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Background Acanthocephalans of the genus Bolbosoma Porta, 1908 are trophically transmitted parasites that infect marine mammals (mostly cetaceans and less frequently pinnipeds) worldwide. There are 12 species currently considered as valid; however, most records lack information on the maturity stage of the specimens. This, coupled with the scarce phylogenetic information available, hinders a correct understanding of their patterns of host specificity, evolutionary history, and taxonomy. A particularly intriguing case is that of Bolbosoma vasculosum (Rudolphi, 1819), which has been frequently reported in odontocetes but rarely as an adult, having been suggested to be synonymous with Bolbosoma capitatum (von Linstow, 1880). Methods We used a comprehensive approach to investigate the concept of Bolbosoma . First, we conducted a bibliographic review of records of Bolbosoma spp. to clarify which are the final hosts for each species. We paid particular attention to B. vasculosum , using morphological and molecular analyses to compare it with B. capitatum . Second, we characterized the complete mitochondrial genome of Bolbosoma balaenae (Gmelin, 1790), Bolbosoma turbinella (Diesing, 1851), B. capitatum , and B. vasculosum . Then, we reconstructed the phylogenetic relationships of Bolbosoma spp. and related taxa using full mitochondrial genomes (or only cox1 when full mitogenomes were unavailable) and nuclear ribosomal genes (18S and 28S). Results Bolbosoma spp. exhibit high specificity for cetaceans, with no confirmed records of adult specimens in other host groups. Within this genus, B. vasculosum appears to be conspecific with B. capitatum based on both morphological and molecular evidence. This species shows high affinity to odontocetes, while the remaining species are specific to mysticetes. Phylogenetic analyses showed strong support for the monophyly of Bolbosoma spp., which appeared as sister taxa to Corynosoma spp. and Andracantha spp.. The resulting topology aligns with the patterns of specificity indicated by host records, revealing two distinct clades for species specific to odontocetes and mysticetes, respectively. Conclusions The phylogenetic relationships obtained support the hypothesis that the association of Bolbosoma spp. with cetaceans originated through a host-switching event from aquatic birds.

Post-mortem examination of a fin whale Balaenoptera physalus stranded in the Mediterranean Sea led to the finding of Bolbosoma balaenae for the first time in this basin. In this work, we describe new structural characteristics of this parasite using light microscopy and scanning electron microscopy approaches. Moreover, the molecular and phylogenetic data as inferred from both ribosomal RNA 18S-28S and the mitochondrial DNA cytochrome oxidase c subunit 1 (cox1) for adult specimens of B. balaenae are also reported for the first time. Details of the surface topography such as proboscis's hooks, trunked trunk spines of the prebulbar foretrunk, ultrastructure of proboscis's hooks and micropores of the tegument are shown. The 18S + 28S rRNA Bayesian tree (BI) as inferred from the phylogenetic analysis showed poorly resolved relationships among the species of Bolbosoma. In contrast, the combined 18S + 28S + mtDNA cox1 BI tree topology showed that the present sequences clustered with the species of Bolbosoma in a well-supported clade. The comparison of cox1 and 18S sequences revealed that the present specimens are conspecific with the cystacanths of B. balaenae previously collected in the euphausiid Nyctiphanes couchii from the North Eastern Atlantic Ocean. This study provided taxonomic, molecular and phylogenetic data that allow for a better characterization of this poor known parasite.

