37 Diverse Strategies for Ion Regulation in Fish Collected from the Ion-Poor, Acidic Rio Negro R. J. Gonzalez 1,2, * R. W. Wilson 1,3 C. M. Wood 1,4 M. L. Patrick 1,5 A. L. Val 1 1 Laboratory of Ecology and Molecular Evolution, National Institute for Amazon Research, Alameda Cosme Ferreira, 1756. 69.083-000 Manaus, Amazonas, Brazil; 2 Department of Biology, University of San Diego, 5998 Alcala ´ Park, San Diego, California 92110; 3 Department of Biological Sciences, Hatherly Laboratories, University of Exeter, Prince of Wales Road, Exeter EX4 4PS, United Kingdom; 4 Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada; 5 Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92697 Accepted 12/12/01 ABSTRACT We measured unidirectional ion fluxes of fish collected directly from the Rio Negro, an extremely dilute, acidic blackwater tributary of the Amazon. Kinetic analysis of Na + uptake re- vealed that most species had fairly similar J max values, ranging from 1,150 to 1,750 nmol g -1 h -1 , while K m values varied to a greater extent. Three species had K m values !33 mmol L -1 , while the rest had K m mmol L -1 . Because of the extremely values ≥ 110 low Na + concentration of Rio Negro water, the differences in K m values yield very different rates of Na + uptake. However, regardless of the rate of Na + uptake, measurements of Na + efflux show that Na + balance was maintained at very low Na + levels (!50 mmol L -1 ) by most species. Unlike other species with high K m values, the catfish Corydoras julii maintained high rates of Na + uptake in dilute waters by having a J max value at least 100% higher than the other species. Corydoras julii also demonstrated the ability to modulate kinetic parameters in response to changes in water chemistry. After 2 wk in 2 mmol L -1 NaCl, J max fell 150%, and K m dropped about 70%. The unusual acclimatory drop in K m may represent a mechanism * Corresponding author; e-mail: gonzalez@sandiego.edu. Physiological and Biochemical Zoology 75(1):37–47. 2002.  2002 by The University of Chicago. All rights reserved. 1522-2152/2002/7501-01091$15.00 to ensure high rates of Na + uptake on return to dilute water. As well as being tolerant of extremely dilute waters, Rio Negro fish generally were fairly tolerant of low pH. Still, there were significant differences in sensitivity to pH among the species on the basis of degree of stimulation of Na + efflux at low pH. There were also differences in sensitivity to low pH of Na + uptake, and two species maintained significant rates of uptake even at pH 3.5. When fish were exposed to low pH in Rio Negro water instead of deionized water (with the same con- centrations of major ions), the effects of low pH were reduced. This suggests that high concentrations of dissolved organic mol- ecules in the water, which give it its dark tea color, may interact with the branchial epithelium in some protective manner. Introduction Recent studies of fish native to the ion-poor, acidic blackwaters of the Rio Negro reveal two basic strategies for maintenance of ion balance. In several characid species, ion regulation is characterized by high-affinity, high-capacity ion transport sys- tems that are relatively insensitive to low pH (Gonzalez et al. 1997; Gonzalez and Preest 1999; Gonzalez and Wilson 2001). At the same time, diffusive ion loss is resistant to the stimu- latory effects of low pH. The highest degree of specialization in this group is found in neon tetras (Paracheirodon innesi), which possess a very high affinity Na + transporter (K p 12 m mmol L -1 ) that is completely insensitive to water pH (Gonzalez and Preest 1999). Consequently, in waters as acidic as pH 3.5, they ion regulate much as they do in circumneutral waters. The alternate strategy is displayed by angelfish (Pterophyllum scalare; Cichlidae), which possess a low-affinity, low-capacity ion transporter (rates of Na + uptake are one-seventh that of neon tetras) that is pH sensitive and completely inhibited at around pH 4.0 (Gonzalez and Wilson 2001). To maintain ion balance, angelfish rely on equally low rates of diffusive ion loss that are resistant to stimulation by low pH. Their strategy is to limit net ion loss at low pH and wait out the exposure with minimal disturbance. The studies describing these two ionoregulatory patterns were largely performed on fish acquired from tropical fish sup- pliers in North America or Europe. Some species were originally collected from the Rio Negro, but the chemistry of the water in which they were subsequently held before acquisition is un- known (Gonzalez and Wilson 2001). Other species were ac-