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All issues Volume 32 (2025) Parasite, 32 (2025) 73 Full HTML
Open Access
Issue
Parasite
Volume 32, 2025
Article Number 73
Number of page(s) 13
DOI https://doi.org/10.1051/parasite/2025069
Published online 28 November 2025

© K.-Y. Wang et al., published by EDP Sciences, 2025

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Introduction

The genus Rhadinorhynchus Lühe, 1911 is a common group of acanthocephalans containing more than 40 described species that parasitize various marine fishes worldwide [1, 3, 4, 16, 38]. However, the true diversity of Rhadinorhynchus remain unclear, because most species in this genus are defined under the morphospecies concept, which does not always yield taxa consistent with the actual biological species in some cases of Acanthocephala [26, 27, 42, 51]. The number and distribution pattern of trunk spines are considered to be key diagnostic characters for traditional species identification of Rhadinorhynchus [2, 38]. However, we are still not sure whether it is reliable to use the number and distribution of trunk spines as critical criteria for delineating the species of Rhadinorhynchus.

Recently, some studies have proven that nuclear and mitochondrial sequence data, including mitochondrial genomes, play important roles in the detection of cryptic species, assessment of phenotypic variation, identification of cystacanths, and clarification of phylogenetic relationships of acanthocephalans [9, 16, 18, 20, 21, 26–29, 37, 45–47, 49–51]. However, the current genetic database for Rhadinorhynchus remains insufficient. To date, a total of 12 species of Rhadinorhynchus have been genetically sequenced [4, 5, 7, 8, 12–14, 16, 18, 21, 28–31, 43]. In the family Rhadinorhynchidae, only R. laterospinosus Amin, Heckmann & Ha, 2011 has had a complete mitochondrial genome documented [47].

Rhadinorhynchus cololabis Laurs & McCauley, 1964 is a poorly known acanthocephalan species, originally described from the Pacific saury Cololabis saira (Brevoort) (Beloniformes: Scomberesocidae) off the coast of the United States (Oregon) [23]. Since then, only Motora (2016) [32] recorded this species from Oncorhynchus masou (Brevoort) (Salmoniformes: Salmonidae) in the Sea of Japan. During a helminthological survey of Chinese marine fishes, large numbers of Rhadinorhynchus specimens identified morphologically as R. cololabis were collected from C. saira in the South China Sea. Examination of specimens using light and scanning electron microscopy revealed the presence of remarkable phenotypic variation in the number and distribution of trunk spines among different individuals. In order to evaluate whether the present specimens of R. cololabis, with a different number and distribution of trunk spines, belong to a complex of sibling species or a single species, Assemble Species by Automatic Partitioning (ASAP) analyses and Bayesian inference (BI) were performed based on different nuclear and mitochondrial sequence data. Furthermore, we also sequenced and annotated the complete mitochondrial genome of this species for the first time, to enrich the mitogenomic data and reveal the pattern of mitogenomic evolution of the family Rhadinorhynchidae.

Materials and methods

Ethics approval

This study was conducted under the protocol of Hebei Normal University (LLSC2024090). All applicable national and international guidelines for the protection and use of animals were followed.

Acanthocephalan collection and morphological observation

A total of 46 individuals of C. saira collected from the South China Sea (off Nanning City, Guangxi Zhuang Autonomous Region, China) in 2023, were examined for helminth parasites, and 57 acanthocephalan specimens were isolated from the intestine of 31 fish hosts [prevalence 67.4%, intensity of infection 1–22 (mean = 4.55) specimens]. Specimens were washed using clear water, and then fixed and stored in 80% ethanol for further study.

For light microscopy studies, acanthocephalans were cleared in lactophenol and made in temporary mounts. Photomicrographs were recorded using a Nikon® digital camera coupled to a Nikon® optical microscope (Nikon ECLIPSE Ni-U, Nikon Corporation, Tokyo, Japan). For scanning electron microscopy (SEM), specimens were post-fixed in 1% OsO4 (Osmium Tetroxide), dehydrated via an ethanol series and acetone, and then critical point dried. The material was coated with gold at about 20 nm and examined using a Hitachi SU8600 scanning electron microscope at an accelerating voltage of 20 kV.

