Taxonomic and biological studies of protozoan parasites in Tasmanian marine fishes

Twelve species of marine fish were sampled over a 22 month period from Dru Point, North-West Bay, south-east of Hobart, Tasmania, Australia, to investigate the protozoan parasites present on the external surface and in the internal organs of fish. A total of 16 species of protozoan parasites belonging to 11 genera (in addition to the collective group Microsporidium), 10 families and 4 phyla were obtained. Among these, 12 new species are described. Two new Australian records and two unidentified forms are reported. Detailed ultrastructural studies were carried out for Zschokkella leptatherinae sp. nov., a myxosporean from the hepatic ducts of the atherinid fish, Leptatherina presbyteroides. The development of Z. leptatherinae can be described by four stages: pansporoblast formation, capsulogenesis, valvogenisis and sporoplasm maturation. The formation of the early pansporoblast of Z. leptatherinae is by the union of two generative cells. Two spores are produced from one pansporoblast at the end of sporogenesis. The surface topography and the adhesive discs of Paratrichodina tasmaniensis sp. nov. from the gills of two species of atherinid fish Leptatherina presbyteroides, Atherinosoma microstoma and Trichodina nesogobii from the gills of gobio, Nesogobius sp.1 were examined using scanning electron microscopy. Both ciliates have two ciliary bands around the body surface. The adoral ciliary spiral turns 150-2000 in P. tasmaniensis, while it is about 370-4000 in T. nesogobii. The aboral ciliary band consists of three distinct sets of cilia in both species; however the length of the cilia, as well as the development of the septum which separates each set of cilia, are different. The adhesive disc of T. nesogobii shows a similar structure as observed by the light microscope. An ellipsoidal foramen is visible clearly in the denticle of P. tasmaniensis which corresponds to a small dot in silver impregnated specimens. Ecological studies were conducted on Zschokkella leptatherinae and Paratrichodina tasmaniensis and their two host species, Atherinosoma microstoma and Leptatherina presbyteroides. The frequency distributions, the seasonal variations of parasite infection as well as the relationships between the parasites and the host fishes were investigated. The frequency distribution of Z. leptatherinae in both fish species and P. tasmaniensis in L. presbyteroide fit the log-normal distribution model. The infection of Z. leptatherinae showed no significantly seasonal variation, while a pronounced seasonal variation was revealed in the infestation of P. tasmaniensis. The prevalence and intensity of infestation were high in summer and spring, low in winter and autumn in both Atherinosoma microstoma and Leptatherina presbyteroides. Infection of Z. leptatherinae occurred more frequently in larger hosts in both fish species. For P. tasmaniensis, the variation of infestation in different size group was closely related to the seasonal variations of infection. There were no significant differences in the prevalence of infection of both parasite species between the male and female samples of A. microstoma and L. presbyteroides. The interspecific associations between six species of protozoan parasites and the spatial distributions of Paratrichodina tasmaniensis on the gills of host fish were also analysed. A positive association was found between P. tasmaniensis and Trichodina australis in A. microstoma. A negative association existed between T. australis and Zschokkella leptatherinae, Trichodina sp. and Z. leptatherinae in A. microstoma; P. tasmaniensis'and Z. leptatherinae, Clausophrya branchialis and Z. leptatherinae in L. presbyteroides. A preliminary investigation revealed that P. tasmaniensis has a significantly high affinity for the left gill arches in both A. microstoma and L. presbyteroides. Both Zschokkella leptatherinae and Paratrichodina tasmaniensis_are pathogenic agents of atherinid fishes. The liver of L. presbyteroides infected by Z. leptatherinae shows clear histopathological change. Proliferation, enlargement and thickening of the hepatic ducts are observed. The epithelial cells of infected fish are lower than the normal cells. The parasites also provoke blockage of the hepatic ducts and atrophy of the neighbouring liver parenchyma. In more advanced infection, over 85% of the liver tissue is replaced by the plasmodia; and necrosis occurs finally. The gills of Atherinosoma microstoma infested by Paratrichodina tasmaniensis show sloughing of the epithelial cells of secondary lamellae, hyperplasia of interlamellar and secondary lamellar epithelial cells and fusion of the adjacent secondary lamellae. Using the transmission electron microscope, hypertrophy of the epithelial cells of secondary lamellar is observed. The pillar cells are irregular and the basement membrane is thinner than those in uninfested fishes. The pattern of surface ridges is often disrupted.