The role of invertebrates in some acanthocephalan life cycles is unclear because juvenile acanthocephalans are difficult to identify to species using morphology. Most reports suggest acanthocephalans from turtle definitive hosts use ostracods as intermediate hosts and snails as paratenic hosts. However, laboratory studies of the life cycle suggest that ostracods and snails are both required hosts in the life cycle. To elucidate the role of ostracods and snails in acanthocephalan life cycles better, we collected 558 freshwater snails of 2 species, including Planorbella cf. Planorbella trivolvis and Physa acuta, from 23 wetlands in Oklahoma, U.S.A., and examined them for acanthocephalan infections. Additionally, we examined 37,208 ostracods of 4 species, Physocypria sp. (morphotype 1), Cypridopsis sp., Stenocypris sp., and Physocypria sp. (morphotype 2) for juvenile acanthocephalans from 2 wetlands in Oklahoma. Juvenile acanthocephalans were morphologically characterized, and the complete internal transcribed spacer (ITS) region of nuclear rDNA was sequenced from acanthocephalans infecting 11 ostracod and 13 snail hosts. We also sampled 10 red-eared slider turtles, Trachemys scripta elegans, and 1 common map turtle, Graptemys geographica, collected from Oklahoma, Arkansas, and Texas and recovered 1,854 adult acanthocephalans of 4 species. The ITS of 17 adult acanthocephalans of 4 species from turtle hosts were sequenced and compared to juvenile acanthocephalan sequences from ostracod and snail hosts from this study and GenBank to determine conspecificity. Of the 23 locations sampled for snails, 7 (30%) were positive for juvenile acanthocephalans in the genus Neoechinorhynchus. The overall prevalence and mean intensity of acanthocephalans in Planorbella cf. P. trivolvis and P. acuta were 20% and 2 (1–6) and 2% and 1 (1), respectively. In contrast, only 1 of 4 species of ostracods, Physocypria sp. (morphotype 1), was infected with larval/juvenile Neoechinorhynchus spp. with an overall prevalence of 0.1% and a mean intensity of 1 (1–2). Although 4 species of acanthocephalans infected turtle definitive hosts, including Neoechinorhynchus chrysemydis, Neoechinorhynchus emydis, Neoechinorhynchus emyditoides, and Neoechinorhynchus pseudemydis, all the ITS sequences from cystacanths infecting snail hosts were conspecific with N. emydis. In contrast, the ITS sequences from larval/juvenile acanthocephalans from ostracods were conspecific with 2 species of acanthocephalans from turtles (N. emydis and N. pseudemydis) and 1 species of acanthocephalan from fish (Neoechinorhynchus cylindratus). These results indicate that N. emydis infects freshwater snails, whereas other species of Neoechinorhynchus appear not to infect snail hosts. We document new ostracod and snail hosts for Neoechinorhynchus species, including the first report of an ostracod host for N. pseudemydis, and we provide novel molecular barcodes that can be used to determine larva, juvenile, and adult conspecificity of Neoechinorhynchus species.

Specimens of 3 species of Neoechinorhynchus Stiles and Hassall, 1905, were collected from a number of species of marine fish along the Pacific coast of Vietnam. New information is added to the descriptions of Neoechinorhynchus (Neoechinorhynchus) longnucleatusAmin, Ha, and Ha, 2011, and its wider host and geographical distribution are reported. Similarly, more descriptive information and host and geographical records are added to our knowledge of Neoechinorhynchus (Hebosoma) manubrianusAmin, Ha, and Ha, 2011, and Neoechinorhynchus (Neoechinorhynchus) dimorphospinusAmin and Sey, 1996. The latter species was previously known only from the Persian Gulf. The presence of the para-receptacle structure was documented in all 3 species of Neoechinorhynchus reported. The molecular characterization of N. dimorphospinus was carried out using a partial 18S rDNA sequence. The phylogenetic analysis showed that most species of Neoechinorhynchus are very closely related, while N. dimorphospinus was distinct from others in the tree. Metal analysis of hooks of N. dimorphospinus using Energy Dispersive X-ray Analysis also distinguished its characteristic finger print of high phosphorus and calcium and low sulfur levels.