According to the number and distribution pattern of trunk spines, the present specimens were divided into two morphotypes (Figs. 1–3, Table 1). Morphometric comparison of the two different morphotypes of R. cololabis are provided in Table 2. Voucher specimens (morphotype I: HBNU–A–F20250710WL, morphotype II: HBNU–A–F20250711WL) were deposited in the College of Life Sciences, Hebei Normal University, Hebei Province, China.

thumbnail Figure 1

Scanning electron micrographs of two different morphotypes of Rhadinorhynchus cololabis. (A) Morphotype I of male: anterior trunk possessing fewer spines, divided by a distinct aspinose zone into anterior and posterior fields, ventral view; (B) Morphotype I of female: anterior trunk possessing fewer spines, divided by a distinct aspinose zone into anterior and posterior fields, ventral view; (C) Morphotype II of male: anterior trunk possessing more spines, lacking distinct aspinose zone (red circle showing two lateral spines connecting the anterior and posterior fields together), ventral view; (D) Morphotype II of female: anterior trunk possessing more spines, lacking a distinct aspinose zone (red circle showing two lateral spines connecting the anterior and posterior fields together), ventral view.

thumbnail Figure 2

Photomicrographs of morphotype I of Rhadinorhynchus cololabis. (A) Male, lateral view; (B) Female, lateral view; (C) Proboscis of male, lateral view; (D) Egg (isolated from body cavity); (E) Anterior part of male, lateral view.

thumbnail Figure 3

Photomicrographs of morphotype II of Rhadinorhynchus cololabis (proboscis not fully evaginated). (A) Male, lateral view; (B) Female, lateral view; (C) Proboscis of male, lateral view; (D) Egg (isolated from body cavity); (E) Anterior part of male, lateral view.

Table 1

Two morphotypes of Rhadinorhynchus cololabis selected for molecular analysis.

Table 2

Morphometric comparisons of Rhadinorhynchus cololabis. Abbreviations: TL, trunk length; SP, size of proboscis; NRP, number of longitudinal rows of proboscis hooks; NHPR, number of hooks per longitudinal row; SAS, size of anterior trunk spines; SPS, size of posterior trunk spines; SPR, size of proboscis receptacle; SAT, size of anterior testis; SPT, size of posterior testis; LL, length of lemnisci; NCG, number of cement glands; SE, size of eggs. All measurements are in millimetres.

Molecular procedures

A total of 13 specimens representing two morphotypes were selected for molecular analysis (Table 1). The genomic DNA of acanthocephalans was extracted using a Magnetic Universal Genomic DNA Kit (DP705) [Sangon Biotech (Shanghai) Co., Ltd., Shanghai, China], according to the manufacturer’s instructions. The primers and cycling conditions used for amplifying the target sequences of ITS, cox1, cox2, and 12S are provided in Table 3 [15, 22]. PCR products were purified, sequenced, and analyzed according to methods reported previously [9, 27, 50, 51]. The sequence data obtained herein were deposited in GenBank (http://www.ncbi.nlm.nih.gov) under the accession numbers provided in Table 1.

Table 3

Primers and cycling conditions used for amplification of target regions of Rhadinorhynchus cololabis.

Species delimitation and phylogenetic analyses

ASAP analyses were used for species partition of different morphotypes of present samples and 10 species of Rhadinorhynchus, including R. decapteri Parukhin & Kovalenko, 1976, R. dorsoventrospinosus Amin, Heckmann & Ha, 2011, R. hiansi Soota & Bhattacharya, 1981, R. johnstoni Golvan, 1969, R. laterospinosus, R. mariserpentis (Steinauer et al., 2019), R. seriolae (Yamaguti, 1963), R. trachinoti Grano-Maldonado et al., 2025, R. villalobosi Martínez-Flores, García-Prieto & Oceguera-Figueroa, 2025 and Rhadinorhynchus sp., based on the partial nuclear ITS region, and mitochondrial cox1, cox2, and 12S sequence data, respectively. The ASAP analyses were executed using the ASAP online server (https://bioinfo.mnhn.fr/abi/public/asap/) under the Kimura (K80) ts/tv model. Among the recommended results by ASAP, the one with the lowest score was considered to be the optimal result according to the ASAP program [34].

