Unique Transcriptomic Profiles in Atlantic Killifish During Salinity Acclimation in Warm Water

The warming effect of climate change threatens aquatic species’ ability to maintain temperature, water, and ion balance. For example, species native to brackish water estuaries are more vulnerable to temperature changes because of frequent salinity changes that are energetically taxing. We used Atlantic killifish (Fundulus heteroclitus) as a model to examine the effects of increased temperature during salinity acclimation on fish physiology and the transcriptomic response at the gill, the major organ for oxygen and ion/water balance. We acclimated killifish to brackish water, then separated the fish into one of six treatment conditions consisting of temperate (22°C) or warm temperature (28°C) and different salinities (freshwater, brackish water, saltwater) for three days.

Using plasma glucose as a general stress indicator, we found that temperature induces stress in both salinity treatments. We also found that changes in salinity altered osmoregulation: plasma osmolality increased in saltwater in both temperatures, and average osmolality in freshwater was lower in warm water than in the control temperature, indicating impaired ionic homeostasis. Using standard RNA-seq approaches, we found that variation in gill gene expression between treatments was driven primarily by salinity and to a lesser extent temperature. Furthermore, we identified unique gene regulatory pathways during salinity acclimation in warm water compared to salinity or temperature acclimation alone. Overall, our results suggest that maintaining homeostasis in warm freshwater is more challenging than in warm saltwater. Climate change may thus pose a particular challenge for brackish water species especially when acclimating to freshwater.