Conservation of the dwarf galaxias, Galaxiella pusilla (Mack 1936) (Teleostei: Galaxiidae), a threatened freshwater fish from south-eastern Australia

© 2014 Dr. Rhys Antony ColemanThe dwarf galaxias, Galaxiella pusilla, is a small native freshwater fish of national conservation significance from south-eastern Australia. Recovery efforts for this species have been restricted by an absence of fundamental biological and ecological knowledge. The purpose of this research was to address key knowledge gaps required for the development of sound management strategies. Firstly, both nuclear and mitochondrial DNA markers were applied to determine broad-scale patterns of genetic structure and diversity across the species entire geographic range. Substantial genetic differences between two distinct geographic regions were found that suggested G. pusilla, as it is currently recognised, comprises two cryptic species. Lower genetic diversity amongst the east region compared to the west region was also found. Following the initial genetic study, a more comprehensive population genetic assessment was conducted to provide robust information on genetic structure and diversity within and between populations. This revealed a strong inverse relationship between genetic uniqueness and genetic diversity within both geographic regions. Furthermore, differentiation within each region was largely due to a loss of diversity and changes in allele frequencies, rather than the presence of unique alleles. This highlighted the influence of genetic drift as a driver of genetic patterns in G. pusilla and may be a reflection of ongoing extreme wetting and drying events (e.g. severe droughts and major floods) that lead to founder events or bottlenecks. In the case of G. pusilla, and mostly likely other freshwater fish species exposed to similar dynamic environments, there is a potential tradeoff between genetic diversity and genetic uniqueness that needs to be balanced when determining conservation priorities. Both habitat drying and respiration experiments demonstrated that Galaxiella pusilla have adaptations that enable them to survive for several days without surface water and that habitats with a greater ability to retain moisture (e.g. detritus, crayfish burrows) increased survival rates and duration. An ability of G. pusilla to shift their mode of respiration to air-breathing when exposed to air was also observed. Parallel experiments with Gambusia holbrooki, an invasive fish species that is commonly cited as a major threat to Galaxiella pusilla, showed comparatively poor survival of Gambusia holbrooki in the absence of surface water and no apparent shift in their mode of respiration in air. These results indicate the potential application of habitat drying to control Gambusia holbrooki in Galaxiella pusilla habitats, with possible additional benefits for Galaxiella pusilla food resources. Targeted revegetation that increases levels of detritus in Galaxiella pusilla habitats may also favour Galaxiella pusilla over Gambusia holbrooki. Considering adaptation to habitat drying, patterns of genetic structure and diversity, and observations of large inter-annual fluctuations in population densities at some sites, G. pusilla conservation strategies are likely to be particularly successful in the context of managing metapopulations. This places emphasis on protecting and restoring multiple habitats, as well as their interconnections across a landscape, so that localised extinctions are balanced by recolonisation and long-term persistence of the population as a whole is achieved. Given the extent of habitat loss and isolation over much of the distribution of Galaxiella pusilla, the restoration of functional metapopulations within river systems is likely to be critical to the recovery of the species. Finally, morphological and meristic measurements on populations from both the east and west regions identified significant differences in a number of characters including total length, number of vertebrae, length of caudal peduncle and dorsal fin-anal fin setback. Ventral surface markings are also likely to aid visual classification. Based on distinct genetic differences, variation in a number of morphometric and meristic characters, and a clear geographic separation, a new species of Galaxiella is proposed: Galaxiella toourtkoourt sp. nov.. Clearly, the separation of populations into two species for a nationally listed species has significant conservation implications. The description of a new Galaxiella species and re-description of Galaxiella pusilla ensures that independent management strategies can be devised in a way that sufficiently accounts for their threatened status (e.g. eastern populations have much lower genetic diversity and are under more immediate threat from habitat loss and isolation) and potential biological and ecological differences