A new species of Acanthocephala is described based on specimens found parasitising the intestine of the gafftopsail pompano Trachinotus rhodopus Gill in the State of Oaxaca, off the Pacific coast of Mexico. Rhadinorhynchus villalobosi sp. n. differs from the other 25 species of the genus known from the Pacific Ocean by a combination of morphological traits, such as the fewer number of rows of hooks along the proboscis, the distribution of spines of the trunk forming two fields, the position of genital pore, subterminal in both sexes, a shorter neck and cement glands, among others. The phylogenetic analyses based on sequences of the nuclear 18S and 28S rRNA genes, as well as the mitochondrial cox1, all agree that R. villalobosi sp. n. is part of the genus, and closely related to Rhadinorhynchus trachinoti Grano-Maldonado, Sereno-Uribe, Hernández-Payán, Pérez-Ponce de León et García-Varela, 2025, a recently described Mexican species, despite the marked morphological differences between the two and being distributed in different areas.

Acanthocephalans of the genus Rhadinorhynchus parasitize various marine fishes worldwide. However, the true diversity of Rhadinorhynchus is still unclear. In this study, we found an example of phenotypic variation in trunk spines of the poorly known rhadinorhynchid species R. cololabis Laurs & McCauley, 1964. According to the number and distribution of trunk spines, the present specimens of R. cololabis can be divided into two distinct morphotypes, which may erroneously be recognized as distinct taxa in the absence of molecular data. However, the Assemble Species by Automatic Partitioning (ASAP) and Bayesian inference (BI) analyses based on different nuclear and mitochondrial sequence data, all confirm that the two distinct morphotypes are conspecific, and do not represent two separate genetic lineages. Our ASAP and BI results of cox1 data also suggest that is R. villalobosi Martínez-Flores et al., 2025 is a synonym of R. trachinoti Grano-Maldonado et al., 2025, and challenge the validity of R. dorsoventrospinosus Amin et al., 2011, and R. hiansi Soota & Bhattacharya, 1981. The present findings also indicate that the number and distribution of trunk spines vary markedly in some species of Rhadinorhynchus, and care must be taken when differentiating Rhadinorhynchus species based on this feature. Additionally, the complete mitogenome of R. cololabis is presented for the first time, which has only 13,567 bp, and displays a very high level of similarity with the mitogenome of R. laterospinosus in both nucleotide sequences (94.6%) and amino acid sequences of 12 protein-coding genes (93.8%). However, comparative mitogenomics support R. cololabis and R. laterospinosus representing two separate taxa.

Rhadinorhynchus hiansi Soota and Bhattacharya, 1981, has remained unknown since its original incomplete description from 2 male specimens collected from the flat needlefish Ablennes hians Valenciennes (Belonidae) off Trivandrum, Kerala, India. Recent collections of fishes along the Pacific coast of Vietnam in 2016 and 2017 produced many specimens of the same species from the striped bonito Sarda orientalis Temminck and Schlegel (Scombridae) off the southern Pacific coast of Vietnam at Nha Trang. We describe females for the first time, assign a female allotype status, and provide an expanded description of males from a larger collection completing missing information on hooks and hook roots, receptacle, lemnisci, cement glands, Saefftigen's pouch, and trunk spines. Specimens of R. hiansi characteristically have no dorsal spines in the posterior field of trunk spines and a long proboscis with 36–48 dorso-ventrally differentiated proboscis hooks per row becoming progressively smaller posteriorly then increasing in size near the posterior end to a maximum at the posterior-most ring. Trunk, testes, and lemnisci in our specimens were considerably larger than those reported in the original description, but the proboscis was relatively smaller. The females had long reproductive system and corrugated elliptic eggs without polar prolongation of fertilization membrane. Energy Dispersive X-ray Analysis (EDXA) demonstrates high levels of calcium and phosphorus in large gallium cut hooks and high levels of sulfur in tip cuts of large and small hooks and in spines. This EDXA pattern is a characteristic fingerprint of R. hiansi. The molecular profile of R. hiansi is described from 18S rDNA and COI genes, and phylogenetic relationships with most closely related species are discussed.

