Subcutaneous merocercoids of Clistobothrium sp. in two Cape fur seals (Arctocephalus pusillus pusillus)

Abstract

Fur seals represent intermediate hosts of the cestode Clistobothrium. Large sharks are definitive hosts for these parasites. Two female, 25– and 27-year-old fur seals, caught in the 1980s at the South African coast, were examined pathomorphologically. Both animals showed multifocal, up to 1 cm in diameter large cavities of the thoracic and abdominal subcutaneous adipose tissue containing intraluminal metacestodes of tapeworms, which were surrounded by a locally extensive, pyogranulomatous panniculitis. The metacestodes (merocercoids) of one fur seal were isolated from the subcutaneous adipose tissue and characterized morphologically and for the first time from this host by molecular techniques. The morphometric data corresponded with ‘delphini'-morphotype merocercoids, but the sequence of the partial 28S ribosomal RNA gene identified them as conspecific with merocercoids of the morphotype ‘grimaldii’. These merocercoid types are morphologically Type XV metacestodes of marine tapeworms and represent different species of Clistobothrium. Sequence data were generated for 18S, ITS1, 5.8S, ITS2, partial 28S ribosomal DNA and partial mitochondrial cox1 gene and phylogenetic analysis of 18S rRNA and partial 28S rRNA genes identified the fur seal merocercoids as Clistobothrium species. However, it cannot yet be assigned to species level because of limited molecular data from adult stages. Most likely, both fur seals were infected as juveniles in their original habitat, the coastal regions of South Africa. The metacestode infection is probably an incidental finding, however, there is a chronic inflammatory reaction next to the subcutaneous merocercoids. It is noteworthy, that the merocercoids remain in a potentially infective stage even after more than 20 years.

Molecular Characterization of Clistobothrium sp. Viable Plerocercoids in Fresh Longfin Inshore Squid (Doryteuthis pealeii) and Implications for Cephalopod Inspection

“…The LSU was selected as elective target for the analysis, as it is the most used molecular marker for identifying Cestoda [ 3 , 26 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ], sometimes in combination with the small subunit (SSU) rRNA gene [ 39 , 40 , 41 ]. Although Olson et al [ 40 ] indicated that the D2 region of the LSU gene exhibited sufficient variability to be useful for species-level identification, a low interspecies variability has been reported by other authors [ 35 ], and other genes, such as COI and ITS, were proposed as alternative markers for distinguishing closely related species of Phyllobothriidae [ 35 , 42 , 43 ]. Thus, the COI was used as additional target to better assess inter and intra-specific variability.…”

“…Thus, the COI was used as additional target to better assess inter and intra-specific variability. However, reference sequences for this target region are scarce [ 35 ], as it can be observed in Table S1 , where 427 sequences were retrieved from databases for the LSU, while only 72 sequences were available for the COI gene. A higher taxonomic coverage of LSU in comparison with COI is factually evident ( Table S1 ).…”

“…Interestingly, Pardo Gandarillas et al [ 17 ] found an unidentified Tetraphyllidea plerocercoid from the jumbo flying squid Dosidicus gigas morphologically very similar to the larvae found in this study and stated that the presence of an apical sucker-like structure, accessory sucker on each bothria and the folded and curled bothrial shape resembled Phyllobothrium tumidum (former name of C. tumidum ) described by Stunkard [ 31 ]. In the work of Klotz et al [ 35 ], genetically identified Clistobothrium sp. merocercoids were found in seals, and tentatively attributed to C. tumidum on the basis of bothridial morphology.…”

“…merocercoids were found in seals, and tentatively attributed to C. tumidum on the basis of bothridial morphology. However, further molecular analysis, ideally investigating also adult specimens, would be needed [ 35 ], also considering that the taxonomy of tetraphyllidean and phyllobothridean has undergone major revision [ 34 , 38 , 46 ]. In fact, the Phyllobothriidae family, which was traditionally included in the Tetraphyllidea order, was recently elevated to ordinal status [ 25 , 30 ].…”

A Perspective Around Cephalopods and Their Parasites, and Suggestions on How to Increase Knowledge in the Field

“…is the most frequently reported species ( Hochberg, 1990 ). A general life cycle for Phyllobothriidae has recently been suggested ( Klotz et al, 2018 ): procercoid development occurs in crustaceans (first intermediate hosts), followed by plerocercoid development in bony fish, sea turtle or squid (second intermediate host). Marine mammals can harbor both plerocercoids and merocercoids, acting as third intermediate or paratenic hosts, and sharks serve as the definitive hosts, harboring the adult parasites.…”