Respiratory structures shape heat and desiccation tolerance in terrestrial isopods

The transition from aquatic to terrestrial life exposed organisms to novel thermal and hydric challenges. In terrestrial isopods, these pressures drove key adaptations in their aerial respiratory system, including the evolution of pseudotracheae (0, 2 or 5 pairs depending on species). However, the functional consequences of these respiratory adaptations for heat and desiccation tolerance remain unclear. We quantified these tolerances in five species differing in pseudotracheae number to test whether a higher number confers higher physiological limits. Heat tolerance was assessed using the thermal death time framework, and desiccation tolerance was tested at 34°C under three relative humidity levels. Species with pseudotracheae showed higher tolerance to both heat and desiccation, supporting their role in terrestrial adaptation. Yet, at low humidity, the 5-pair species was more sensitive than the 2-pair species, suggesting that other morpho-behavioral traits such as volvation also shape desiccation tolerance. Our study highlights the functional role of pseudotracheae in the transition between aquatic and terrestrial environments by improving physiological limits.