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Abstract Theory predicts relaxed host specificity and high host vagility should contribute to reduced genetic structure in parasites while strict host specificity and low host vagility should increase genetic structure. Though these predictions are intuitive, they have never been explicitly tested in a population genomic framework. Trypanorhynch tapeworms, which parasitize sharks and rays (elasmobranchs) as definitive hosts, are the only order of elasmobranch tapeworms that exhibit considerable variability in their definitive host specificity. This allows for unique combinations of host use and geographic range, making trypanorhynchs ideal candidates for studying how these traits influence population-level structure and genetic diversity. Multiplexed shotgun genotyping (MSG) data sets were generated to characterize component population structure and infrapopulation diversity for a representative of each trypanorhynch suborder: the ray-hosted Rhinoptericola megacantha (Trypanobatoida) and the shark-hosted Callitetrarhynchus gracilis (Trypanoselachoida). Adults of R. megacantha are more host-specific and less broadly distributed than adults of C. gracilis, allowing correlation between these factors and genetic structure. Replicate tapeworm specimens were sequenced from the same host individual, from multiple conspecific hosts within and across geographic regions, and from multiple definitive host species. For R. megacantha, population structure coincided with geography rather than host species. For C. gracilis, limited population structure was found, suggesting a potential link between degree of host specificity and structure. Conspecific trypanorhynchs from the same host individual were found to be as, or more, genetically divergent from one another as from conspecifics from different host individuals. For both species, high levels of homozygosity and positive FIS values were documented. 

The rhinebothriidean tapeworm family Escherbothriidae has recently been expanded to include the genus Ivanovcestus, species of which parasitise arhynchobatid skates. Similarities in morphology and host associations between Ivanovcestus and Semiorbiseptum – a genus yet to be assigned to one of the families in the order Rhinebothriidea – led us to explore the possibility that Semiorbiseptum might also belong in the Escherbothriidae. Morphological similarities with Scalithrium ivanovae, Scalithrium kirchneri and Rhinebothrium scobinae, all of which also parasitise arhynchobatid skates, raised questions regarding the generic placements of these species. In addition, new collections from the skate Sympterygia brevicaudata revealed two new species that morphologically resemble species of Ivanovcestus. A combination of morphological and molecular data were used to assess the generic placement of the newly discovered species and refine our understanding of the membership of the family Escherbothriidae. Sequence data for the D1–D3 region of the 28S rDNA gene were generated de novo for 14 specimens of 7 rhinebothriidean species and combined with comparable published data to represent all 6 families in the Rhinebothriidea in the analysis. The phylogenetic tree resulting from maximum likelihood analysis strongly supports the inclusion of the genus Semiorbiseptum in the family Escherbothriidae. Our work also suggests that the skate-hosted species previously assigned to Scalithrium and Rhinebothrium are also members of Semiorbiseptum and that Ivanovcestus is a junior synonym of Semiorbiseptum. Six species are transferred to Semiorbiseptum, bringing the total number of species in the genus to ten. The diagnosis of Semiorbiseptum is amended to accommodate the additional species. A second species in the previously monotypic type genus of the family, Escherbothrium, is described. The diagnosis of the Escherbothriidae is amended to include the new and transferred species. This study underscores the importance of integrating morphological and molecular data in bringing resolution to cestode systematics. We believe our findings provide a robust foundation for future research into the evolutionary history and host associations of cestodes within the order Rhinebothriidea and beyond. These also highlight the importance of expanding our understanding of skate-hosted cestodes. ZooBank: urn:lsid:zoobank.org:pub:8052AFCA-5FBD-4430-95F4-0E5E368DEA3D 

