Intrigue surrounding the life-cycles of species of Clistobothrium (Cestoda: Phyllobothriidea) parasitising large pelagic sharks

Parasite assemblages in volatile host stocks: inter- and intra-cohort variability restrict their value as biological tags for squid stock assessment

The Argentine shortfin squid, Illex argentinus, inhabits in the southwest Atlantic; it is a semelparous species which grows rapidly along its 1 year lifespan. The identification of its stocks is critical for sustainable fishery exploitation. Parasites have been used as biological indicators in a lower number of studies dealing with squids, therefore a validation of this methodology is necessary. The intra- and inter-cohort variability of parasite assemblages in the summer-spawning stock of I. argentinus was analysed to assess their value as indicators of stock structure. Four squid samples from the continental shelf of central Patagonia, corresponding to 3 consecutive cohorts, were examined for metazoan parasites. Results evidenced heterogeneity in terms of parasite assemblage composition and structure, dominated by short-lived gastrointestinal parasites, with a strong influence of host size, but no effect of squid sex. These changes are related to their recent habitats and diets, which change with ontogeny and migrations, clouding any interpretation of patterns when samples spatially or temporally separated are compared. Many squid species share these characteristics; therefore, it is recommended that the use of parasites as biological tags should be restricted to simultaneous sampling, while size or age must be considered for deriving proper conclusions.

Larvae of Clistobothrium grimaldii (Cestoda: Phyllobothriidea) from a Cape fur seal (Arctocephalus pusillus pusillus) kept in a zoo in Japan

The larval form of the Phyllobothriidea cestode was found in the blubber of a Cape fur seal (Arctocephalus pusillus pusillus) from a zoo in Japan. Bladder-bearing larval cestodes with a scolex have been occasionally reported from blubbers of pinnipeds and morphologically identified as Clistobothrium delphini (formerly known as Phyllobothrium delphini) or rarely Clistobothrium grimaldii (Monorygma grimaldii). Although the larvae here morphologically resembled C. delphini, the 28S rDNA sequence was 100% (1,430/1,430 bp) homologous to the registered sequence of C. grimaldii (GenBank Accession No. KU724058). This discrepancy between morphological and molecular analyses confirms the difficulty of identifying C. delphini and C. grimaldii larvae based solely on morphology, and the need for molecular data to elucidate the morphological variations in Clistobothrium parasites.

Parasites and prey of the nursehound shark Scyliorhinus stellaris (Linnaeus, 1758): Insights into hidden trophic web interactions in the Mediterranean Sea

The metazoan parasite community and the stomach contents of the nursehound shark Scyliorhinus stellaris from the Gulf of Naples (central Mediterranean Sea) were studied. The nursehound shark harboured a poor parasite community composed of a species of gill monogenean (Hexabothrium appendiculatum) and three intestinal cestode taxa (Acanthobothrium coronatum and two unidentified species of the genera Yamaguticestus and Scyphophyllidium), all represented by adult stages. Hosts were mostly parasitized by individuals of A. coronatum, which was the most abundant species and contributed to almost 80% of the total number of parasites found. Conversely, other trophically transmitted parasites (i.e., Yamaguticestus sp. and Scyphophyllidium sp.) showed low prevalence and abundance. The parasite infracommunity was poor, showing low values of species richness, total mean abundance, and diversity indices. Overall, 52 prey items belonging to 13 taxa were identified in the stomach contents. Cephalopods were the most important prey items (represented by nine taxa) and the most diverse and abundant group. In the multivariate space provided by a principal component of mixed data (PCAmix), nursehound sharks distributed along two main axes, related to individual traits (first axis) and stomach contents (including empty ones, second axis). A logistic regression based on the first two axes of the PCAmix showed a significant influence of host individual traits and, to a lesser extent, of stomach contents, regarding the probability of being infected by A. coronatum. Alongside specific traits already associated with parasites transmission, our results highlight the importance of cephalopods in transferring cestode infections through trophic web interactions in the top-predator nursehound shark.

