Patterns of ion regulation in acidophilic fish native to the ion-poor, acidic Rio Negro

The Na+ uptake mechanism of cardinal tetras Paracheirodon axelrodi displaye d specialization for operation in dilute waters of the amazonian Rio Negro. Kinetic analysis revealed low K-m and high J(max) values which ensure high rates of uptake even in very dilute waters. In contrast, Nat uptake of ang elfish Pterophyllum scalare did not appear to be specialized fur dilute wat ers at all, with much higher K-m and lower J(max) values. Na+ uptake in car dinal tetras was high and completely unaffected down to pH 3.5, while uptak e in angelfish was much lower and progressively inhibited by dropping pH it was completely shut down at pH3.5. During chronic exposure to pH 4.0 and 3 .5, angelfish showed no ability to restore Na+ uptake and at pH 3.5 all ind ividuals died between day 3 and 6 presumably due to small, but steady loss of Na+. At low pH, both species displayed a strong ability to prevent stimu lation of diffusive Na+ losses. In angelfish, the ability to control diffus ive ion losses at low pH was related to a high branchial affinity for Ca2+. For cardinal tetras, the rate of Na+ loss appeared to be independent of wa ter Ca2+ concentration. The specializations of cardinal tetras are similar to other characid fishes tested, but the results for angelfish reveal a new pattern of ion regulation in acidophilic species from the Rio Negro. The d ifferences between the species may be the result of their different distrib utions, cardinal tetras are found in the central Rio Negro region while ang elfish are more peripheral, or they may reflect phylogenetic differences. ( C). 2001 The Fisheries Society of the British Isles.