This paper aims to expand understanding of a poorly known group of cestodes that parasitize an intriguingly diverse suite of elasmobranchs. The group’s three currently described members (i.e., Pentaloculum macrocephalum, Pentaloculum hoi, and Zyxibothrium kamienae) parasitize an electric ray, a carpet shark, and a skate, respectively. Pentaloculum grahami n. sp. is described from a second genus of carpet shark, specifically Parascyllium collare, in Australia. Zyxibothrium duffyi n. sp. and Zyxibothrium healyae n. sp. are described from the deep-sea skates Brochiraja asperula and Brochiraja spinifera, respectively off New Zealand. The three new species share distinctive bothridia that bear a small number of large, circular, facial loculi and lateral bands of vitelline follicles that converge posterior to the ovary—features which are found in all other members of these genera. Zyxibothrium healyae n. sp. is unique in possessing three, rather than four or five, facial loculi. Zyxibothrium duffyi n. sp. possesses a combination of five facial loculi and vitelline follicles that stop short of the anterior margin of the proglottid. Pentaloculum grahami n. sp. is the largest member of the group with the greatest number of proglottids. Based on striking similarities in scolex morphology, Pentaloculum and Zyxibothrium have been hypothesized to belong to a distinct subgroup of “tetraphyllideans” provisionally designated as Clade 1. Based on sequence data for the D1–D3 region of the 28S rDNA gene generated for species of Zyxibothrium for the first time, we confirm the reciprocal monophyly of both genera as well as the monophyly of Clade 1 and its status as a distinct lineage among the “Tetraphyllidea”. This work also suggests that the presence of five facial loculi is homoplasious given this character state is found in members of both genera. The new species expand the host associations of Clade 1 to include additional skate and carpet shark genera. Moving forward we would expect to find additional members of this group parasitizing other species of parascyliid carpet sharks as well as other species of the rajid genus Malacoraja and the arhynchobatid genus Brochiraja. Here we have doubled the number of described species in the taxon referred to as Clade 1 while simultaneously expanding our understanding of the morphology and anatomy of its members. This additional information will help inform the ultimate revision of the ordinal classification of the cestodes to address the highly polyphyletic nature of the order “Tetraphyllidea” as it is currently configured. 

The three members of the lecanicephalidean tapeworm family Eniochobothriidae are unusual among tapeworms in that they lack a vagina and possess a series of expanded proglottids forming a trough at the anterior end of their body. They exclusively parasitise cownose rays of the genus Rhinoptera (Myliobatiformes: Rhinopteridae). New collections from six of the nine known species of cownose rays from the waters off Australia, Mexico, Mozambique, Senegal, Taiwan and the United States (off Mississippi, Louisiana and South Carolina) yielded eight new species and a new genus of eniochobothriids. Here we erect Amiculucestus, gen. nov. and describe six of the eight new species – four in the new genus and two in Eniochobothrium – expanding the number of genera in the family to two and the number of described species in the family to nine. Morphological work was based on light and scanning electron microscopy. The tree resulting from a maximum likelihood analysis of sequence data for the D1–D3 region of the 28S rDNA gene for 11 species of eniochobothriids supports the reciprocal monophyly of both genera. The mode of attachment to the mucosal surface of the spiral intestine of the host was investigated using histological sections of worms in situ. These cestodes appear to use the anterior trough-like portion of their body, which consists of an unusual series of barren proglottids, rather than their scolex, to attach to the mucosal surface. Based on our new collections, we estimate that the total number of eniochobothriids across the globe does not exceed 27 species. ZooBank LSID: urn:lsid:zoobank.org:pub:0740EC72‐AC3F‐43AA‐BD41‐B9820BA9D0CE 