BI analyses were performed to clarify the phylogenetic relationships of the present specimens and Rhadinorhynchus spp. using MrBayes 3.2.7 [36] with two parallel runs (2,000,000 generations) under the optimal models: GTR+F+G4 for cox1, HKY+F+I for cox2, HKY+F+I for 12S, and HKY+F for ITS. Pseudoacanthocephalus sichuanensis Zhao et al., 2024 (Palaeacanthocephala: Echinorhynchida) was treated as the outgroup.

Assembly and annotation of the mitogenome

A total of 50 Gb of clean genomic data were generated using the Pair-End 150 sequencing method on the Illumina NovaSeq 6000 platform by Novogene (Tianjin, China). The methods and procedures used for assembly, annotation and comparative analysis of the complete mitochondrial genome of R. cololabis are according to the previous studies [25, 45, 49–51], using the following software programs or tools, including GetOrganelle v1.7.2a [19], MitoS web server (http://mitos.bioinf.uni-leipzig.de/index.py), MitoZ v3.6, ORF finder web server (https://www.ncbi.nlm.nih.gov/orffinder/), ViennaRNA module [17], MitoS2 [6], RNAstructure v6.3 [35], Codon Adaptation Index (CAI) [24], and CGView online server V1.0 (http://stothard.afns.ualberta.ca/cgview_server/). The mitogenome of R. cololabis was deposited in GenBank (http://www.ncbi.nlm.nih.gov) under accession number PV866798.

Results

Molecular characterization

Partial ITS region

The 13 partial ITS sequences of R. cololabis obtained herein were all 546 bp in length, with no nucleotide divergence detected. There are only two Rhadinorhynchus species with ITS data registered in GenBank. Pairwise comparison of nucleotide differences in the ITS sequences of R. cololabis obtained herein with that of Rhadinorhynchus spp. available in GenBank ranged from 9.60% (R. mariserpentis, MK014834) to 15.3% (R. dorsoventrospinosus, MH384822).

Partial cox1 region

The 13 partial cox1 sequences of R. cololabis obtained herein were all 655 bp in length, representing 13 different genotypes with 0.61–2.44% nucleotide divergence detected. There are 10 Rhadinorhynchus species with cox1 data registered in GenBank. Pairwise comparison of nucleotide differences in the cox1 sequences of R. cololabis with that of Rhadinorhynchus spp. available in GenBank ranged from 5.80% (R. laterospinosus, OR625531) to 26.5% (R. decapteri, KJ590125).

Partial cox2 region

The 13 partial cox2 sequences of R. cololabis obtained herein were all 548 bp in length, representing 13 different genotypes with 0.18–2.55% nucleotide divergence detected. To date, only R. laterospinosus with the cox2 sequence (PV590110) is available in GenBank. Pairwise comparison of nucleotide differences in the cox2 sequences of R. cololabis with that of R. laterospinosus ranged from 3.83% to 4.93%.

Partial 12S region

The 13 partial 12S sequences of R. cololabis obtained herein were all 416 bp in length, representing 10 different genotypes with 0.24–1.68% nucleotide divergence detected. To date, only R. laterospinosus with the 12S sequence (PV590110) is available in GenBank. Pairwise comparison of nucleotide differences in the 12S sequences of R. cololabis with that of R. laterospinosus ranged from 2.21% to 3.19%.

