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

The Na + uptake mechanism of cardinal tetras Paracheirodon axelrodi displayed 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, Na + uptake of angelfish Pterophyllum scalare did not appear to be specialized for dilute waters at all, with much higher K m and lower J max values. Na + uptake in cardinal tetras was high and completely unaffected down to pH 3·5, while uptake in angelfish was much lower and progressively inhibited by dropping pH; it was completely shut down at pH 3·5. During chronic exposure to pH 4·0 and 3·5, angelfish showed no ability to restore Na + uptake and at pH 3·5 all individuals 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 stimulation of diffusive Na+ losses. In angelfish, the ability to control diffusive ion losses at low pH was related to a high branchial affinity for Ca 2+ . For cardinal tetras, the rate of Na + loss appeared to be independent of water Ca 2+ 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 differences between the species may be the result of their different distributions, cardinal tetras are found in the central Rio Negro region while angelfish are more peripheral, or they may reflect phylogenetic differences.