Gill and intestinal Na+-K+ ATPase activity, and estimated maximal osmoregulatory costs, in three high energy-demand teleosts: yellowfin tuna (Thunnusalbacares), skipjack tuna (Katsuwonus pelamis), and dolphin fish (Coryphaena hippurus)

We hypothesize that the morpho-physiological adaptations that permit tunas to achieve maximum metabolic rates (MMR) that are more than double those of other active fishes should result in high water and ion flux rates across the gills and concomitant high osmoregulatory costs. The high standard meta bolic rates (SMR) of tunas and dolphin fish may, therefore, be due to the e levated rates of energy expenditure for osmoregulation (i.e. teleosts capab le of achieving exceptionally high MMR necessarily have SMR). Previous inve stigators have suggested a link between activity patterns and osmoregulator y costs based on Na+-K+ ATPase activity in the gills of active epipelagic a nd sluggish deepsea fishes. Based on these observations, we conclude that h igh-energy-demand fishes (i.e, tunas and dolphin fish) should have exceptio nally elevated gill and intestinal Na+-K+ ATPase activity reflecting their elevated rates of salt and water transfer. To test this idea and estimate o smoregulatory costs, we measured Naf-Kf ATPase activity (V,,,) in homogenat es of frozen samples taken from the gills and intestines of skipjack and ye llowfin tunas, and the gills of dolphin fish. As a check of our procedures, we made similar measurements using tissues from hybrid red tilapia (Oreoch romis mossambicus x O. niloticus). Contrary to our supposition, we found no difference in Na+-K+ ATPase activity per unit mass of gill or intestine in these four species. We estimate the cost of osmoregulation to be at most 9 % and 13% of the SMR in skipjack tuna and yellowfin tuna, respectively. Our results, therefore, do not support either of our original suppositions, an d the cause(s) underlying the high SMR of tunas and dolphin fish remain une xplained.