ASAP and BI analyses

The present results of the ASAP analyses of the cox1, cox2, 12S, and ITS sequences all showed R. cololabis representing a distinct species from the other Rhadinorhynchus species (Fig. 4). Additionally, the ASAP result of cox1 data did not support the current species partition of R. seriolae, R. hiansi, and R. dorsoventrospinosus, together with R. villalobosi and R. trachinoti (Fig. 4). All the present BI results revealed R. cololabis forming a separate branch from the other Rhadinorhynchus species, but none of the results supported the current two morphotypes of R. cololabis belonging to distinct genetic lineages (different samples of the two morphotypes of R. cololabis mixed together) (Fig. 5). The BI using cox1 data displayed samples of R. villalobosi nested into R. trachinoti, and R. seriolae and R. dorsoventrospinosus nested into R. hiansi (Fig. 5).

thumbnail Figure 4

Assemble Species by Automatic Partitioning (ASAP) analyses of Rhadinorhynchus spp. conducted based on different nuclear and mitochondrial genetic markers. Pseudoacanthocephalus sichuanensis was chosen as the outgroup. The asterisk (*) indicates the best result according the lowest score and optimal recommendation by ASAP.

thumbnail Figure 5

Bayesian inference (BI) results of Rhadinorhynchus spp. based on different nuclear and mitochondrial genetic markers. Pseudoacanthocephalus sichuanensis was chosen as the outgroup.

Characterization of the mitogenome

The complete mitogenome of R. cololabis is 13,567 bp in size, and includes 36 genes, containing 12 protein-coding genes (PCGs) (cox1–3, cytb, nad1–6, nad4L and atp6; missing atp8), 22 transfer RNA (tRNAs) genes, and 2 ribosomal RNA (rRNA) genes (rrnL is 916 bp, located between tRNA-Tyr and tRNA-Leu1; rrnS is 534 bp, located between tRNA-Met and tRNA-Phe), plus two non-coding regions (NCRs) (NCR1 is 242 bp, located between tRNA-Gln and tRNA-Tyr; NCR2 is 377 bp, located between tRNA-Trp and tRNA-Lys) (Fig. 6, Table 4). All genes are transcribed from the same strand in the same direction. The overall A+T content in the mitogenome of R. cololabis is 63.1%, displaying a strong nucleotide compositional bias toward A+T. The nucleotide content of the R. cololabis mitogenome is provided in Tables 4 and 5.

thumbnail Figure 6

Gene map of mitochondrial genome of Rhadinorhynchus cololabis. All genes are transcribed in the clockwise direction on the same strand.

Table 4

Organization of the mitochondrial genome of Rhadinorhynchus cololabis.

Table 5

Base composition and skewness of Rhadinorhynchus cololabis.

The total size of the 12 PCGs of the present mitogenome is 9,880 bp (excluding stop codons), varied from 237 bp (nad4L) to 1617 bp (nad5) for each gene, which encode 3,292 amino acids. Among the 12 PCGs, four genes (cox1, cox2, nad5, and nad6) use GTG as the start codon, whereas three genes (atp6, nad4, and cytb) use ATG, and the other three genes (nad2, nad3, and nad4L) use TTG. The remaining two genes, cox3 and nad1, use ATT as the start codon. TAG is the most commonly used stop codon for 6 genes (cox1, nad2, nad5, nad6, atp6, and cytb). The remaining three genes (cox2, cox3, and nad4L) use TAA as the stop codon. The incomplete stop codon T is inferred for two genes (nad1 and nad3), while only nad4 uses the incomplete stop codon TA (Table 4). Detailed information on overall codon usage and RSCU for the 12 PCGs is shown in Figure 7. A total of 22 tRNAs were identified with lengths ranging from 51 bp (trnY and trnP) to 66 bp (trnW). The lengths of 22 tRNAs and their anticodon secondary structures are provided in Table 4 and Supplementary file: Figure S1.

thumbnail Figure 7

Relative synonymous codon usage (RSCU) of Rhadinorhynchus cololabis. Codon families (in alphabetical order, from left to right) are provided below the horizontal axis. Values on top of each bar represent amino acid usage in percentage.

The gene arrangement of the 36 genes in the mitogenome of R. cololabis is in the following order: cox1, tRNA-Gly, tRNA-Gln, tRNA-Tyr, rrnL, tRNA-Leu1, nad6, tRNA-Asp, atp6, nad3, tRNA-Trp, tRNA-Lys, tRNA-Glu, tRNA-Thr, tRNA-Ser2, nad4L, nad4, tRNA-Val, tRNA-His, nad5, tRNA-Leu2, tRNA-Pro, cytb, nad1, tRNA-Ile, tRNA-Met, rrnS, tRNA-Phe, cox2, tRNA-Cys, cox3, tRNA-Ala, tRNA-Arg, tRNA-Asn, tRNA-Ser1, and nad2 (Fig. 6 and Supplementary file: Fig. S2).

