| Summary: | The energetic aspects of osmoregulation in several species of fish were examined, using an
experimental approach on both a whole-animal and tissue level. The first series of experiments
examined the metabolic response of temperate and tropical fish species to acute and gradual
salinity change, using whole-animal oxygen consumption rates and gill Na⁺,K⁺-ATPase activity
as indicators of osmoregulatory energetics. Juvenile dolphin fish (Coryphaena hippurus) were
exposed for 24 h to a reduced water salinity (34 to 20 ppt). They responded by decreasing
oxygen consumption and gill Na⁺,K⁺-ATPase activity, suggesting a decrease in osmoregulatory
costs. Mozambique tilapia (Oreochromis mossambicus) transferred from fresh water (FW) to
seawater (SW), showed an elevation in plasma growth hormone levels, gill Na⁺,K⁺-ATPase
activity, and a 20% increase in oxygen uptake after 4 d. No increases in these variables were seen
in tilapia transferred from FW to isosmotic salinity (ISO). These results indicated that the
physiological changes associated with SW entry represent a significant short-term cost, whereas
ISO did not impose (or reduce) an energy demand in tilapia during the acclimation process. In a
long-term study (6 wk), coho salmon (Oncorhynchus kisutch) smolts did not show any
differences in metabolic rate between FW, ISO and SW, whereas gill Na⁺,K⁺-ATPase activity
was lowest in ISO, higher in FW and highest in SW. In this case, there was no correlation
between whole-animal oxygen consumption rates and the relative activity of ion transport
enzymes in the gills. An acute (24 h) transfer of cutthroat trout (O. clarki clarki) from FW to SW
resulted in a significant elevation of both oxygen uptake and plasma Cortisol levels.
To further examine the influence of Cortisol on oxygen consumption and osmoregulatory
variables, cutthroat trout parr were given Cortisol implants that elevated plasma Cortisol titres to a level similar to that found in fish following SW exposure. Cortisol significantly increased
oxygen consumption rates and plasma glucose levels of trout in FW, consistent with its
glucocorticoid role. This study suggests that some of the increases in oxygen consumption that
occurred during the intitial stages of SW exposure may have been related to the metabolic effects
of Cortisol, rather than the direct costs usually associated with osmoregulation.
To separate the energy costs of NaCl transport from other whole-animal metabolic responses
to salinity change, experiments were conducted using isolated preparations of osmoregulatory
tissues. Oxygen consumption and Na⁺,K⁺-ATPase activity were measured in excised rectal gland
and gill tissue of the spiny dogfish (Squalus acanthias), using ouabain to estimate the portion of
tissue respiration required by the Na⁺/K⁺-pump. Ouabain-sensitive oxygen consumption of the
rectal gland accounted for 55% of tissue respiration, compared to 22% for the gill. On a wholemass
basis, the cost of NaCl secretion in the rectal gland was estimated to be 0.5% of whole animal
oxygen uptake. A similar approach was used on excised gill tissue from FW-adapted
cutthroat trout, to assess the oxygen cost of NaCl uptake in the FW trout gill. In that study,
bafilomycin was also used to inhibit H⁺-pump activity in the gill tissue. A similar portion of gill
tissue respiration was required by the Na⁺/K⁺-pump (18%) and H⁺-pump (19%), and the cost of
NaCl uptake in the FW trout gill was estimated at 1.8% of resting metabolic rate. Finally, an
isolated, perfused gill arch preparation was used to compare gill energetics in FW- and SWadapted
cutthroat trout. The total gill oxygen consumption of FW gills was significantly (33%)
higher than SW gills, and accounted for 3.9% and 2.4% of resting metabolic rate, respectively.
The results of those experiments indicate that the energy demands of ion transport in
osmoregulatory organs, such as the rectal gland and gill, represent a relatively small portion of
the total energy budget in fish.
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