Stimulation of sodium efflux in air-breathing fish exposed to low pH

Sodium efflux and survival times at low pHs were measured in two species of facultative air-breathing fish. Mudminnows (Umbra pygmaea) and gar (Lepisosteus platyrhincus) were quite tolerant of exposure to acidic waters. This tolerance was associated with a striking capacity for resisting the acceleration in sodium efflux usually characteristic of low pH toxicity. Such a specialization for ionic regulation may have evolved within the context of life in naturally acidic swamps low in sodium concentration. The presence of a very strong degree of resistance to low pH-induced sodium loss in these species affirms the crucial importance of net sodium loss as a mechanism of acid toxicity. Both species showed similar relationships between sodium efflux and low pH. Survival times at low pHs were inversely related to sodium efflux. At death, body sodium concentrations in both species were severely depressed, reflecting large net losses of sodium. An acid resistant salmonid, Salvelinus fontinalis, shows a correlation between sodium efflux and survival time similar to those of the air-breathers studied here. Possession of the capacity for aerial respiration is obviously not the only or perhaps even the primary factor that confers increased resistance to acid toxicity in these two air-breathing fish.