Discussion

Laurs & McCauley (1964) [23] originally described R. cololabis from C. saira from waters off Oregon, United States, with a description of R. cololabis that is rather good for its time. Subsequently, Motora (2016) [32] found R. cololabis from O. masou in the Sea of Japan. This study is the first record of this species in Chinese waters. The morphology and measurements of our specimens are almost identical to the original description of R. cololabis, including the trunk length, the size, shape and armature of the proboscis, the size of the proboscis receptacle and testis, the shape and size of the trunk spines, the number of cement glands, and the morphology and size of eggs (Table 2). Additionally, the present specimens were also collected from the type host C. saira. Consequently, we have no hesitation in considering the present material being conspecific with R. cololabis.

In this study, striking variability in the number and distribution of trunk spines was observed among different individuals of R. cololabis for the first time. According to the number and distribution of trunk spines (presence or absence of a distinct aspinose zone separating the trunk spines into anterior and posterior fields), the present specimens were divided into two distinct morphotypes, which might be treated as separate taxa under the traditional morphospecies concept. However, molecular analysis of the nuclear ITS region showed no nucleotide divergence among different individuals of the two morphotypes. By contrast, the mitochondrial sequence data displayed a relatively broad range of intraspecific nucleotide divergence among different individuals of R. cololabis (0.61–2.44%, 0.18–2.55%, and 0.24–1.68% detected in the cox1, cox2, and 12S, respectively). Additionally, the ASAP and BI results using different sequence data all revealed that the two distinct morphotypes of R. cololabis are conspecific and do not represent two separate genetic lineages.

Recent molecular phylogenetic studies revealed that R. seriolae, R. hiansi, and R. dorsoventrospinosus have very close relationships within Rhadinorhynchus [2, 21, 31, 40, 48]. However, the present ASAP and BI analyses of cox1 data did not support the current species partition of R. hiansi, R. dorsoventrospinosus, and R. seriolae. The validity of R. hiansi and R. dorsoventrospinosus were challenged. In fact, R. seriolae and R. dorsoventrospinosus both mainly parasitize fishes of the Scombridae and Carangidae, which also share the same or similar distribution regions, and have similar morphological characters, except for different number and distribution of trunk spines [2, 8, 21, 38, 48]. Additionally, some previous studies reported the presence of a broad range of variation in the number and distribution of trunk spines in R. seriolae [21, 38]. Consequently, we speculate that R. dorsoventrospinosus simply represents a particular morphotype of R. seriolae, and suggest to list it as a junior synonym of R. seriolae. However, the type host of R. hiansi is Ablennes hians Valenciennes (Beloniformes: Belonidae), and this species has 21–24 longitudinal alternating rows of 36–48 hooks each, which are distinctly different from those of R. seriolae. The taxonomical status of R. hiansi needs further study in the future.

Grano-Maldonado et al. (2025) [16] described the adult of R. trachinoti from the Gafftopsail pompano Trachinotus rhodopus Gill (Carangiformes: Carangidae) from off the coast of Mazatlán, Mexico and the cystacanth from the mysid crustacean, Metamysidopsis frankfiersi (Hendrickx & Hernández-Payán) from Playa Novillero, Mexico. Later, Martínez-Flores et al. (2025) [31] described R. villalobosi from the same definitive host of R. trachinoti. These authors mentioned that R. villalobosi differs from R. trachinoti on the number of trunk spines, position of the genital pore, and size of the cement glands. However, our ASAP and BI results of cox1 data did not support that R. villalobosi and R. trachinoti represent two distinct species. In fact, the measurements of cement glands provided by Martínez-Flores et al. (2025) [31] seem to be erroneous and are not concordant with the illustration. The differences in the number of trunk spines and the position of genital pores between the two species should belong to intraspecific variation. Consequently, we suggest to treat R. villalobosi as a new synonym of R. trachinoti. The present findings also indicate that the number and distribution of trunk spines vary markedly in some cases of Rhadinorhynchus, and care must be taken when differentiating Rhadinorhynchus species using this feature.