Two new species of Andracantha (Polymorphidae) are described from the intestine of the shags Leucocarbo chalconotus (Gray) and Phalacrocorax punctatus (Sparrman), and the penguin Eudyptula minor (Forster) from southern South Island, New Zealand. Andracantha leucocarboi n. sp. is distinguished from its congeners by having no genital or ventral trunk spines, but possessing a scattering of small spines between the anterior fields of spines. This is the first record of a species of Andracantha from a penguin. Circumbursal papillae are illustrated in a scanning electron micrograph for the first time in the polymorphids. Andracantha sigma n. sp. is distinguished by the sigmoid shape of its largest proboscis hook, hook VIII, and having the ventral field separated from the posterior disc field by an aspinous gap. A Maximum Likelihood tree from cox1 and large ribosomal subunit (LSU) data shows A. leucocarboi n. sp. to be more closely related to A. gravida than A. sigma n. sp. and the genus Andracantha as sister to Corynosoma spp. Genetic distances between species of Andracantha are comparatively large. A key to the species of Andracantha is provided.

Adult specimens of Pomphorhynchus fuhaiensis were identified from common carp (Cyprinus carpio L.) in Ulungur Lake of northwest China, and acanthors, acanthellae, cystacanths dissected from Gammarus lacustris in a small tributary of Ulungur River for the first time. The acanthocephalans were also found in crucian carp (Carassius carassius L.), tench (Tinca tinca L.), oriental bream (Abramis brama orientalis Berg), and ide (Leuciscus idus L.) in the lake. This species is distinguished from other species in Pomphorhynchus by its large, spherical bulb and very long neck as well as by a cylindrical proboscis armed with 15–17 longitudinal rows of 9–12 hooks each. The anterior proboscis hooks are almost uniform in size and shape, the sixth hook in a short row and the seventh hook in long row decrease abruptly in size posteriorly with the last end hook being a little larger than the prebasal hook, and in a ring; posterior proboscis hooks much more widely spaced. Furthermore, the lemnisci are claviform. The mean neck:trunk ratio is about 0.5, which is larger than most other species in Pomphorhynchus. Females are larger than males. In males, the testes are in one-third to one-half of the trunk, equal, ovoid–spheroid, usually contiguous, and small relative to the body size, and there are 6 ovoid cement glands. Pomphorhynchus fuhaiensis is similar to Pomphorhynchus laevis but can be distinguished by the number of longitudinal rows of hooks. Pomphorhynchus laevis is armed with 18–20 longitudinal rows of 11–13 hooks, P. fuhaiensis is armed with 15–17 longitudinal rows of 9–12 hooks.

Background Thorny-headed worms (Acanthocephala) occur worldwide in gnathostome vertebrates feeding on mandibulate arthropods. They can manipulate host behavior, accumulate heavy metals, and have lately gained economic relevance as a pest in fish aquaculture. Yet, despite their ecological and economic significance, little is known about the gene-expressional background of acanthocephalan development, maturation, and reproduction in the definitive host. To fill this gap in knowledge, we studied Neoechinorhynchus agilis (Eoacanthocephala) specimens sampled from the digestive tracts of naturally infected thin-lipped mullets ( Chelon ramada ). Results We generated a nuclear draft genome and a whole-body transcriptome assembly. Differential expression analysis based on transcript abundances of 36 males and 30 females revealed that 30% of the transcripts had sex-biased expression. Gene ontologies relating to energy metabolism and microtubules were enriched with male-biased genes; female-biased genes indicated increased cell division and transcription activity. Only 0.19% of genes were differentially expressed as a function of female size (using whole-body RNA weight as a proxy for size), versus 5.4% in males. Conclusions Transcriptome annotations underlined energy metabolism and reproduction as major tasks in N. agilis life. Our results suggest that males, smaller than females and thus supposedly less competitive, struggle for sufficient energy to produce large quantities of sperm. Female-biased genes were consistent with the production and development of numerous eggs. Finally, we identified genes with particular importance in the growth or reproduction of N. agilis , that could be investigated as candidate targets for acanthocephalan control in fish aquaculture.