Changes in oxidative stress of white epaxial muscle in sheepshead minnows and sailfin mollies acclimated to chronic temperatures when acutely exposed to a critical thermal maximum (CTmax) trial.

Rising global temperatures are intensifying thermal stress in aquatic environments, summarily increasing the intensity and frequency of acute thermal stress events for numerous ectothermic organisms. Acute thermal stressors may increase the potential for oxidative damage to tissue, among other risk factors. Estuarine fish that inhabit shallow, thermally variable habitats are especially likely to experience such thermal stress events and, thereby, potential oxidative damage. The oxidative stress system is responsible for quenching reactive oxygen species (ROS) endogenous to aerobic respiration. Enzymatic antioxidants, such as catalase (CAT) and glutathione peroxidase (GPx), and low molecular antioxidants can thwart accumulation of oxidative damage, as in the case of lipid peroxidation (LPO) damage. Here, we investigated how chronic thermal acclimation and acute heat stress influence the oxidative stress system in white epaxial muscle of two thermally resilient estuarine species, the sheepshead minnow (Cyprinodon variegatus) and the sailfin molly (Poecilia latipinna). Fish were acclimated for 1 month to ecologically relevant temperatures of 10 °C, 20 °C, or 30 °C, then subjected to a critical thermal maximum (CTmax) trial designed to simulate an acute heat stress event. We found that CTmax values increased significantly with acclimation temperature in both species, with sailfin mollies achieving higher thermal limits than sheepshead minnows within the 10 and 30 °C chronic acclimation thermal treatments. We also found species-level differences in CAT and GPx activities as well as peroxyl scavenging capacity, with sailfin mollies having higher values for each compared with sheepshead minnows regardless of temperature treatment. However, LPO damage in white muscle did not significantly vary across species, chronic acclimation temperatures, or CTmax timepoints. These findings suggest that both species possess robust, thermally flexible antioxidant systems, capable of protecting cells from LPO damage even with increasing heat. The absence of elevated LPO under chronic and acute thermal challenge supports the idea that eurythermic estuarine fishes are physiologically equipped to withstand rapid temperature fluctuations, a trait that may become increasingly vital as climate change drives more extreme and frequent thermal events in coastal ecosystems.