Hepatoxylon trichiuri (Cestoda: Trypanorhyncha) plerocercoids in cage-reared northern bluefin tuna, Thunnus thynnus (Osteichthyes: Scombridae)
Northern bluefin tuna (Thunnus thynnus) were caught by purse seine boats and brought to a farm for a rearing cycle of 1.5 years. Mortalities occurred in the first weeks of acclimatization at the facility. Parasitological examination revealed plerocercoids of the cestode Hepatoxylon trichiuri embedded in the stomach mucosa. The larvae were found in 28.4% of the fish and the mean abundance was 3.12 per fish. Histopathology revealed disseminated erosion at the site of attachment, atrophy of fundic glands, lymphocytic migration, and hyperplasia of connective tissue in the lamina propria. These symptoms, however, together with the parasite abundance, could not have triggered the mortalities. This is the first record of this parasite in northern bluefin tuna.
Does host diet govern the structure and diversity of tapeworm assemblages in sharks? Insights from the literature and a model shark species; Cephaloscyllium isabellum
Previous research has shed some light on what phylogenetic and ecological factors may be important determinants of tapeworm parasite diversity in elasmobranchs (sharks, skates and rays). However, several potentially key factors for tapeworm transmission, i ncluding the breadth and composition of host’s diets, have been recognised as crucial gaps in our understanding. The main objective of this research was to investigate the relative importance of sharks’ diets for the structure and diversity of their tapeworm assemblages. First, the literature was searched for information on tapeworms and host features for a large subset of different shark species, and aspects of shark’s diets (including their diet breadth, diet composition and trophic level) were assessed for their relative importance as predictors of tapeworm diversity. Second, literature records were used to conduct a comprehensive analysis of the relationship between shark’s diet compositions and tapeworm compositions. Finally, the importance of host diet was examined as a potential encounter filter for restricting tapeworm diversity in a model shark species, Cephaloscyllium isabellum. The results of this study revealed diet breadth to be a key predictor of tapeworm richness in sharks, indicating that sharks with broader diets generally harbour more tapeworm species. The composition of tapeworms infecting a shark species was found to be related to its diet composition, and moreover, certain tapeworm taxa were found to be useful indicators of the host species' ecology and evolutionary history. The research on C. isabellum here offered only limited insights into the potential importance of diet as an encounter filter for the shark, but provided some new important data on both the diet and parasites of this species. Ultimately, the observational studies carried out within this research emphasise that aspects of sharks’ diets can have important implications for their tapeworm parasite assemblages. Further exploration of these patterns with experimental research may be able to validate the influence of these patterns in nature.
Comparative histopathology of Longicollum (Acanthocephala: Pomphorhynchidae) infection in the alimentary tract and spleen of Acanthopagrus australis (Pisces: Sparidae).
The pomphorhynchid acanthocephalan Longicollum alemniscus infected the rectum of 16 and the intestine of five of 628 Acanthopagrus australis from Moreton Bay, Queensland. The neck and proboscis penetrated the entire gut wall and were surrounded by a layer of compact, rounded fibroblasts inside a layer of elongated fibroblasts and scattered connective tissue fibres. These layers extended across the entire gut wall and into the capsule that enclosed the proboscis outside of the gut. A similar parasite occurred extraintestinally in three fish; the proboscis of one penetrated into the spleen. The spleen was shrunken, with the proboscis and anterior part of the neck enclosed in a capsule of proliferated connective tissue and cellular elements similar to that in the response to infection of the gut wall.