EFFECTS OF WATER PH AND CALCIUM-CONCENTRATION ON ION BALANCE IN FISH OF THE RIO-NEGRO, AMAZON

We examined the effects of acute low-pH exposure on ion balance (Na+, Cl-, K+) in several species of fish captured from the Rio Negro, a dil ute, acidic tributary of the Amazon. At pH 5.5 (untreated Rio Negro wa ter), the four Rio Negro species tested (piranha preta, Serrasalmus rh ombeus; piranha branca, Serrasalmus cf. holandi; aracu, Leporinus fasc iatus; and pacu, Myleus sp.) were at or near ion balance; upon exposur e to pH 3.5, while Na+ and Cl- loss rates became significant, they wer e relatively mild. In comparison, tambaqui (Colossoma macropomum), whi ch were obtained from aquaculture and held and tested under the same c onditions as the other fish, had loss rates seven times higher than al l the Rio Negro species. At pH 3.0, rates of Na+ and Cl- loss for the Rio Negro fish increased three-to fivefold but were again much less th an those observed in tambaqui, Raising water Ca2+ concentration from 1 0 mu mol L-1 to 100 mu mol L-1 during exposure to the same low pH's ha d no effect on rates of ion loss in the three species tested (piranha preta, piranha branca, aracu), which suggests that either they have su ch a high branchial affinity for Ca2+ that all sites are saturated at 10 mu mol L-1 and additional Ca2+ had no effect, or that Ca2+ may not be involved in regulation of branchial ion permeability. For a final R io Negro species, the cardinal tetra (Paracheirodon axelrodi), we moni tored body Na+ concentration during 5 d of exposure to pH 6.0, 4.0, or 3.5. These pH's had no effect on body Na+ concentration. These data t ogether suggest that exceptional acid tolerance is a general character istic of fish that inhabit the dilute acidic Rio Negro and raise quest ions about the role of Ca2+ in regulation of branchial ion permeabilit y in these fish.