Adaptation to climate change: trade‐offs among responses to multiple stressors in an intertidal crustacean

Trade-offs may influence both physiological and evolutionary responses to co-occurring stressors, but their effects on both plastic and adaptive responses to climate change are poorly understood. To test for genetic and physiological trade-offs incurred in tolerating multiple stressors, we hybridized two populations of the intertidal copepod Tigriopus californicus that were divergent for both heat and salinity tolerance. Starting in the F2 generation, we selected for increased tolerance of heat, low salinity, and high salinity in replicate lines. After five generations of selection, heat-selected lines had greater heat tolerance but lower fecundity, indicating an energetic cost to tolerance. Lines selected for increased salinity tolerance did not show evidence of adaptation to their respective environments; however, hypo-osmotic selection lines showed substantial loss of tolerance to hyperosmotic stress. Neither of the salinity selection regimes resulted in diminished heat tolerance at ambient salinity; however, simultaneous exposure to heat and hypo-osmotic stress led to decreased heat tolerance, implying a physiological trade-off in tolerance to the two stressors. When we quantified the transcriptomic response to heat and salinity stress via RNA sequencing, we observed little overlap in the stress responses, suggesting the observed synergistic effects of heat and salinity stress were driven by competing energetic demands, rather than shared stress response pathways.