Effects of Salinity and pH Change on the Physiology of an Estuarine Fish Species, Fundulus heteroclitus heteroclitus

Sapelo Island, Georgia is a pristine environment characterized by many variable salt marsh habitats. Throughout the island, salinity ranges from freshwater to full strength seawater and acidity levels range from low pH (pH \u3c 4.4) to above neutral (pH \u3e 8). Fundulus heteroclitus (Mummichog) is a resident organism of the salt marsh environment that has evolved coping strategies to withstand fluctuations in temperature, salinity and pH. Field and laboratory studies have shown F. heteroclitus to exhibit broad tolerance of individual environmental stressors; however, there is limited information on their ability to handle multiple stressors simultaneously. The goal of this study was to determine if exposure to low salinity and low pH combined is more stressful to F. heteroclitus than the factors individually. In a laboratory based experiment, wild caught F. heteroclitus were exposed to one of four treatments: ambient (pH 6.8 and 16 ppt salinity); low salinity (2 ppt), low pH (4.5), and low salinity and low pH combined as a multi-stressor treatment. Six fish were sampled from each treatment group at 1, 5, and 7 days post transfer for the analysis of whole-body cortisol as an indicator of stress or rate of oxygen consumption to estimate metabolic costs. In summary, the results indicate cortisol concentration is unaffected by exposure to low salinity and/or low pH at 1, 5, and 7 days post exposure. However, the resting metabolic rate of F. heteroclitus increased due to low pH at 1 and 7 days post exposure in both the single and multi-stressor exposures. Increased oxygen consumption is likely due to the elevated ATP demand to facilitate changes at the cellular level in attempt to maintain acid-base balance. These results suggest that F. heteroclitus exposed to low pH in natural environments may exhibit energetic tradeoffs that could affect their overall fitness