Thermal conductivity modulation as a mechanism of inducible thermotolerance in the eutardigrade <i>Paramacrobiotus</i> sp.

Many organisms, including tardigrades, can survive extreme conditions. However, the mechanisms underlying this extremotolerance remain unclear. Here, we demonstrate inducible thermotolerance in the eutardigrade Paramacrobiotus sp., linked to its anhydrobiotic 'tun' state. While active tardigrades did not survive exposure to 45°C for 1 h, approximately 90% of those in the anhydrobiotic tun state did, and some even withstood temperatures up to 85°C. To explore the physical basis of this inducible thermotolerance, we measured thermal conductivity in both active and anhydrobiotic tardigrades using a custom-built vacuum apparatus. Anhydrobiotic tardigrades exhibited significantly higher thermal resistance than active forms, suggesting that reduced heat transfer during this state helps shield internal structures from heat-induced damage. Thus, these findings identify modulation of thermal conductivity as a physical mechanism contributing to heat tolerance in anhydrobiotic tardigrades, offering new insight into how these animals survive thermal stress.