sampled around host midgut and thoracic ganglia. Genes associated with lipid uptake were strongly expressed by the internal tissues. We identified candidate genes probably involved in host manipulation (suppression of ecdysis and gonad development) including those encoding crustacean neurohormones and the juvenile hormone binding protein. The evolution of Rhizocephala therefore appears to have involved a rapid turnover of genes (losses and expansions) as well as the fine tuning of gene expression. [ABSTRACT FROM AUTHOR]

Background: Rhizocephalan barnacles stand out in the diverse world of metazoan parasites. The body of a rhizocephalan female is modified beyond revealing any recognizable morphological features, consisting of the interna, the system of rootlets, and the externa, a sac-like reproductive body. Moreover, rhizocephalans have an outstanding ability to control their hosts, literally turning them into “zombies”. Despite all these amazing traits, there is no genomic and transcriptomic data about any Rhizocephala. Methods: We collected transcriptomes from four body parts of an adult female rhizocephalan Peltogaster reticulata: externa and main, growing, and thoracic parts of the interna. We used all prepared data for the de novo assembly of the reference transcriptome. Next, a set of encoded proteins was determined, the expression levels of protein-coding genes in different parts of the parasite body were calculated and lists of enriched bioprocesses were identified. We also in silico identified and analyzed sets of potential excretory / secretory proteins. Finally, we applied phylostratigraphy and evolutionary transcriptomics approaches to our data. Results: The assembled reference transcriptome included transcripts of 12,620 protein-coding genes and was the first for both P. reticulata and Rhizocephala. Based on the results obtained, the spatial heterogeneity of protein-coding genes expression in different regions of P. reticulata adult female body was established. The results of both transcriptomic analysis and histological studies indicated the presence of germ-like cells in the lumen of the interna. The potential molecular basis of the interaction between the nervous system of the host and the parasite's interna was also determined. Given the prolonged expression of development-associated genes, we suggest that rhizocephalans “got stuck in the metamorphosis”, even in their reproductive stage. Conclusions: The results of the first comparative transcriptomic analysis for Rhizocephala not only clarified but also expanded the existing ideas about the biology of this amazing parasites.

Rhizocephala is a group of highly modified parasitic crustaceans. The body of an adult female is formed de novo, and lacks most traits of free-living crustaceans. Despite the large number of studies dedicated to different aspects of rhizocephalan biology, the structure of the interna of these parasites is still poorly understood. The muscular systems specifically were described only for representatives of three families. The aim of our study is to visualize and describe the muscular system in the interna of Peltogasterella gracilis from the family Peltogasterellidae. P. gracilis has a colonial type of organization, with a common part of the interna and numerous externa-bearing modules. Interestingly, the muscular system has dissimilar organization in different parts of the interna. Muscular system in modules resembles that of Peltogaster paguri , while in the common part of the interna it's more similar to the muscular system of sacculinids and polyascids. We propose that the muscular system of rhizocephalans is a new structure in terms of evolution, and as such can be easily modified in respect to the functional characteristics of the different interna's parts. [ABSTRACT FROM AUTHOR]

Background: Rhizocephalan barnacles stand out in the diverse world of metazoan parasites. The body of a rhizocephalan female is modified beyond revealing any recognizable morphological features, consisting of the interna, the system of rootlets, and the externa, a sac-like reproductive body. Moreover, rhizocephalans have an outstanding ability to control their hosts, literally turning them into “zombies”. Despite all these amazing traits, there is no genomic and transcriptomic data about any Rhizocephala. Methods: We collected transcriptomes from four body parts of an adult female rhizocephalan Peltogaster reticulata: externa and main, growing, and thoracic parts of the interna. We used all prepared data for the de novo assembly of the reference transcriptome. Next, a set of encoded proteins was determined, the expression levels of protein-coding genes in different parts of the parasite body were calculated and lists of enriched bioprocesses were identified. We also in silico identified and analyzed sets of potential excretory / secretory proteins. Finally, we applied phylostratigraphy and evolutionary transcriptomics approaches to our data. Results: The assembled reference transcriptome included transcripts of 12,620 protein-coding genes and was the first for both P. reticulata and Rhizocephala. Based on the results obtained, the spatial heterogeneity of protein-coding genes expression in different regions of P. reticulata adult female body was established. The results of both transcriptomic analysis and histological studies indicated the presence of germ-like cells in the lumen of the interna. The potential molecular basis of the interaction between the nervous system of the host and the parasite's interna was also determined. Given the prolonged expression of development-associated genes, we suggest that rhizocephalans “got stuck in the metamorphosis”, even in their reproductive stage. Conclusions: The results of the first comparative transcriptomic analysis for Rhizocephala not only clarified but also expanded the existing ideas about the biology of this amazing parasites.

We studied the population and reproductive characteristics of the stone crab Hapalogaster dentata (De Haan, 1849) (Anomura: Lithodidae) parasitized by Briarosaccus hoegi (Rhizocephala: Peltogastridae) from Peter the Great Bay (northwestern Sea of Japan) in 2023. The sex structure of the H. dentata population was female-biased and represented by 39.1% males and 60.9% females. The size-frequency distribution differed between males and females. The prevalence of parasitization of H. dentata by B. hoegi reached 6.7%. No relationship between crab sex and parasitization prevalence was found. Most of the parasitized specimens (94.8%) had only a single parasitic externa. The non-parasitized and parasitized crabs did not differ in main carapace length. The pleopods in most of the externa-bearing females were shortened or completely atrophied. H. dentata produced 1 clutch per year with ovigerous females occurring from October to May. An obvious delay in embryonic development was observed in winter. None of the parasitized H. dentata females were ovigerous. Externae with embryos of B. hoegi were found from May to July.

