Ion transport physiology and its interaction with trace element accumulation and toxicity in inanga (Galaxias maculatus)

Inanga (Galaxias maculatus) are a culturally and economically important fish species in New Zealand and abroad. However, very little is known about their ability to deal with trace element contamination. As a scaleless fish with the ability to survive in relatively extreme environments, they may not fit toxicity models (such as the biotic ligand model; BLM) based on other fish species. The aim of this study was to determine how this fish responds to elevated trace elements in both the laboratory and field in order to determine the applicability of these toxicity models. In order to determine the impacts of stress on ion transport and subsequent metal toxicity, inanga were exposed to handling stress and measures of ion uptake were collected. Handling stress was shown to result in increased ventilation rates, resulting in stimulated sodium (Na+) efflux. A compensatory increase in Na+ influx was also measured as a result of this stress. Inanga largely recovered from this ionoregulatory stress within 2 hours, with full recovery after 24 hours. This was indicative of a rapid homeostatic response for maintaining ion balance. Enhanced Na+ uptake in response to this stress resulted in increased copper (Cu) uptake in Cu-contaminated water, suggesting stressed fish will accumulate more Cu (and likely other Na+ mimics) than an unstressed fish. These results suggest a heightened vulnerability of inanga to this type of contaminant as a result of exercise stress during migrations. A combination of field and laboratory studies was used in order to measure trace element accumulation in inanga. In situ field studies showed changes to aluminum (Al) and iron (Fe) body burdens when inanga were placed in streams of varying trace element concentrations along the West Coast of the South Island. However, other trace elements measured did not alter over the period of exposure (9-10 days). Biochemical biomarker analysis showed no changes in the activity of Na+/K+-ATPase (NKA), but a marker of lipid peroxidation (thiobarbituric acid reactive substances; TBARS) was elevated in one stream. Analysis suggested that stream pH was the major driver of this effect, whether directly or via changes to metal bioavailability. Subsequent laboratory exposures (96 h) of inanga to 1.2, 2.7, 10.8, and 44 µg L-1 dissolved Fe and 5.6, 23.3, 60.7, and 128.7 µg L-1 dissolved zinc (Zn) showed no difference in whole body trace element accumulation, ammonia excretion, ion influx (Ca2+ and Na+), and TBARS. There were significant differences in oxygen consumption (MO2) after Fe exposures, with increases in the 2.7 and 44 µg L-1 dissolved Fe exposures. Laboratory exposure results suggest inanga are relatively insensitive to short-term Fe and Zn exposures. Both in vivo (whole body partitioning) and in vitro (Ussing chamber) techniques were used to determine the influence of cutaneous ion transport on preventing trace element accumulation. Results suggest inanga use their skin as an additional site of calcium (Ca2+) and Na+ uptake. This is the first study to confirm these ion transport capabilities in inanga, and revealed that up to 48% of Na+ uptake may occur across the skin. Pharmacological inhibition of Ca2+ uptake was achieved by known Ca2+ channel blockers (verapamil and lanthanum). Furthermore Fe and Zn impaired cutaneous Ca2+ transport, indicating that ion transport pathways in the skin modulate in response to these metals