Black spot disease in freshwater fishes ofsouth-western Australia: identification of the parasite, host range and potential as a bioindicator for water quality

The salinisation of freshwater ecosystems by anthropogenic influences is recognised as one of the largest threats to the highly endemic freshwater fish fauna of south-western Australia. There has also been some recent evidence that secondary salinisation is affecting the parasite community of freshwater fishes. A parasitic trematode, which causes black spot disease in the musculature of native fishes, has previously been tentatively identified as Diplostomum galaxiae, a species first described from a galaxiid fish (Galaxias auratus) in Tasmania. The aims of the current study were to confirm this specific identification using genetic analyses; investigate the host and geographic range of the parasite in south-western Australia; and determine whether the parasite could be a suitable bioindicator of secondary salinisation. Encysted metacercariae were extracted from preserved fishes in the collection of the Freshwater Fish Group & Fish Health Unit, Murdoch University, and from a specimen of Galaxias truttaceus from Tasmania, and a section of the 18S rRNA gene sequenced. All parasites from south-western Australia and Tasmania were genetically identical, but did not group with other species in the Diplostomum genus in phylogenetic analyses. Instead, the Australian parasite aligned more closely to a clade containing Posthodiplostomum spp. , and several genera from the Strigeidae family. Reclassification of the parasite causing black spot disease in south-western Australian freshwater fishes is thus recommended, following more extensive comparisons with the parasite throughout Australia. In the interim, the parasite has been designated with the temporary name Dip01. Historical collections of fishes from the West Australian Museum and from the Freshwater Fish Group & Fish Health Unit, Murdoch University were assessed, to collate records of Dip01 metacercariae. The parasite appears to preferentially infect Galaxias maculatus and Galaxias occidentalis over other species; although it was also found in two percichthyids (a single Nannatherina balstoni and several Bostockia porosa from a single catchment) and has been reported previously from two estuarine fishes (Leptatherina wallacei and Pseudogobius olorum) that have colonised secondarily salinised rivers of the region. The overall prevalence of Dip01 in G. maculatus was 11.7% (95% CI 9.3-14.4%), and in G. occidentalis was 6.1% (4.8-7.7%). The mean intensity of infection in G. maculatus was 3.5 (2.0-7.3) parasites/infected fish, while the mean intensity in G. occidentalis was 7.8 (5.9-10.3) parasites per infected fish. The geographic range of Dip01 closely matches that of the preferred hosts, G. maculatus and G. occidentalis. Prevalence and intensity data were compared against historical environmental data from the Water Information Reporting database of the Department of Water and Environmental Regulation, where available. There was a significant inverse relationship between parasite prevalence and salinity, with Dip01 only occurring in habitats with a conductivity of less than 1,000 μS/cm. Based on these data, the parasite appears to be a useful bioindicator of salinity, as it is restricted to low salinity waters, even though its preferred galaxiid hosts are tolerant of a wide range of salinities.