Rhizocephala is a group of endoparasitic barnacles, the morphological characteristics of which are degenerated, and which has recently undergone active molecular identification. Despite several recent studies of Korean rhizocephalans, a comprehensive analysis of rhizocephalan fauna has not yet been conducted. In this study, we analyzed morphological and molecular characteristics of 64 rhizocephalan samples from 15 decapod hosts sampled across the Korean coast. We found 16 Rhizocephala species of six genera from four Rhizocephala families, i.e. , Peltogasterellidae, Peltogastridae, Polyascidae, and Sacculinidae. Combining morphological examination and molecular analysis of mitochondrial cytochrome c oxidase I revealed three new species candidates, i.e. , Peltogasterella sp., Peltogaster sp., and Parasacculina sp. 1, and three rhizocephalans that have expanded their distribution range to the Korean coast, i.e. , Parasacculina oblonga , Sacculina angulata , and Sacculina gracilis , whose expanded their distribution range to Korean coast. A synthetic update of Korean Rhizocephala fauna including a species checklist and remarks regarding distribution and taxonomy is also presented.

[Display omitted] • Cancer-like neoplasma was found in the interna of parasitic barnacle. • Tumor is located in the spherical widening of the interna's rootlets. • Tumor has disrupted tissue organization. • The tumor seems to originate from the floating cell lineage. Cancer-like neoplasms are extremely rarely present in arthropods, particularly in crustaceans. Thus, it is assumed that these animals have some efficient cancer-preventing mechanisms. However, several cases of cancer-like neoplasms are described in crustaceans, though only for the Decapoda. We identified a tumor in the parasitic barnacle Peltogaster paguri (Cirripedia: Rhizocephala), and described its histological structure. A spherical cell mass consisting mostly of roundish cells with big translucent nuclei, prominent nucleoli, and sparse chromatin, and of cells with condensed chromosomes, was found in the main trunk of the P. paguri rootlet system. Numerous mitoses were observed in this area. Such tissue organization is utterly uncharacteristic of the Rhizocephala. Based on acquired histological data, we assume that this tumor is a cancer-like neoplasm. This is the first report of a tumor identified in the rhizocephalans, as well as in non-decapod crustaceans as a whole. [ABSTRACT FROM AUTHOR]

One of the most conspicuous traits of parasitic organisms is a well-developed reproductive system. In Rhizocephala ("Crustacea": Cirripedia) it is believed to be nested in the externa-a "reproductive part" located outside of the host. However, it is not clear how nutrients are transported to the externa. Several authors described a system of lacunae in the externa, and muscular contractions probably enable transport through these cavities. The aim of our study was to visualize (using microcomputed tomography and confocal laser scanning microscopy) and describe lacunar and muscular systems in the externa of Peltogasterella gracilis (fam. Peltogasterellidae). The lacunar system consists of "ventral" lacuna and several protrusions. The "ventral" lacuna is probably responsible for visceral mass nutrition, and mantle protrusions are associated with the mantle nutrition. The gross organization of the muscular system mostly corresponds to previous descriptions in other rhizocephalan species. Nonetheless, we observed several features of the externa morphology that had not been described before such as a muscular thickening in the proximal externa's part and a stalk plug disk. The muscular thickening might play a role of a propulsatory organ, helping to transport liquid through the lacunar system. The plug disk might fill the hole in the host's cuticle after the old externa drop off. The results allow us to make first assumptions on transport mechanisms in Rhizocephala., (© 2023 Wiley Periodicals LLC.)

Specialized morphology of diverse parasitic crustaceans reflects their adaptations to an endoparasitic lifestyle. Rhizocephalan barnacles are one of the most highly modified obligatory parasites of other crustaceans. Comprehension of the functional morphology of rhizocephalans could elucidate the main evolutionary trends not only inside parasitic barnacles, but in parasitism as a whole. Despite that, the available morphological information on the rhizocephalans is very fragmented. In this study, we examined the organization and ultrastructural features in different parts of the interna of Peltogaster reticulata (fam. Peltogastridae). The main trunk cuticle is much thicker than that of the side branches due to the different functions of these body parts. The central lumen in the main trunk is lined by an extracellular matrix, while the side branches are not. Muscular fibers are only present in the body wall of the main trunk, where they are organized as a "wicker basket". Furthermore, functional differentiation can be found at the ultrastructural level in the cells of the rootlets: there are distinct cell types both in hypodermal and axial cell layers. The rootlets of P. reticulata are covered by a network of the host's neurons and capillaries. • The interna of Peltogaster reticulata is morphologically and ultrastructurally heterogeneous. • The central lumen of the main trunk is lined with extracellular matrix. • The muscular system resembling a "wicker basket" is present in the main trunk. • The rootlets are enlaced by a network of the host neurons and capillaries. [ABSTRACT FROM AUTHOR]