In the Rhadinorhynchidae, only R. laterospinosus had its complete mitochondrial genome sequenced prior to this study [47]. The size of the mitogenomes of R. cololabis and R. laterospinosus are both 13,567 bp, which are the smallest among the reported mitogenomes of Echinorhynchida [10, 11, 33, 39, 41, 44–47, 49–51]. The 36 gene arrangement order of mitogenomes of R. cololabis and R. laterospinosus are also identical, which are different from those of the known mitogenomes of Acanthocephala so far [47]. Comparative mitogenomic analysis of R. laterospinosus and R. cololabis also displayed a low level of divergence in both nucleotide sequences of whole mitogenomes (5.40%) and amino acid sequences of 12 protein-coding genes (PCGs) (6.20%), which are distinctly higher than the intraspecific variation level of mitogenome of Pomphorhynchus pagrosomi (vs nucleotide sequences = 2.40%, amino acid sequences = 1.20%) [51]. Consequently, comparative mitogenomics also supported that R. cololabis and R. laterospinosus belong to two separate taxa.

Conclusions

In this study, we found an example of phenotypic variation in trunk spines of the poorly known rhadinorhynchid species R. cololabis collected from C. saira in the South China Sea. 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 ASAP and BI results using different nuclear and mitochondrial sequence data all confirm that the two distinct morphotypes of R. cololabis are conspecific, and do not represent two separate genetic lineages. Our ASAP and BI results of cox1 data also question the validity of R. villalobosi, R. hiansi, and R. dorsoventrospinosus, and suggest to treat R. villalobosi as a synonym of R. trachinoti. The present findings also indicate that the number and distribution of trunk spines vary markedly in some cases, and care must be taken when differentiating Rhadinorhynchus species using this feature. Additionally, the complete mitogenome of R. cololabis is present for the first time, 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%). The results of ASAP, BI analyses, and comparative mitogenomics all support R. cololabis and R. laterospinosus representing two separate taxa.

Acknowledgments

The authors are grateful to Dr. Xin Gao (Chinese Research Academy of Environmental Sciences, China) for collecting and identifying fish hosts.

Funding

This study was supported by the National Natural Science Foundation of China (Grant No. 31872197) and the National Key R&D Program of China (Grant No. 2022YFC2601200).

Conflicts of interest

The authors declare that they have no competing interests.

Data availability statement

The datasets generated in the present study are available in the GenBank repository.

Author contribution statement

K.-Y. W. and L. L. contributed to the study design and species identification. K.-Y. W. and H.-X. C. collected acanthocephalan specimens. K.-Y. W., Y.-Y. X., and L. L. sequenced and analyzed genetic data, annotated the mitogenome, and conducted the ASAP and BI analyses. K.-Y. W., Y.-Y. X., and L. L. wrote the manuscript. All authors read and approved the final manuscript.

Supplementary materials

thumbnail Figure S1:

The predicted secondary structures of 22 tRNAs in the mitogenome of Rhadinorhynchus cololabis (Watson-Crick bonds indicated by lines, GU bonds indicated by dots, grey bold bases representing anticodons). The tRNAs are labelled with the abbreviations of their corresponding amino acids according to IUPAC-IUB code.

thumbnail Figure S2:

Comparison of the linearized gene arrangement of acanthocephalan mitogenomes. All genes are transcribed in the same direction from left to right. The tRNAs are labelled by a single-letter code for the corresponding amino acid. The non-coding regions are not included. Rhadinorhynchus cololabis indicated using an asterisk (*).

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Cite this article as: Wang K-Y, Xie Y-Y, Chen H-X & Li L. 2025. Molecular evidence on phenotypic variation in the poorly known acanthocephalan species Rhadinorhynchus cololabis Laurs & McCauley, 1964 (Echinorhynchida: Rhadinorhynchidae). Parasite 32, 73. https://doi.org/10.1051/parasite/2025069.

