Temperatures of inland aquatic habitats are increasing with climate change, and understanding how fishes respond physiologically to thermal stress will be critical for identifying species most susceptible to these changes. Desert fishes may be particularly vulnerable to rising temperatures since many species occupy only a fraction of their historic range, and remaining populations often occur in habitats with already high temperatures. Here, we examined endocrine and metabolic responses to elevated temperature in Amargosa pupfish, Cyprinodon nevadensis amargosae. We studied C. n. amargosae from two habitats with distinct thermal conditions: the Amargosa River, which experiences diurnally and seasonally variable temperatures (0.2 to 40°C), and Tecopa Bore, a spring and marsh fed by hot groundwater (47.5°C) from an artesian borehole. These allopatric populations differ in morphology, and prior evidence suggests that temperature may contribute to these differences via altered thyroid hormone (TH) regulation of morphological development. Here, we document variation in hepatic iodothyronine deiodinase type 2 (dio2) and type 3 (dio3) and TH receptor b (trb) gene transcript abundance between the Amargosa River and Tecopa Bore wild populations. Fish from these populations acclimated to 24°C or 34°C retained differences in hepatic dio2, dio3 and trb mRNA levels, and also varied in transcripts encoding TH membrane transporters monocarboxylate transporter 8 (mct8) and organic anion-transporting protein 1c1 (oatp1c1). Tecopa Bore pupfish also exhibited elevated dio2 and trb mRNAs in skeletal muscle relative to Amargosa River fish. Muscle citrate synthase (CS) activity was reduced at 34°C for both populations, while lactate dehydrogenase (LDH) activity and LDH A chain (ldhA) transcripts were elevated and T₃-responsive in Tecopa Bore pupfish only. These findings reveal that local population variation and thermal experience interact to shape how pupfish respond to elevated temperatures, and point to the need to consider such interactions in management actions for desert fishes under a changing climate. 

Endocrine and metabolic impacts of warming aquatic habitats on pupfish