Since 2010, the trypanorhynch tapeworm family Rhinoptericolidae Carvajal & Campbell, 1975 has housed just two distinctive, monotypic genera ( Rhinoptericola Carvajal & Campbell, 1975 and Nataliella Palm, 2010). However, global collections of tapeworms from sharks and rays over the last more than three decades brought to light the need for major revision of the family by suggesting a much greater species-level diversity for the nominal genus Rhinoptericola . Through synonymy and the description of new species, the number of species in the genus is increased from one to eight. A phylogenetic analysis of the D1–D3 gene region of 28S rRNA (28S), including seven of the now nine species of rhinoptericolids, and a broad sampling of the other Trypanobatoida is the first to recover a monophyletic Rhinoptericolidae. In addition to systematic revision, this study allowed for the first evaluation of the degree of intraspecific vs interspecific variation in 28S for adult trypanorhynchs across the various hosts and geographic localities from which they have been reported, suggesting a relatively consistent boundary for Rhinoptericola . It is further suggested that detailed scanning electron microscopy (SEM) images of both the basal and metabasal armatures greatly aid in the interpretation of hook arrangement and shape. A schematic to streamline determination of the tentacular surface presented in scanning electron micrographs and line drawings of trypanorhynchs is presented for species with both two and four bothria. In combination, these methodological refinements can now be used as a model to resolve issues of classification and non-monophyly within both major lineages of the Trypanorhyncha. As a result of the taxonomic work, Rhinoptericola megacantha Carvajal & Campbell, 1975 (previously only known from the American cownose ray from the Chesapeake Bay and the Ticon cownose ray from the Gulf of Mexico, Venezuela, and Brazil) is now known from an additional species of cownose ray and a species of stingray, and is revealed to have a transatlantic distribution. Data from SEM suggest a simpler interpretation of hook arrangement in the metabasal armature for Rhinoptercola and—in combination with 28S sequence data—support Shirleyrhynchus Beveridge & Campbell, 1988 (a former rhinoptericolid) as its junior synonym. The three species formerly assigned to Shirleyrhynchus are thus transferred to Rhinoptericola . Data from light microscopy on whole-mounted specimens and histological sections, SEM, and 28S showed the eutetrarhynchid Prochristianella jensenae Schaeffner & Beveridge, 2012b to be morphologically consistent with species of Rhinoptericola and it is thus transferred to the genus. The type series of P. jensenae was determined to be mixed, representing two distinct species which are here redescribed and described as new, respectively. Two additional novel species of Rhinoptericola are described from cownose rays from off Mozambique and the Gulf of California. 

Abstract Three new species of ‘tetraphyllideans’ in the family Serendipeidae are described from cownose rays off Senegal. Duplicibothrium jeannettae n. sp. and Duplicibothrium collosum n. sp. parasitize both Rhinoptera marginata and Rhinoptera peli . Duplicibothrium jillae n. sp. parasitizes R. marginata . Duplicibothrium jeannettae n. sp. and D. jillae n. sp. differ from one another and their described congeners in size, number of proglottids, and locular configuration. The generic assignment of D. collosum n. sp. was initially unclear because its scolex more closely resembles that of Serendip than Duplicibothrium . To help inform generic placement, sequence data were generated for the 28S rDNA gene (D1–D3 region) for the three new species as well as for Duplicibothrium minutum and Duplicibothrium n. sp. 2 from Rhinoptera bonasus off North Carolina, Duplicibothrium n. sp. 3 from Rhinoptera brasiliensis off Belize, Duplicibothrium n. sp. 4 and Duplicibothrium n. sp. 5 from Rhinoptera jayakari off Mozambique, and Duplicibothrium n. sp. 6 from Rhinoptera neglecta off Australia. Comparable data were obtained from GenBank for adults and larvae of Duplicibothrium collected from the Gulf of Mexico. The tree resulting from a maximum likelihood analysis (MLA) placed D. collosum n. sp. robustly within Duplicibothrium ; the generic diagnosis is emended accordingly. This raises a question regarding the independence, and thus also validity, of the three genera of the Serendipeidae – a question that must await a molecular analysis that includes Serendip and Glyphobothrium . These results extend the hosts of Duplicibothrium to include R . brasiliensis , R . jayakari , R . marginata , R . neglecta and R . peli , and the distribution to include the western Pacific Ocean, eastern Atlantic Ocean, and western Indian Ocean (Zoobank Registration: 97BB5020-BFFF-4FEA-AE07-B4711D1110FC).