All Tables

Table 1

Two morphotypes of Rhadinorhynchus cololabis selected for molecular analysis.

In the text
Table 2

Morphometric comparisons of Rhadinorhynchus cololabis. Abbreviations: TL, trunk length; SP, size of proboscis; NRP, number of longitudinal rows of proboscis hooks; NHPR, number of hooks per longitudinal row; SAS, size of anterior trunk spines; SPS, size of posterior trunk spines; SPR, size of proboscis receptacle; SAT, size of anterior testis; SPT, size of posterior testis; LL, length of lemnisci; NCG, number of cement glands; SE, size of eggs. All measurements are in millimetres.

In the text
Table 3

Primers and cycling conditions used for amplification of target regions of Rhadinorhynchus cololabis.

In the text
Table 4

Organization of the mitochondrial genome of Rhadinorhynchus cololabis.

In the text
Table 5

Base composition and skewness of Rhadinorhynchus cololabis.

In the text

All Figures

thumbnail Figure 1

Scanning electron micrographs of two different morphotypes of Rhadinorhynchus cololabis. (A) Morphotype I of male: anterior trunk possessing fewer spines, divided by a distinct aspinose zone into anterior and posterior fields, ventral view; (B) Morphotype I of female: anterior trunk possessing fewer spines, divided by a distinct aspinose zone into anterior and posterior fields, ventral view; (C) Morphotype II of male: anterior trunk possessing more spines, lacking distinct aspinose zone (red circle showing two lateral spines connecting the anterior and posterior fields together), ventral view; (D) Morphotype II of female: anterior trunk possessing more spines, lacking a distinct aspinose zone (red circle showing two lateral spines connecting the anterior and posterior fields together), ventral view.
In the text
thumbnail Figure 2

Photomicrographs of morphotype I of Rhadinorhynchus cololabis. (A) Male, lateral view; (B) Female, lateral view; (C) Proboscis of male, lateral view; (D) Egg (isolated from body cavity); (E) Anterior part of male, lateral view.
In the text
thumbnail Figure 3

Photomicrographs of morphotype II of Rhadinorhynchus cololabis (proboscis not fully evaginated). (A) Male, lateral view; (B) Female, lateral view; (C) Proboscis of male, lateral view; (D) Egg (isolated from body cavity); (E) Anterior part of male, lateral view.
In the text
thumbnail Figure 4

Assemble Species by Automatic Partitioning (ASAP) analyses of Rhadinorhynchus spp. conducted based on different nuclear and mitochondrial genetic markers. Pseudoacanthocephalus sichuanensis was chosen as the outgroup. The asterisk (*) indicates the best result according the lowest score and optimal recommendation by ASAP.
In the text
thumbnail Figure 5

Bayesian inference (BI) results of Rhadinorhynchus spp. based on different nuclear and mitochondrial genetic markers. Pseudoacanthocephalus sichuanensis was chosen as the outgroup.
In the text
thumbnail Figure 6

Gene map of mitochondrial genome of Rhadinorhynchus cololabis. All genes are transcribed in the clockwise direction on the same strand.
In the text
thumbnail Figure 7

Relative synonymous codon usage (RSCU) of Rhadinorhynchus cololabis. Codon families (in alphabetical order, from left to right) are provided below the horizontal axis. Values on top of each bar represent amino acid usage in percentage.
In the text
thumbnail Figure S1:

The predicted secondary structures of 22 tRNAs in the mitogenome of Rhadinorhynchus cololabis (Watson-Crick bonds indicated by lines, GU bonds indicated by dots, grey bold bases representing anticodons). The tRNAs are labelled with the abbreviations of their corresponding amino acids according to IUPAC-IUB code.
In the text
thumbnail Figure S2:

Comparison of the linearized gene arrangement of acanthocephalan mitogenomes. All genes are transcribed in the same direction from left to right. The tRNAs are labelled by a single-letter code for the corresponding amino acid. The non-coding regions are not included. Rhadinorhynchus cololabis indicated using an asterisk (*).
In the text

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