Ionoregulatory responses to temperature change in two species of freshwater fish.

Abstract

We examined the ionoregulatory responses to temperature changes in two species of freshwater fish that differ in thermal preferences; the stenothermal, cold-water rainbow trout (Oncorhynchus mykiss) and the more eurythermal, warm-water common shiner (Notropis cornutus). We found that rainbow trout maintained constant plasma Na+ levels over the entire temperature regime (5–20 °C). Upon transfer from 15 °C (holding temperature) to 5 and 10 °C, rainbow trout experienced a significant drop in Na+ uptake (Jin Na), but after two weeks Jin Na had upregulated to warm temperature levels. Further, Na+ efflux (Jout Na) fell significantly at the colder temperatures. As a result, trout at the lowest temperatures were still in ion balance. When trout were exercised to exhaustion both O2 consumption (MO2) and Jout Na rose significantly at all temperatures, but while MO2 continued to be dependent upon temperature, Jout Na was high and constant. In contrast to the trout, common shiners experienced a 20% drop in plasma Na+ at 5 °C. Upon exposure to cold temperatures they experienced a reduced Jin Na, and showed no signs of acclimation during the subsequent two weeks. Likewise Jout Na was constant at all temperatures. These findings raise questions regarding the degree to which fish employ homeostatic mechanisms designed to defend a set- point (i.e., steady-state) osmolarity and ionic composition.