THE RELATIONSHIP BETWEEN OXYGEN-UPTAKE AND ION LOSS IN FISH FROM DIVERSE HABITATS

A recent examination of the relationship between O-2 uptake (M(O2)) an d diffusive sodium loss (J(out)(Na)) in a freshwater fish showed that Na+ losses after exhaustive exercise exceeded those expected on the ba sis of M(O2), probably due to distortion of the paracellular tight jun ctions (the primary site of diffusive ion loss) and/or glomerular-type filtration caused by increased lamellar pressure. In the present stud y, an examination of this relationship in nine species of fish from di verse habitats supports this model. Under routine conditions, the rate of Na+ loss per unit of O-2 consumed (termed the ion/gas ratio or IGR ) was similar in all the species tested, averaging 61.6 pmol Na+ nmol( -1) O-2. After exhaustive exercise, the degree to which the IGR of eac h species increased relative to its routine levels (post-exercise IGR/ routine IGR) was exponentially related to the relative rise in M(O2), i.e. greater rates of O-2 uptake led to even greater ion losses. Furth er analysis revealed that, although naturally active species had the l owest proportionate increase in M(O2) by virtue of their high routine rates, they had the highest post-exercise rates of O-2 uptake. In fact , there was an inverse correlation between post-exercise IGR and M(O2) , i.e. species with low M(O2) values lost more Na+ per mole of O-2 tak en up than did species with high ones. Thus, naturally active species, such as the common and golden shiner, were able to achieve higher rat es of O-2 uptake while avoiding high rates of ion loss. Surprisingly, species such as banded sunfish, yellow perch and smallmouth bass did n ot limit ion loss associated with exercise despite their apparent abil ity to do so. They demonstrated a strong ability to limit ion losses c aused by a brief osmotic shock and by exposure to soft water (both of which distort tight junctions).