Temperature-dependent aerobic scope and Hsp70 expression in the sea cucumber Holothuria scabra

The aerobic scope, which reflects the functional capacity for biological fitness, is a highly relevant proxy to determine thermal tolerance in various taxa. Despite the importance of this method, its implementation is often hindered, due to lacking techniques to accurately measure standard- (SMR) and maximal- (MMR) metabolic rates, especially in sluggish marine invertebrates, such as sea cucumbers. This study targets temperature-induced maximal- and minimal- O2 consumption rates in the sea cucumber Holothuria scabra, through acute temperature change (2°C per hour; 17–41°C). In addition to O2 consumption, mRNA expression of Hsp70 was measured to define critical threshold temperatures on an interlinked basis. O2 consumption of H. scabra peaked distinctly at 38°C (33.2 ± 4.7 µgO2 g-1h-1) and reached a clear bottom line at 22°C (>3 µgO2 g-1 h-1). Within the thermal window of 22–38°C H. scabra sustained positive aerobic capacity, with theoretic optimal performance in the center of the aerobic window (30.5°C; 17.5 µgO2 g-1 h-1). Between 39–41°C H. scabra decreased respiration progressively, while gene expression levels of Hsp70 were significantly increased, indicating lethal conditions due to functional disruption of homeostasis. At the cold end (17–22°C) H. scabra exhibited a narrow minimal respiration range, but no Hsp70 induction. We identified for the first time critical and optimal temperature levels for H. scabra, on the basis of aerobic performance. The characterization of temperature-induced maximal- (≈MMR) and minimal- (≈SMR) O2 consumption rates may serve as reliable alternative to precisely determine aerobic scope in sea cucumbers and other slow-moving marine species. Moreover, we showcase the important linkage between energy-related physiological markers and molecular defense mechanisms to predict the effects of environmental stress, such as climate change.