The effect of temperature acclimation on thermal tolerance, hypoxia tolerance and aerobic scope in two subspecies of sheepshead minnow; Cyprinodon variegatus variegatus and Cyprinodon variegatus hubbsi.

The freshwater teleost Cyprinodon variegatus hubbsi (Cvh) diverged from its euryhaline relative Cyprinodon variegatus variegatus (Cvv) ~150 kya and these subspecies are physiologically distinct in their osmoregulatory capabilities. Cvv inhabits intertidal estuaries and saltwater marshes along the Gulf of Mexico and Atlantic coast, where they experience a broad temperature range from -1.9 to 43 °C and frequent bouts of hypoxia. In contrast, Cvh lives in several lakes in central Florida, where temperature is more stable (12-31 °C) and hypoxia is uncommon. To assess whether relaxed selective pressure on Cvh has resulted in reduced temperature and hypoxia tolerance, a comparative study on the effects of acclimation to 25, 30 and 35 °C on critical thermal tolerance (CTMax), hypoxia tolerance, and aerobic scope was performed. The CTMax was similar between subspecies and positively correlated with acclimation temperature. Neither subspecies, however, could survive at 38 °C for a prolonged period of time. In general, Cvv displayed greater hypoxia tolerance and aerobic scope relative to Cvh over the range of acclimation temperatures. Routine metabolic rate was significantly lower while maximum metabolic rate and aerobic scope were significantly higher in Cvv, but only in fish acclimated to 30 °C. Overall, the different responses of Cvh to relaxed selective pressure suggest these traits are weakly linked physiologically in these fishes.