Effect of salinity on osmoregulation, metabolism and nitrogen excretion in the amphidromous fish, inanga (Galaxias maculatus)

•The amphidromous inanga is an excellent osmoregulator.•Energy expenditures remained constant (5.4 ± 0.1 J g fish− 1 h− 1) in all salinities.•Oxygen to nitrogen ratios did suggest changes in fuel use at different salinities.•Estuaries might also have bioenergetics advantages for its inhabitants.•No evidence of osmorespiratory compromise was found.

Inanga (Galaxias maculatus) is an amphidromous fish, capable of moving freely between waters of different salinities. The impact of environmental salinity on inanga physiology was investigated by examining ionic and osmotic regulation, nitrogen excretion, oxygen metabolism and energy expenditure. After 16 days of acclimation to salinities ranging from freshwater to 43, inanga showed only minor changes in plasma osmolality and no significant changes in metabolic rate or calculated energy expenditure, underlining their high salinity tolerance. Decreases in ammonia excretion at salinities close to the isosmotic point, and variation in oxygen to nitrogen ratios did, however, suggest changes in fuel use. Partitioning studies isolating the head from the body indicated that there was no trade-off between oxygen consumption and nitrogen excretion functions in the gills of inanga with salinity. Although extra-branchial epithelia (i.e. skin and/or kidney) were significant contributors to both gas exchange and ammonia excretion (e.g. ~ 35% of ammonia excretion was extra-branchial), these contributions were independent of salinity. These data suggest that inanga do not incur significant physiological costs following acclimation to a wide range of environmental salinities, but that there may be subtle physiological advantages to inhabiting salinities close to their isosmotic point.