Myxosporean (Myxozoa: Myxosporea) parasites infect different lineages of vertebrate hosts with the overwhelming majority described to date from teleost fishes. A few myxosporean species have been described from elasmobranchs, and on rare occasion from invertebrates, amphibians, reptiles, octopus, moles, shrews, and waterfowl. This study investigates the myxosporean parasites of elasmobranchs (i.e. sharks and rays), the cartilaginous fishes of the Class Chondrichthyes, subclass Elasmobranchii, since little is known either about the richness of these parasites, the host/parasite relationships (i.e. host specificity, prevalence, pathogenicity, and distribution), or the genetic relatedness of elasmobranch-infecting myxosporeans compared to that of teleosts.
Prior to this study 124 host/parasite combination of myxosporeans infections in elasmobranchs were known globally, of which only five records are from Australia. Representatives of both the myxosporean orders have been reported from elasmobranchs. The Bivalvulida, by far the richest, is represented by the genera Ceratomyxa, Chloromyxum, Ellipsomyxa, Sinuolinea, Sphaerospora and Myxidium. The Multivalvulida is represented by the genera Kudoa and Unicapsula. These records include thirty-three formally described species, of which there are no multivalvulidans characterised, and only three species, Chloromyxum noblei, Chloromyxum pristiophori and Sinuolinea lesteri, reported from Australian elasmobranchs.
The first section of this thesis examines the diversity of myxozoan parasites in Australian elasmobranchs. A total evidence taxonomic approach using a combination of morphological, biological and genetic data was used to characterise the parasites and thus explore host/parasite relationships. Myxosporeans were detected in elasmobranchs from all geographic regions within Australian waters: tropical; sub-tropical; and temperate. This study provides the first taxonomic descriptions of multivalvulidan myxosporeans infecting elasmobranchs. Two genera of Multivalvulida were detected. Two species of Kudoa were recognised: Kudoa carcharhini n.sp. described from Carcharhinus cautus (and two other carcharhinid species); and Kudoa hemiscylli n.sp. described from Hemiscyllium ocellatum (and eight other host species across two orders). While, Unicapsula sp. was reported infecting Orectolobus hutchinsi but not formally described here due to limited material. The Bivalvulida was far richer than the Multivalvulida, and comprised representatives of three genera. Three novel species of Ceratomyxa were detected: Ceratomyxa carcharhini n.sp. and Ceratomyxa melanopteri n.sp. from Carcharhinus melanopterus; and Ceratomyxa negaprioni n.sp. from Negaprion acutidens. Chloromyxum was the richest genus in the study and comprised six novel species and another reported but not described: Chloromyxum hemiscyllii n.sp. from Hemiscyllium ocellatum; Chloromyxum kuhlii n.sp. from Neotrygon kuhlii; Chloromyxum lesteri n.sp. from Cephaloscyllium laticeps; Chloromyxum mingazzinii n.sp. from Pristiophorus nudipinnis; Chloromyxum myliobati n.sp. from Myliobatis australis; Chloromyxum squali n.sp. from Squalus acanthias; and Chloromyxum sp. infecting Squalus acanthias was reported but not formally described due to limited material. In addition, two potentially novel Myxidium sp. were reported infecting Neotrygon kuhlii and Squalus acanthias, however, due to poor quality samples not formally described.
Phylogenetic analysis of SSU rDNA data demonstrate that each parasite genus that infects elasmobranchs grouped to the exclusion of other congeneric teleost infecting species. This evidence strongly suggests that host evolution contributed to the distinct parasitic fauna observed in elasmobranchs. The species partitioning may be attributed to a disparate host evolution of elasmobranch species to that of teleost species, as such myxosporean species radiated within these hosts without genetic mixing (i.e. hence elasmobranch-infecting myxosporeans clade to the exclusion of teleost-infecting species).
The host specificity of elasmobranch-infecting kudoids tend towards euryxenous, infecting a number of unrelated host species, while the bivalvulidans display oioxenous specificity, with each being found only in a single host species. The host specificity of elasmobranch-infecting myxosporeans closely reflects that of teleost-infecting species.
Parasite species have evolved numerous elaborate means of transmission to enable survival in a range of environments. The typical myxosporean life cycle is no exception and involves two hosts to complete development, with the actinosporean stage a critical link in the survival of the species. Although, no known elasmobranch-infecting myxosporean (i.e. both multivalvulidan and bivalvulidan) or kudoid (including teleost-infecting species) has been determined. An examination of the intrinsic and extrinsic factors involved in the susceptibility of hosts to infection by myxosporeans was explored, and demonstrates that parasitism in this system is complex and involves factors relating to host immune response, host evolution, viability of infectious stages in response to environmental conditions (i.e. temperature, hydrology), and host encounter with infectious stages; all of which play a role in structuring the myxosporean fauna of elasmobranchs but to what degree each contributes remains unknown.
Myxosporeans of elasmobranchs are more prevalent than previous records suggest, they may not be as diverse as those of teleosts, but they are certainly not an inconsequential part of the fauna. Clearly, there remains a fertile area for research into myxosporeans of both elasmobranchs and teleosts, particularly in the determination of life cycles and understanding of epizootiology of myxosporeans. Indeed, the remarkably high prevalence of infection in elasmobranchs demonstrated in this study may well provide an ideal system for such an investigation.