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 (J_in ^Na), but after two weeks J_in ^Na had upregulated to warm temperature levels. Further, Na⁺ efflux (J_out ^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 J_out ^Na rose significantly at all temperatures, but while MO₂ continued to be dependent upon temperature, J_out ^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 J_in ^Na, and showed no signs of acclimation during the subsequent two weeks. Likewise J_out ^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.