Upper thermal tolerance of the euryhaline fish Fundulus heteroclitus is lower in freshwater and shaped by environmental salinity–acclimation time interaction

Abstract

This study investigated whether salinity affects upper thermal tolerance (CTmax) in the euryhaline mummichog (Fundulus heteroclitus). Fish were acclimated to seawater (38 ppt, SW) or freshwater (<1 ppt, FW) at 18.5 °C before being transferred to the opposite salinity, and CTmax together with physiological responses were assessed on days 0, 1, 3, 6, and 13 post-transfer. CTmax remained consistently lower in FW than in SW regardless of prior acclimation. Relative to acclimation controls, CTmax exposure decreased plasma ion concentrations in FW-transferred fish but increased them in SW-transferred fish, while elevating plasma cortisol, glucose, lactate, and hematocrit in both salinities. Model selection using the second-order Akaike Information Criterion (AICc) identified the interaction between salinity and acclimation time as the strongest predictor of CTmax. Multivariate analyses further indicated that the physiological mechanisms underlying thermal tolerance differed between environments. In FW, CTmax progressively declined over time, reflecting a trade-off between maintaining ionic homeostasis through energetically costly residual (ouabain-insensitive) ATPase activity and a distinct stress-induced erythropoietic response. In contrast, thermal performance improved over time in SW and was closely associated with osmolality, glucose, and cortisol, whereas lactate accumulation represented a common response across both salinities. Overall, these findings demonstrate that ion-poor freshwater environments impose a persistent thermal penalty, suggesting that the combined effects of low salinity and increasing temperatures may heighten the climate vulnerability of estuarine fish populations beyond current ecological predictions.