Increased thermal tolerance under anoxic conditions in an extremophile fish from hot sulfur springs in Mexico

The thermal ecology of ectotherm animals has gained considerable attention in the face of human induced climate change. Particularly in aquatic species the experimental assessment of critical thermal limits (CTmin and CTmax) may help to predict possible effects of global warming on habitat suitability and ultimately species survival. Here we present data on the thermal limits of two endemic and endangered extremophile fish species, inhabiting a geothermally-heated and sulfur-rich spring system in Southern Mexico: The sulfur molly (Poecilia sulphuraria) and the widemouth gambusia (Gambusia eurystoma). Besides physiological challenges induced by toxic hydrogen sulfide and related severe hypoxia during the day, water temperatures have been previously reported to exceed those of nearby clearwater streams. We now present temperature data for various locations and years in the sulphur spring complex and conducted laboratory thermal tolerance tests (CTmin and CTmax) both under normoxic as well as sever hypoxic conditions in both species. Average CTmax limits did not differ between species under normoxic conditions. Surprisingly P. sulphuraria was found to reach a higher critical temperature (CTmax = 43.2{degrees}C) when tested under hypoxic conditions, while G. eurystoma on average had a lower CTmax when oxygen was absent. Based on this data we calculated both species thermal safety margins and used a TDT (thermal death time) model framework to relate our experimental data to observed temperatures in the natural habitat. Our findings suggest, that both species live near their thermal limits during the annual dry season and are locally already exposed to temperatures above their critical thermal limits. We discuss these findings in the light of possible physiological adaptions of the sulfur-adapted fish species and the anthropogenic threats for this unique system.