Multiple sub-lethal thresholds for cellular responses to thermal stressors in an estuarine fish

Understanding physiological limits and the ability to acclimatize to changing conditions will determine whether species will be able to cope with further increases in water temperature. Changes in temperature may be considered to become stressful for an ectotherm when it results in reduced performance, which can lead to fitness level consequences. The relative intensity of the stressor as well as the duration of the exposure to the stressor will determine the response observed. Transcriptomic responses can potentially indicate thresholds where physiological performance begins to decline. An understanding of the cellular shifts throughout the temperature range that an organism experiences in the wild is often lacking, especially for species of conservation concern such as the delta smelt (Hypomesus transpacificus). We examined the expression of 15 genes that represented cellular responses related to stress, growth, cell proliferation and osmoregulation to show how the response patterns change to acute increases in temperatures that occur throughout the thermal distribution of the species. Several genes showed U-shaped or inverted U-shaped response patterns suggesting the presence of sub-lethal thresholds as temperatures increase. We also highlight the importance of including a temporal component to exposure studies as several genes showed a delay in the recovery to control levels at extreme temperatures. We propose that the non-linear response patterns represent sub-lethal thermal thresholds that can predict the severity of the response to thermal stressors. Identifying these sub-lethal thresholds can help differentiate between responses to routine increases in water temperature and responses that can lead to longer-term fitness impacts.