Ion and acid-base balance in three species of Amazonian fish during gradual acidification of extremely soft water

Sensitivity to acid water was assessed in three species of Amazonian fish t hat encounter naturally acidic blackwaters to differing degrees in the wild : tambaqui (Colossoma macropomum), matrincha (Brycon erythropterum), and ta moata (Hoplosternum littorale), in decreasing order of occurrence in blackw ater. Fish were exposed to a graded reduction in water pH, from pH 6 to 5 t o 4 to 3.5, followed by return to pH 6. Fish were exposed to each new pH fo r 24 h. During these exposures, net transfers of ions (Na+, K+, Cl-, and Ca 2+) and acid-base equivalents to and from the external water were used as p hysiological indicators of acid tolerance. Exposure to pH 5 had a minimal e ffect on net ion fluxes. Significant net losses of all ions (except Ca2+) w ere recorded in an three species during the first few hours of exposure to pH 4. However, ion balance was usually restored within 18 h at pH 4. Exposu re to pH 3.5 caused even greater ion losses in all three species and proved to be acutely lethal to tamoata. Matrincha sustained irreversible physiolo gical damage at pH 3.5, as ion fluxes did not recover following return to p H 6 and there was some mortality. Tambaqui suffered the least ionoregulator y disturbances at pH 3.5 and was the only species to make a full recovery o n return to pH 6. In ail species, there was a tendency for ammonia excretio n to increase at low water pH, but even at pH 3.5, there was no significant net uptake of acid from the water. Overall, there was a strong relationshi p between the magnitude of ionic disturbances and the lethality of exposure to low pH. The relative insensitivity of the ionoregulatory system of tamb aqui to low pH indicates that this is a feature of fish native to blackwate r systems rather than one that is common to all Amazon fish.