Inanga (Galaxias maculatus) is an amphidromous fish species that is able to successfully inhabit a variety of salinities. Using an integrated approach this thesis has characterised for the first time the physiological characteristics that facilitate acclimation in inanga. Structural studies using scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM) revealed freshwater-acclimated inanga have a high density of apical pits and freshwater-type mitochondria-rich cells (MRCs) that can facilitate ion absorption from the hypo-osmotic environment. In seawater, inanga remodel their gills by increased proliferation of seawater-type MRCs to facilitate ion secretion in the hyper-osmotic environment. Concentration-dependent sodium (Na+) kinetic analysis revealed that at a whole body level, inanga regulate Na+ using a saturable, high affinity, low capacity uptake system which makes them extremely adept at extracting Na+ from very dilute freshwater environments. In fact inanga displayed an uptake affinity (Km) of 52 ± 29 µM, which is one of the lowest ever recorded in freshwater fish. The sodium/potassium ATPase transporter (NKA) is central to Na+ regulation within the gill. In high salinties inanga displayed increased NKA activity (6.42 ± 0.51 µmol ADP mg protein-1 h-1) in an effort to excrete the excess Na+, diffusively gained from the hyper-osmotic environment. This increase in NKA was most likely a reflection of the proliferation of NKA-containing MRCs. The NKA activities seen in freshwater- and 50% seawater-acclimated inanga were similar (2.54 ± 0.19 and 2.07 ± 0.22 µmol ADP mg protein-1 h-1 respectively) to each other suggesting the inanga gill is capable of supporting ion regulation in brackish waters without a significant increase in NKA activities, and the energetically-expensive changes in gill structure and function that accompany such a change. Molecular investigation of NKA isoform expression using quantitative PCR (qPCR) showed that inanga displayed salinity-induced changes in the expression of the three α NKA isoform variants investigated. Isoform α1a exhibited a pattern consistent with an important role in freshwater, confirming results from other fish species. While it is generally accepted that α1b isoform is the predominant NKA isoform in seawater, inanga did not display this pattern with a freshwater dominance seen. None of the salinity-induced changes could quantitatively explain the increased NKA activity in seawater suggesting that different isoforms may convey different activities, that there is also regulation of NKA at a post-transcriptional level, and/or other isoforms or subunits may have a significant role. The importance of the osmoregulatory hormone cortisol and prolactin is widely accepted and inanga were treated with cortisol, prolactin and a combination of the two in an effort to further elucidate their role. NKA activity and NKA isoform expression were assessed but no specific patterns were deduced, except for a decrease in both NKA activity and isoform expression in 100% seawater-acclimated inanga treated with cortisol and prolactin. The reasons for this decrease were not evident, although the impact of stress induced by the injection protocol was likely to be a confounding factor. The development of a new confocal-based technique in this study was able to describe, for the first time, intracellular sodium levels ([Na+]i) as a function of salinity in an intact euryhaline fish gill cell. Using the fluorescent Na+ indicator dye CoroNa Green this study demonstrated the ability of inanga gill cells to maintain [Na+]i in the face of environmental change. Freshwater-acclimated inanga displayed basal [Na+]i of 5.2 ± 1.8 mM, with 12 ± 2.3 mM and 16.2 ± 3.0 mM recorded in 50% seawater- and 100% seawater-acclimated cells, respectively. Low [Na+]i is advantageous in hypo-osmotic environments as it provides a gradient between the cell and the blood which is essential for generating electrochemical gradients cell volume regulation and other cellular homeostatic mechanisms. A slightly elevated [Na+]i seen at the higher sanities would help minimise the diffusive gradient for passive influx from the environment which would be of benefit in hyper-osmotic environments. Upon salinity challenge 50% seawater cells were equally adept at maintaining a constant [Na+]i at any salinity, suggesting these cells are have the necessary constituents to regulate Na+ in both lower and higher salinities. This novel LSCM approach is advantageous relative to existing transport models as it will allow the observation of cellular ion transport in real time, within a native filament structure displaying functional interaction of different cell types. The extreme ion uptake characteristics of the inanga and their amenability to in situ confocal-based studies demonstrated in this study, confirm inanga as a valuable model species for future research.