Effects of salinity acclimation on oxygen consumption in the freshwater blenny, Salaria fluviatilis, and the marine peacock blenny, S-pavo

The effect of salinity on standard metabolic rate was studied in a freshwater blenny, Salaria fluviatilis, and a marine congener, S. pavo. These blennies tolerate both fresh water (FW, <30 mOsm L-1) and full-strength sea water (SW, 1000 mOsm L-1). Fish of both species were acclimated to FW, iso-osmotic concentration (IOC, 375 mOsm L-1) and SW at 21 +/- 2 degrees C for three months before oxygen consumption rates were measured. Slopes of the regressions relating oxygen consumption rates to body mass did not differ significantly between species and among salinity treatments. Oxygen consumption rates of S. fluviatilis in FW were significantly higher than in IOC and SW, whereas oxygen consumption rates of S. pavo were similar in IOC and SW salinity but were significantly lower in fish in FW. These results do not seem to be accounted for by different costs of osmoregulation or by different concentrations of oxygen in the water among different salinities, and they may demonstrate reduction in metabolic activities as a reaction to chronically unfavourable, sub-optimal environmental conditions.