Thermal survival limits of young and mature larvae of a cold stenothermal chironomid from the Alps (Diamesinae: Pseudodiamesa branickii [Nowicki, 1873]).

The threats posed by climate change make it important to expand knowledge concerning cold and heat tolerance in stenothermal species from habitats potentially threatened by temperature changes. Thermal limits and basal metabolism variations were investigated in Pseudodiamesa branickii (Diptera: Chironomidae) under thermal stress between -20 and 37 °C. Supercooling point (SCP), lower (LLTs) and upper lethal temperatures (ULTs), and oxygen consumption rate were measured in overwintering young (1st and 2nd instar) and mature (3rd and 4th instar) larvae from an Alpine glacier-fed stream. Both young and mature larvae were freezing tolerant (SCPs = -7.1 °C and -6.4 °C, respectively; LLT -20 °C) and thermotolerant (ULT = 31.7 ± 0.4, 32.5 ± 0.3, respectively). However, ontogenetic differences in acute tolerance were observed. The LLT calculated for the young larvae (= -7.4 °C) was almost equal to their SCP (= -7.1 °C) and the overlapping of the proportion of mortality curve with the CPIF curve highlighted that the young larvae are borderline between freezing tolerance and freezing avoidance. Furthermore, a lower ULT in the young larvae (of ca. 1 °C), suggests that they are less thermotolerant than mature larvae. Finally, young larvae exhibit a higher oxygen consumption rate (mgO /gAFDM/h) at any temperature tested and are overall less resistant to oxygen depletion compared to mature larvae at ≥10 °C. These findings suggest that mature larvae enter into a dormant state by lowering their basal metabolism until environmental conditions improve in order to save energy for life cycle completion during stressful conditions.