Fish tapeworms (Cestoda) in the molecular era: achievements, gaps and prospects

The tapeworms of fishes (Chondrichthyes and Actinopterygii) account one-third (1670 from around 5000) of the total tapeworm (Platyhelminthes: Cestoda) species diversity. In total 1186 species from 9 orders occur as adults in elasmobranchs (sharks, rays and chimaeras), and 484 species from 8 orders mature in ray-finned fishes (referred to here as teleosts). Teleost tapeworms are dominated by freshwater species (78%), but only 3% of elasmobranch tapeworms are known from freshwater rays of South America and Asia (Borneo). In the last 2 decades, vast progress has been made in understanding species diversity, host associations and interrelationships among fish tapeworms. In total, 172 new species have been described since 2017 (149 from elasmobranchs and 23 from teleosts; invalidly described taxa are not included, especially those from the Oriental region). Molecular data, however, largely limited to a few molecular markers (mainly 28S rDNA, but also 18S and cox1), are available for about 40% of fish tapeworm species. They allowed us to significantly improve our understanding of their interrelationships, including proposals of a new, more natural classification at the higher-taxonomy level (orders and families) as well as at the lower-taxonomy level (genera). In this review, we summarize the main advances and provide perspectives for future research.

A synergistic, global approach to revising the trypanorhynch tapeworm family Rhinoptericolidae (Trypanobatoida)

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.

Molecular Identification of Plerocercoids of Clistobothrium montaukensis (Cestoda: Phyllobothriidea) Parasitizing the King of Herrings Regalecus glesne

PURPOSE

Endo-parasites of the bathypelagic king of herrings Regalecus glesne and oarfish Regalecus russelii are only known from few specimens opportunistically examined. As a consequence, there are few records of parasites from either Regalecus species. We report plerocercoid larvae of phyllobothriidean cestodes parasitizing an adult R. glesne stranded in Bahía de La Paz, Baja California Sur, Mexico.

METHODS

Sixty-three plerocercoids were obtained from the intestine of R. glesne and characterized using morphological and molecular methods (nuclear 28S rDNA and mitochondrial cytochrome c oxidase I gene sequences).

RESULTS

Following the morphological diagnostic criteria of scolex and muscle bands in the strobila, plerocercoids specimens were preliminary assigned to the genus Clistobothrium. Mitochondrial and nuclear DNA sequences indicate these plerocercoids correspond to Clistobothrium montaukensis Ruhnke, 1993.

CONCLUSION

Regalecus glesne is a new host known for C. montaukensis and this report is a new geographical record of C. montaukensis parasitizing species of the genus Regalecus previously known only from California and Florida, USA.

A revision of the Rhoptrobothriidae (Cestoda: Tetraphyllidea)

The Rhoptrobothriidae are one of the more enigmatic families of cestodes of elasmobranchs. Opinions on the taxonomic status of the familys three original genera (i.e., Myzophyllobothrium, Rhoptrobothrium, and Myzocephalus) have varied over the 115 years since they were erected. Some authors have considered all three valid, others have considered Rhoptrobothrium to be a synonym of Myzopyllobothrium or a genus inquirendum, yet others have considered Myzocephalus to be a synonym of the phyllobothriid genus Thysanocephalum. All three genera were established for specimens collected from eagle rays off Sri Lanka. The erection of Mixophyllobothrium for two specimens from a cowtail stingray off India three decades ago added additional confusion to the situation, with some authors considering it valid and others a synonym of Myzocephalus. These disagreements stem largely from differences in interpretation of the complex morphology of the scolex of members of these genera. Furthermore, with the exception of Rhoptrobothrium comprising four species, each genus is monotypic. All but Rhoptrobothrium has not been considered in detail for nearly a century, largely because of a lack of available material. The taxonomic status of these genera is assessed here based on light and scanning electron microscopy, and molecular data generated from new material collected from eagle rays off Indonesian and Malaysian Borneo, Japan, Sri Lanka, and Viet Nam. Morphological work indicates that the genera differ largely only in the degree of folding of the four remi that extend from the cephalic peduncle. A molecular phylogeny based on sequence data for the D1D3 region of the 28S rRNA gene, which include new data for eight specimens of four species, indicates that Myzophyllobothrium, Myzocephalus, and Rhoptrobothrium are not mutually monophyletic. The latter two genera and Mixophyllobothrium are considered synonyms of Myzophyllobothrium and five species are transferred to that genus. Myzophyllobothrium okamuri n. comb. is considered a species inquirendum. Myzophyllobothrium nagasawai n. sp. is described from Aetobatus narutobiei off Japan. Myzophyllobothrium narinari n. comb. is re-described based on newly collected cestodes from the type host and locality (i.e., Aetobatus ocellatus off Sri Lanka). Despite consisting of only a single genus, the family status of the group is retained in recognition of the unusual configuration of the scolex, which bears four biloculate bothridia and four remi extending from the cephalic peduncle. The ordinal placement of the family remains uncertain, but affinities with the Phyllobothriidea, rather than Tetraphyllidea are considered.