OBJECTIVE: Molecular phylogenetics has been improving the acanthocephalan systematics, yet numerous taxa remain unexplored. The palaeacanthocephalan Metarhadinorhynchus Yamaguti, 1959 and its type species M. lateolabracis Yamaguti, 1959 are such to-be-explored taxa. We aim to refine (i) the systematic placement, (ii) the morphological circumscription, and (iii) the taxonomic components of the genus. We also aim to examine the taxonomic status of the species that have been assigned to the genus.
METHODS: Morphological observations on newly collected specimens as well as the type material of Metarhadinorhynchus lateolabracis were conducted. Also, molecular phylogenetic analyses with maximum-likelihood method and Bayesian inference were performed based on freshly collected specimens. Nominal species that have at least once been assigned in Metarhadinorhynchus, as well as a related form, Gorgorhynchus lateolabri Yin and Wu, 1984, are taxonomically re-evaluated based on literature information.
RESULTS: Our re-examination of the type material of M. lateolabracis revealed that the number of cement glands is six, instead of eight as described and illustrated in the original description. In the resulting phylogenetic tree, M. lateolabracis was nested in Isthmosacanthidae. Gorgorhynchoides Cable and Linderoth, 1963 was found to be a junior synonym of Metarhadinorhynchus. Taxonomic re-evaluations of six nominal species that have once belonged in Metarhadinorhynchus led to modifications of generic diagnoses for Indorhynchus Golvan, 1969 and Neotegorhynchus Lisitsyna, Xi, Orosová, Barčák, and Oros, 2022.
CONCLUSIONS: Metarhadinorhynchus has been assigned to Leptorhynchoididae (Echinorhynchida), but our study now locates it in Isthmosacanthidae (Polymorphida). We propose 13 new combinations of specific names in Metarhadinorhynchus and three in Indorhynchus. Metarhadinorhynchus lateolabri (Yin and Wu, 1984) comb. nov. may be synonymous with M. orientalis (Wang, 1966) comb. nov.

BACKGROUND: Classification of the Acanthocephala, a clade of obligate endoparasites, remains unresolved because of insufficiently strong resolution of morphological characters and scarcity of molecular data with a sufficient resolution. Mitochondrial genomes may be a suitable candidate, but they are available for a small number of species and their suitability for the task has not been tested thoroughly.
METHODS: Herein, we sequenced the first mitogenome for the large family Rhadinorhynchidae: Micracanthorhynchina dakusuiensis. These are also the first molecular data generated for this entire genus. We conducted a series of phylogenetic analyses using concatenated nucleotides (NUC) and amino acids (AAs) of all 12 protein-coding genes, three different algorithms, and the entire available acanthocephalan mitogenomic dataset.
RESULTS: We found evidence for strong compositional heterogeneity in the dataset, and Micracanthorhynchina dakusuiensis exhibited a disproportionately long branch in all analyses. This caused a long-branch attraction artefact (LBA) of M. dakusuiensis resolved at the base of the Echinorhynchida clade when the NUC dataset was used in combination with standard phylogenetic algorithms, maximum likelihood (ML) and Bayesian inference (BI). Both the use of the AA dataset (BI-AAs and ML-AAs) and the CAT-GTR model designed for suppression of LBA (CAT-GTR-AAs and CAT-GTR-NUC) at least partially attenuated this LBA artefact. The results support Illiosentidae as the basal radiation of Echinorhynchida and Rhadinorhynchidae forming a clade with Echinorhynchidae and Pomporhynchidae. The questions of the monophyly of Rhadinorhynchidae and its sister lineage remain unresolved. The order Echinorhynchida was paraphyletic in all of our analyses.
CONCLUSIONS: Future studies should take care to attenuate compositional heterogeneity-driven LBA artefacts when applying mitogenomic data to resolve the phylogeny of Acanthocephala.

The phylum Acanthocephala is a small group of endoparasites occurring in the alimentary canal of all major lineages of vertebrates worldwide. In the present study, the complete mitochondrial (mt) genome of Cavisoma magnum (Southwell, 1927) (Palaeacanthocephala: Echinorhynchida) was determined and annotated, the representative of the family Cavisomidae with the characterization of the complete mt genome firstly decoded. The mt genome of this acanthocephalan is 13,594 bp in length, containing 36 genes plus two non-coding regions. The positions of trnV and SNCR (short non-coding region) in the mt genome of C. magnum are different comparing to those of the other acanthocephalan species available in GenBank. Phylogenetic analysis based on amino acid sequences of 12 protein-coding genes using Bayesian inference (BI) supported the class Palaeacanthocephala and its included order Polymorphida to be monophyletic, but rejected monophyly of the order Echinorhynchida. Our phylogenetic results also challenged the validity of the genus Sphaerirostris (Polymorphida: Centrorhynchidae). The novel mt genomic data of C. magnum are very useful for understanding the evolutionary history of this group of parasites and establishing a natural classification of Acanthocephala.

In the present study, Neorhadinorhynchus nudus (Harada, 1938) is reported from the frigate tuna Auxis thazard (Lacepéde) (Perciformes: Scombridae), in the South China Sea for the first time. The detailed morphology of N. nudus was studied using light and scanning electron microscopy based on the newly collected material. The results showed some morphometric variability between our specimens and previous studies, including the number of hooks per longitudinal row and the size of copulatory bursa and eggs. Our SEM observations also revealed all proboscis hooks emerged from elevated round rims on proboscis surface. In addition, N. nudus was firstly characterised using molecular methods by sequencing and analysing the ribosomal ITS and mitochondrial cox1 regions. There is no nucleotide divergence found in the ITS sequences, but a low level of nucleotide variability detected in the cox1 regions (the level of intraspecific nucleotide variability being 0.75% to 2.54%). The DNA sequence data obtained herein will indeed be a useful reference for rapid and accurate species identification of Neorhadinorhynchus.