Living in ice: Examining the effects of temperature on thermal and metabolic physiology of glacier ice worms ( <i>Mesenchytraeus solifugus</i> )

Animals employ many strategies to survive in extreme cold. Glacier ice worms (Mesenchytraeus solifugus) are the largest animals that spend their entire life cycle in ice. Indeed, they spend most of their lives near 0°C and accumulate adenosine trisphosphate (ATP) to mitigate the impacts of cold on their performance. However, the degree to which ice worms can survive temperatures above and below freezing has not been thoroughly investigated. Furthermore, the role of cold temperatures in shaping whole-body metabolism in ice worms is largely unknown. Here, we assessed thermal limits of ice worms as well as their whole-body metabolic rates. Notably, we found that though ice worms can survive short-term exposures to surprisingly warm temperatures (~26°C), they cannot tolerate freezing, including internal ice formation. Ice worm metabolic rates also significantly increased with temperature up to 16°C where a significant break point occurred. Taken together, our results further illuminate how ice worms survive their unique life in ice and highlight how close they live to their lower thermal limits. Looking ahead, we note the clear risks that anthropogenic climate change, glacier recession, and loss of mountain snowpack pose to the future of ice worms.